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A coder for pulse code modulation using circulated pulses Hafer, Rodney Arnold 1959

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A CODER FOR PULSE CODE MODULATION USING CIRCULATED PULSES by RODNEY ARNOLD HAFER B . A . S c , U n i v e r s i t y of B r i t i s h C o l u m b i a , 1 9 5 8 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE i n the Department o f E l e c t r i c a l E n g i n e e r i n g We a c c e p t t h i s t h e s i s as c o n f o r m i n g to the st a n d a r d s r e q u i r e d from c a n d i d a t e s f o r the degree o f Maste r o f A p p l i e d S c i e n c e . Members o f the Department o f E l e c t r i c a l E n g i n e e r i n g THE UNIVERSITY OF BRITISH COLUMBIA AUGUST, 1 9 5 9 I ABSTRACT Thi s t h e s i s i s concerned w i t h the c o n s t r u c t i o n o f a p u l s e code m o d u l a t i o n c o d e r s the purpose o f w h i c h i s to code the i n f o r m a t i o n c o n t a i n e d i n a s i g n a l h a v i n g no f r e -quency components g r e a t e r t h a n l\. kc i n t o a s e r i e s o f b i n -a r y p u l s e s . The major o b j e c t i v e was the c o n s t r u c t i o n o f a t r a n s i s -t o r i z e d coder u s i n g the c i r c u l a t e d p u l s e p r i n c i p l e w h i c h i n v o l v e d o n l y s i m p l e c i r c u i t s and r e a d i l y a v a i l a b l e com-ponents. The c o n s t r u c t i o n and o p e r a t i o n o f t h i s system are d e s c r i b e d i n f u l l . The r e s u l t s o f t e s t s made on the c i r c u i t u s i n g d-c i n p u t s i g n a l s are a l s o g i v e n . In presenting t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study. I f u r t h e r agree that permission f o r extensive copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s r e p r e s e n t a t i v e s . I t i s understood that copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l gain s h a l l not be allowed without my w r i t t e n permission. Department of E < l / r e 1 = //6>. The U n i v e r s i t y of B r i t i s h Columbia, Vancouver 8, Canada. I I TABLE OP CONTENTS Page ,A.L")S tr*9 Ct o o o o o o o o o o o o o o o « * < » o o o o o o o o « » o o * » o « » « » » » » » « < > * i L i s t o f I l l u s t r a t i o n s .... o . . o . . . . . . . . . . . . . . . . . . . . . . « i v Acknowledgements ................................ v l o I n t r o d u c t i o n . o o . o . o o . o o o . o . . . . . . . . . 1 2 . P r i n c i p l e s o f P u l s e Code M o d u l a t i o n . ............. i | 2 o 1 S 9 Hip X XII ^ o » o o o « o o * » a o o o o o o o e o » o < > o e o » » © » o » © » * L|_ 2 © 2 Q u. s n t i z s t i o n » o © « o o © o o o o * o o o o » o © o » « « « « » » « o » < > ]_|. 2 o 3 Coding » o o O O » « 0 * 0 » 0 9 0 0 0 0 < > « « » « » 0 « 0 « 0 » » 0 » « o i » ' » 0 ^ 2 o Ij. RSgSIlG t l O I l o o o < > o o o o o o o o o o 9 o o o < > o * o » * » » o 9 0 * « 3 2 o 5 Mill t>XpX©Xin§ • a e o « < > o o o o o o o < » » e o o o o < » o o « « « o « * o © 8 3 . Performance C h a r a c t e r i s t i c s o f P u l s e Code MO dull 8 tlOIl. 0 0 0 0 4 > 0 0 » 0 0' O O O O O O O O 0 O O O » O O O O « * » * O » O O O O O XO 3 » 1 T r a n s m i s s i o n Requirements 1 0 3 . 1 ° 1 Ba ndwi d t h o . o . . o « . . o . « . 9 . . . . « . o . . e . . . 1 0 3 . 1 . 2 POWer » 0 9 . . o . o « . o « o o . . . o « a o . « . . . . » » . . 1 1 3 . 2 I n t e r f e r e n c e and C r o s s t a l k ................. 1 2 3 . 3 S i g n a l = to =Noise R a t i o o. 1 3 3 . i f Channel C a p a c i I4.. P u l s e Code M o d u l a t i o n Equipment 1 7 k»l Cod • e 6 111 Page 5<> O p e r a t i o n and C o n s t r u c t i o n o f the Coder .... 2 0 5 » 1 O p e r a t i o n o f the Coder ...•••••..»••...«.... 2 1 5 . 2 Tuning C i r c u i t s o o . « o . o o . o o . « o o o o . « « . « » « o « « « 2I4. 5 o 3 DlOde GateS . o . . o o . o o © . . o o o o . o o « . . . . o e . . . o . © 2 ^ Comparator P u l s e G e n e r a t o r ................. 3 2 5 « 5 S u b t r a c t i n g C i r c u i t 0 0 0 0 . 0 0 0 . 0 . 0 0 . . 0 0 0 0 . . 0 . 0 3^ 4-5 . 6 E m i t t e r ~ F o l l o w e r Gate .......... 3 5 5 » 7 H o l d i n g C i r c u i t 3 6 5 * 8 Double Emitter™Follower * . o o © © o o o © o © o © o o 0 © © o 3 9 6) o A d j u s tine n t s © o o » © « e © o o » o © » o o « © o « © * * © o o © o « o © o © © o » o i|.2 7o Performance o f the Coder o © O o » o < > « « o . © o © © © < » o o o . o « 0 8 » C o n C l t l S l O r i S o O O O 6 © O O O O O O O O O O O e O O 0 * « O O O » O © O O » O O O < > O 4^" 9 Q o R e f e r e n c e s • • c o o o 9 o o o o o o o o o o © * o o o » * o © o * © o © o © Q © o « e 5 1 i v LIST OP ILLUSTRATIONS F i g u r e Page l e i P u l s e M o d u l a t i o n Methods ............... 2 2.1 Input-Output C h a r a c t e r i s t i c o f a L i n e a r Q \X 8 IT t 1 Z6T oooceooooooooooooooooooooooooooo* 3 2.2 I n p u t - O u t p u t C h a r a c t e r i s t i c o f a Tapered Q,UantlZ©r ©o.o.oooooooo.oooo.ooo.o.oo..ooo. 6 2o3 P u l s e Code M o d u l a t i o n oooooooooo»..o»ooooo...o 7 s i n 2/Twt 3 o 1 Tin © P U. 1 S © ,y, ©oooooooooooooooooooaooooo 10 ^7r vvt 3.2 Graph o f S i g n a l - t o ~ N o i s e R a t i o v s . P r o b a b i l i ty o f E r r o r O o . o o o o o o o . . o . o . . 0 . 0 . 0 1 3 5.1 S i m p l i f i e d B l o c k Diagram o f Coder . . 0 0 0 0 . 0 0 . 0 . 22 5.2 Complete B l o c k Diagram o f Coder . o » . . . 0 0 0 0 0 . . . 2 3 5.3 B l o c k Diagram o f Timing C i r c u i t .............. 2 5 5 . I f C i r c u i t Diagram o f Master C l o c k and Frequency D i v i d e r . 0....................... 2 6 5 . 5 C i r c u i t Diagram o f G a t i n g P u l s e G e n e r a t o r .... 2 7 5 . 6 C i r c u i t Diagram o f Dela y e d P u l s e A m p l i f i e r ... 2 7 5 • 7 Timing Wsv©JTonns ooo*eooooo«ooooo«ooo«oooooooo 28 5 . 8 C i r c u i t Diagram o f Diode Gate No. 1 . .. .->. . •. •. . 3 1 5 » 9 C i r c u i t Diagram o f Diode Gate No. 2 ........... 31 5.10 C i r c u i t Diagram of Comparator P u l s e G e n e r a t o r and S u b t r a c t o r 0 0 . 0 0 0 0 . . 0 0 0 0 . 0 0 0 0 33 5.11 C i r c u i t Diagram o f E m i t t e r - F o l i o w© r* Gst© 0 0 0 0 0 3 ^  5.12 C i r c u i t Diagram o f H o l d i n g C i r c u i t ........... 37 5.13 C i r c u i t Diagram o f Double E m i t t e r - F o l l o w e r o.. I4.O 6.1 C i r c u i t Diagram o f d-c I n p u t C i r c u i t ......... 1+2 7 o l I n p u t - O u t p u t C h a r a c t e r i s t i c o f the Coder ...... i+6 V ACKNOWLEDGEMENTS Acknowledgement i s g r a t e f u l l y g i v e n to the N a t i o n a l Re-s e a r c h C o u n c i l f o r s p o n s o r i n g t h i s p r o j e c t under t h e i r b l o c k term g r a n t BT 6 8 and a l s o f o r a S t u d e n t s h i p g r a n t e d the a u t h o r . Acknowledgement i s a l s o g i v e n to P r o f e s s o r P.K. Bowers under whose gu i d a n c e the p r o j e c t was c a r r i e d o u t and to Dr. P. Noakes f o r h i s a s s i s t a n c e to the a u t h o r . Thanks are a l s o extended to the o t h e r members o f the s t a f f and to the gra d u a t e s t u d e n t s o f the E l e c t r i c a l E n g i n e e r i n g Department f o r t h e i r h e l p f u l s u g g e s t i o n s . INTRODUCTION S e v e r a l systems f o r t r a n s m i t t i n g the i n f o r m a t i o n con-t a i n e d i n a c o n t i n u o u s s i g n a l by means o f p u l s e s have been developed i n an e f f o r t t o improve the performance and e f f i -c i e n c y o f communication systems. S i n c e a p u l s e has s e v e r a l parameters s u c h as a m p l i t u d e , d u r a t i o n , and p o s i t i o n t h e r e are many m o d u l a t i o n methods a v a i l a b l e . Some o f the b e t t e r known methods are p u l s e a m p l i t u d e m o d u l a t i o n (PAM) , p u l s e w i d t h m o d u l a t i o n (PWM), p u l s e p o s i t i o n m o d u l a t i o n (PPM), and p u l s e f r e q u e n c y m o d u l a t i o n (PPM). These m o d u l a t i o n methods are i l l u s t r a t e d i n f i g u r e 1 . 1 , As w e l l as the u s u a l f r e q u e n c y d i v i s i o n m u l t i p l e x i n g s most p u l s e m o d u l a t i o n methods a l s o l e n d themselves to time d i v i s i o n m u l t i p l e x i n g s or the s e n d i n g o f s e v e r a l s i g n a l s o v e r o s i n g l e c a r r i e r f r e q u e n c y by i n t e r l e a v i n g t h e i r p u l s e t r a i n s i n t i m e . One o f the most s e r i o u s problems o f communications i s the a c c u m u l a t i o n o f n o i s e i n each l i n k o f a l o n g t r a n s m i s s i o n system. This n o i s e d e t e r m i n e s the maximum l e n g t h o f a p r a c -t i c a l communication c h a n n e l . Any m o d u l a t i o n method, i n c l u d -i n g t he p u l s e methods mentioned above, w h i c h uses a c o n t i n u -o u s l y v a r i a b l e parameter i s s u b j e c t t o t h i s l i m i t a t i o n . How-e v e r , by u s i n g a parameter w h i c h i s l i m i t e d to a s e t o f d i s -c r e t e v a l u e s t h i s a c c u m u l a t i o n o f n o i s e can be overcome. As l o n g as the n o i s e i s l i m i t e d t o l e s s than h a l f the d i f f e r e n c e between two a d j a c e n t a l l o w e d v a l u e s , the r e c e i v e r w i l l be a b l e M o d u l a t i n g S i g n a l J l J l P u l s e A m p l i t u d e M o d u l a t i o n P u l s e W i d t h M o d u l a t i o n P u l s e P o s i t i o n M o d u l a t i o n P u l s e Frequency M o d u l a t i o n F i g u r e l o l P u l s e M o d u l a t i o n Methods 3 . to i d e n t i f y c o r r e c t l y the v a l u e s e n t and so c a n r e c o n s t r u c t the e x a c t s i g n a l . I n a l o n g t r a n s m i s s i o n system e a c h r e -p e a t e r would be a b l e t o r e g e n e r a t e the t r a n s m i t t e d s i g n a l ex-a c t l y , removing any n o i s e p i c k e d up i n the t r a n s m i s s i o n med-ium. T h e r e f o r e , the q u a l i t y o f r e p r o d u c t i o n would be in d e p e n d -ent o f the number o f l i n k s i n the system. Such d i s c r e t e systems are u s u a l l y r e f e r r e d to as d i g i t a l s ystems. I f t h e r e are o n l y two- a l l o w e d v a l u e s o f the t r a n s -m i t t e d s i g n a l s , we have a b i n a r y d i g i t a l system. These a r e o f f a r s i m p l e r c o n s t r u c t i o n t h a n those a l l o w i n g s e v e r a l v a l u e s s i n c e they r e q u i r e o n l y t w o - s t a t e , or b i n a r y , d e v i c e s . Of c o u r s e , i f t h e o r i g i n a l i n f o r m a t i o n i s c o n t a i n e d i n a c o n t i n u o u s s i g n a l , some p r o c e s s w i l l have to be d e v i s e d t o code i t i n t o d i g i t a l f o rm. Two p r a c t i c a l b i n a r y d i g i t a l s y s -tems are i n use toda y , d e l t a m o d u l a t i o n (DM) and p u l s e code m o d u l a t i o n (PCM). The freedom from a c c u m u l a t i v e n o i s e w h i c h th e s e systems a c h i e v e i s p a i d f o r i n t h r e e ways. The c o d i n g p r o c e s s , when a c o n t i n u o u s s i g n a l i s changed to a d i s c r e t e one, i n t r o d u c e s n o i s e . The b a n d w i d t h r e q u i r e d f o r t r a n s m i s s i o n i s g r e a t e r than f o r d i r e c t t r a n s m i s s i o n o r c o n v e n t i o n a l AM. The term-i n a l equipment, c o n s i s t i n g o f coders and d e c o d e r s , i s more complex t h a n f o r c o n v e n t i o n a l systems. T h i s i s e s p e c i a l l y t r u e o f PCM. PRINCIPLES OF PULSE CODE MODULATION 2.1 SAMPLING The b a s i c p r i n c i p l e o f a l l p u l s e communication systems i s t h a t the i n f o r m a t i o n c o n t a i n e d i n a c o n t i n u o u s f u n c t i o n o f l e n g t h T c o n t a i n i n g no f r e q u e n c y component g r e a t e r than W0 i s c o n t a i n e d i n 2W0T + 1 a m p l i t u d e samples o f the f u n c -1 1 t i o n spaced T Q seconds' a p a r t i n time where T 0 = . ' 2W0 T h e r e f o r e , to t r a n s m i t a c o n t i n u o u s s i g n a l o f bandwidth WQ i t i s o n l y n e c e s s a r y to send 2WQ sample a m p l i t u d e s from 1 samples taken a t 0,, seconds i n t e r v a l s . 2W0 The s i g n a l i s r e c o n s t r u c t e d a t the r e c e i v e r by g e n e r -a t i n g an i m p u l s e p r o p o r t i o n a l to e a c h sample a m p l i t u d e and p a s s i n g these i m p u l s e s t h r o u g h an i d e a l low-pass f i l t e r . 2.2 QUANTIZATION Any complex s i g n a l can be approximated by a s i g n a l hav-i n g a d i s c r e t e s e t o f a l l o w e d a m p l i t u d e l e v e l s . This p r o c e s s i s c a l l e d q u a n t i z a t i o n s , a quantum b e i n g the d i f f e r e n c e be-tween two a d j a c e n t d i s c r e t e l e v e l s . The degree o f a p p r o x i -m a t i o n d e s i r e d d e t e r m i n e s the number o f a m p l i t u d e l e v e l s needed and hence the quantum s i z e . The i n p u t - o u t p u t c h a r a c -t e r i s t i c o f a l i n e a r q u a n t i z e r i s a s e r i e s o f s t e p s as i n f i g u r e 2.1. The maximum e r r o r o f t h i s a p p r o x i m a t i o n I s p l u s o r minus one h a l f quantum. This e r r o r causes a d i s t o r t i o n i n the r e -c o n s t r u c t e d s i g n a l c a l l e d q u a n t i z i n g n o i s e . The d i s t o r t i o n c o n s i s t s o f harmonics and m o d u l a t i o n p r o d u c t s between s i g n a l components and the s a m p l i n g f r e q u e n c y . F o r a complex s i g n a l , such as s p e e c h , the q u a n t i z i n g n o i s e i s an e s s e n t i a l l y f l a t band o f n o i s e t h a t sounds much l i k e t h e r m a l n o i s e . I n o r d e r to pass the l o w e s t a m p l i t u d e sounds, the quantum s i z e must be made s m a l l . However, I f the quantum s i z e i s s m a l l , then t o have a system c a p a b l e o f h a n d l i n g h i g h l e v e l s a p r o h i b i t i v e number o f s t e p s , and hence p u l s e s p e r code group, i s r e q u i r e d . The two c o n f l i c t i n g r e q u i r e m e n t s are made compatable by u s -i n g a t a p e r e d s t e p f u n c t i o n as i n f i g u r e 2.2 a s s i g n i n g a g i v -en number o f s t e p s i n g r e a t e r p r o p o r t i o n to the low a m p l i -tudes than to the h i g h s . The o v e r a l l e f f e c t i s to i n c r e a s e the s i g n a l - t o - n o i s e r a t i o o f the s m a l l s i g n a l s b u t a t the same time d e c r e a s i n g t h a t o f the l a r g e ones. F i g u r e 2.2 I n p u t - O u t p u t C h a r a c t e r i s t i c o f a Tapered Q u a n t i z e r 2.3 CODING Each q u a n t i z e d a m p l i t u d e sample o f the i n p u t s i g n a l , s i n c e t h e r e are now o n l y a f i n i t e number o f p o s s i b l e a m p l i -tude v a l u e s , c a n be r e p r e s e n t e d by a group o f b i n a r y p u l s e s . The n p u l s e s i n a code group a r e w e i g h t e d a c c o r d i n g to t h e i r p o s i t i o n i n the group, the m t h p u l s e b e i n g w e i g h t e d by 2 n m . The a m p l i t u d e r e p r e s e n t e d by a code group i s t h e w e i g h t e d sum o f the p u l s e s p r e s e n t . That i s , each a m p l i t u d e sample i s coded i n t o the form o f an n = d i g i t b i n a r y number w i t h p u l s e s f o r m i n g the n d i g i t s , a p u l s e s t a n d i n g f o r the 1 a nd a space f o r the 0 o f the b i n a r y n o t a t i o n . A code h a v i n g n p u l s e s per code group can r e p r e s e n t 2 n a m p l i t u d e l e v e l s . I n some c o d e r s , the q u a n t i z a t i o n i s n o t a s e p a r a t e o p e r a t i o n p r e -c e d i n g c o d i n g b u t i s a r e s u l t o f the c o d i n g o p e r a t i o n i t s e l f . M o d u l a t i n g S i g n a l _ J I fl Q u a n t i z e d A m p l i t u d e Samples [(.-Digit B i n a r y P u l s e Code F i g u r e 2.3 P u l s e Code M o d u l a t i o n 8. Of c o u r s e s a code to any base b c o u l d be used by employ-i n g code p u l s e s h a v i n g b a l l o w e d a m p l i t u d e s . A base b code h a v i n g n p u l s e s per code group c o u l d r e p r e s e n t b n a m p l i t u d e l e v e l s . The b i n a r y code i s the s i m p l e s t s i n c e i t i s e a s i l y r e p r e s e n t e d by b i n a r y o r two p o s i t i o n d e v i c e s , 2,1+ REGENERATION I n most communication systems, the n o i s e i n t r o d u c e d by the i n d i v i d u a l l i n k s accumulates,, A 100 l i n k system w o u l d r e q u i r e a s i g n a l - t o - n o i s e r a t i o i n each l i n k 20 db b e t t e r than the o v e r a l l system r e q u i r e m e n t . O b v i o u s l y , t h e n , the l e n g t h o f a p r a c t i c a l system i s l i m i t e d by the s i g n a l - t o -n o i s e r a t i o r e q u i r e m e n t s . PGMp s i n c e i t s r e c e p t i o n depends o n l y on the r e c o g n i -t i o n o f a s t a n d a r d p u l s e or s p a c e , c a n be r e g e n e r a t e d a t each r e p e a t e r . The r e p e a t e r s must o n l y send out a s t a n -dard p u l s e when a d i s t o r t e d one i s r e c e i v e d and n o t h i n g o t h e r w i s e . Thus, as l o n g as the n o i s e i n e a c h l i n k i s l i m i -ted to s u c h a value t h a t the p u l s e s a r e s t i l l r e c o g n i z a b l e , each r e p e a t e r w i l l send out e x a c t l y the same s i g n a l as was o r i g i n a l l y produced by the c o d e r . T h e r e f o r e , the q u a l i t y o f t r a n s m i s s i o n does not depend on the number o f l i n k s i n the system and so the system l e n g t h i s n o t l i m i t e d by the s i g -n a l - t o - n o i s e r a t i o r e q uirementso 2,5 MULTIPLEXING Modulated, c a r r i e r systems such as AM and PM a r e m u l t i -p l e x e d i n f r e q u e n c y s s e v e r a l c a r r i e r f r e q u e n c i e s b e i n g s e n t s i m u l t a n e o u s l y o v e r the same c h a n n e l . P u l s e communication systems, s i n c e the p u l s e groups can be made to use o n l y a f r a c t i o n o f the time a v a i l a b l e , can be m u l t i p l e x e d i n time as w e l l as i n f r e q u e n c y . That i s , the p u l s e s from s e v e r a l mod-u l a t i n g s i g n a l s can be i n t e r l e a v e d i n time and s e n t o v e r one cha n n e l o r a common c a r r i e r f r e q u e n c y . Of c o u r s e , the time m u l t i p l e x i n g o f s e v e r a l s i g n a l s im-poses a d d i t i o n a l r e s t r a i n t s on p u l s e b u i l d — u p and decay times and p u l s e w i d t h s w h i c h i m p l i e s a s u f f i c i e n t l y wide pass band w i t h good phase l i n e a r i t y . 10, PERFORMANCE CHARACTERISTICS OF PULSE CODE MODULATION 3 . 1 TRANSMISSION REQUIREMENTS 3 . 1 . 1 BANDWIDTH To t r a n s m i t a s i g n a l o f bandwidth W0 by PCM, i t i s n e c e s s a r y to send 2W0 code groups p e r second. T h e r e f o r e , f o r an n - d i g i t c ode s 2nW 0 p u l s e s p e r second must be t r a n s m i t -ted o v e r the c h a n n e l . I f the t r a n s m i s s i o n c h a n n e l has the c h a r a c t e r i s t i c s o f an i d e a l low-pass f i l t e r o f bandwidth nWQ and i f the code p u l s e s a r e s h o r t , t h e n the r e c e i v e d p u l s e s would each be o f the f o r m ( f i g u r e 3 . 1 ) s i n 2 7 T W t V = V o 2 t r w t where W = nW, F i g u r e 3«1 The P u l s e s i n 2 TT Wt 2TTwt A s e r i e s o f such p u l s e s o c c u r r i n g a t r e g u l a r i n t e r v a l s — i — 2W a p a r t has the p r o p e r t y t h a t when one p u l s e i s a t i t s maxi-mum a l l the o t h e r s a re z e r o . Thus, by s a m p l i n g the s i g n a l 11. a t a p p r o p r i a t e i n t e r v a l s , the r e c e i v e r can i d e n t i f y the i n -d i v i d u a l p u l s e s w i t h o u t any I n t e r f e r e n c e from a d j a c e n t p u l -s e s . I n a p r a c t i c a l system, th e t r a n s m i s s i o n medium would not have t h i s i d e a l c h a r a c t e r i s t i c and the r e c e i v e d p u l s e s would n o t have the e x a c t form g i v e n above. As a r e -s u l t s t h e r e would be some i n t e r f e r e n c e between a d j a c e n t p u l -s e s . However, by u s i n g an e f f e c t i v e b a n d w i d t h somewhat g r e a -t e r than nWQ, t h i s i n t e r f e r e n c e can be made s m a l l and t h e r e -f o r e can be c o n s i d e r e d merely as a d d i t i o n a l n o i s e . I t i s then s t i l l p o s s i b l e f o r the r e c e i v e r to d e c i d e whether a p u l s e was s e n t by s a m p l i n g the s i g n a l at a s i n g l e p o i n t . F o r p r a c t i c a l purposes then, the bandwidth r e -q u i r e d f o r the t r a n s m i s s i o n o f one p u l s e t r a i n i s somewhat g r e a t e r than nWQ. I t i s i n t e r e s t i n g to n o t e , however, t h a t much s m a l l e r t r a n s m i s s i o n bandwidths c o u l d be used as l o n g as the s i g n a l - t o - n o i s e r a t i o i s good enough and the r e c e i -v e r I s c a p a b l e o f d i s e n t a n g l i n g the r e s u l t i n g i n t e r f e r e n c e between a d j a c e n t p u l s e s . 3.1.2 POWER I f the r e c e i v e d p u l s e s a r e o f t h e form g i v e n i n the l a s t s e c t i o n and a r e sampled at the a p p r o p r i a t e t i m e s , then a p u l s e i s s a i d to be p r e s e n t i f the sampled s i g n a l i s g r e a t e r t h a n — and absent I f l e s s than — . The d e c i s i o n 2 2 w i l l be wrong i f the n o i s e p r e s e n t a t t h a t i n s t a n t i s g r e a -v o t e r than •— I n the r i g h t d i r e c t i o n . Assuming g a u s s i a n n o i s e the p r o b a b i l i t y o f an e r r o r i s p r o p o r t i o n a l to the comple-mentary e r r o r f u n c t i o n of 2CT V I s I4.N where (f = rms n o i s e a m p l i t u d e P s = s i g n a l "power" = V 0 N = n o i s e "power" i n b a n d w i d t h W=(/ This i s a r a p i d l y d e c r e a s i n g f u n c t i o n o f Ps N so t h a t n e a r l y p e r f e c t t r a n s m i s s i o n can be a c h i e v e d w i t h a v a l u e o f — j u s t s l i g h t l y l a r g e r t h a n i s r e q u i r e d f o r o n l y f a i r t r a n s m i s s i o n . I n f a c t , as can be seen from the g r a p h ( f i g u r e 3.2), t h e r e i s a t h r e s h o l d at about 20 db above w h i c h i n t e r -f e r e n c e i s n e g l i g i b l e . T h e r e f o r e , compared w i t h the 60-70 db r e q u i r e d f o r h i g h q u a l i t y AH t r a n s m i s s i o n , PCM r e q u i r e s much l e s s s i g n a l power even though the n o i s e power i s i n c r e a s e d by the n - f o l d i n c r e a s e i n bandwidth. 3.2 INTERFERENCE AND CROSSTALK One o f the o u t s t a n d i n g c h a r a c t e r i s t i c s o f PCM i s i t s r e s -i s t a n c e to i n t e r f e r e n c e and c r o s s t a l k . I n t e r f e r e n c e i n a b i n a r y PCM system must r e a c h a peak a m p l i t u d e o f g r e a t e r than h a l f the p u l s e h e i g h t to have any e f f e c t on the t r a n s m i s s i o n . S i m i l a r l y * , c r o s s t a l k from nearby channels has no e f f e c t un-l e s s the sum o f the n o i s e and c r o s s t a l k i s g r e a t e r t h a n h a l f a p u l s e h e i g h t o Because o f t h i s , and the r a p i d i n c r e a s e o f — I J J 1 1 I I I 1 2 1 5 16 18 2 0 2 2 2 % 2 6 S i g n a l - t o - N o i s e R a t i o i n db F i g u r e 3 ° 2 Graph o f S i g n a l - t o - N o i s e R a t i o v s , P r o b a b i l i t y o f E r r o r s i g n a l - t o - n o i s e r a t i o w i t h power, i f an adequate power mar-g i n o v e r t h r e s h o l d i s p r o v i d e d the system w i l l be e s s e n t i a l l y u n a f f e c t e d by i n t e r f e r e n c e and c r o s s t a l k . 'The use o f regen-e r a t i o n at r e p e a t e r s a l l o w s the system r e q u i r e m e n t f o r i n t e r -f e r e n c e and c r o s s t a l k to be used f o r each l i n k s i n c e the n o i s e does n o t a c c u m u l a t e . 3 . 3 SIGNAL-TO-NOISE RATIO There a r e two t y p e s o f n o i s e I n t r o d u c e d by a PCM system, q u a n t i z i n g n o i s e i n t r o d u c e d at the coder and f a l s e p u l s e n o i s e caused by i n c o r r e c t i n t e r p r e t a t i o n o f the s i g n a l a t a r e p e a t e r or the r e c e i v e r . F a l s e p u l s e n o i s e can a r i s e any-where a l o n g the system and i s c u m u l a t i v e . However, a s was seen i n s e c t i o n 3 ° 1 ° 2 , t h i s n o i s e i s s u c h a r a p i d l y d e c r e a s -i n g f u n c t i o n o f the s i g n a l power t h a t i t can be made n e g l i -14. g i b l e by d e s i g n . Therefore., the s i g n a l - t o - n o i s e r a t i o o f a PGM system i s s e t by the q u a n t i z i n g n o i s e a l o n e . For s i g n a l s w h i c h are l a r g e compared to the s i z e o f a quantum s t e p , the e r r o r s i n t r o d u c e d by q u a n t i z a t i o n w i l l be e s s e n t i a l l y u n c o r r e l a t e d . A l l po s s i b l e . v a l u e s o f e r r o r up to the maximum o f p l u s o r minus one h a l f quantum are e q u a l l y 1 p r o b a b l e . T h e r e f o r e , the rms e r r o r i n t r o d u c e d i s 2-JT °^ a 2 quantum. The r e c o n s t r u c t e d s i g n a l c o n s i s t s o f the o r i g i n a l s i g n a l p l u s a n o i s e o f u n i f o r m spectrum o u t to WQ and o f rms i a m p l i t u d e 2-y/~3 °^ a Quantum. The r a t i o o f peak-to-peak s i g -n a l to rms n o i s e i s , t h e r e f o r e , R = 2 V"3~ b n' s i n c e b n i s the number o f l e v e l s . E x p r e s s i n g t h i s r a t i o i n d e c i b e l s , we have 20 l o g 1 0 R = 20 l o g 1 0 2 V T + n(20 l o g 1 0 b ) T h e r e f o r e , the s i g n a l - t o - n o i s e r a t i o i n d e c i b e l s f o r a b i n a r y system, b = 2 2 i s 20 l o g 1 Q R = 10.8 + 6n S i n c e W = nWQ, the s i g n a l - t o - n o i s e r a t i o i s a l i n e a r f u n c t i o n o f the b a n d w i d t h used i n t r a n s m i s s i o n . Of c o u r s e , i n c r e a s e d bandwidth a l l o w s i n c r e a s e d n o i s e power and the s i g n a l power would need to be i n c r e a s e d to keep the f a l s e p u l s e n o i s e neg-l i g i b l e . 1 5 . 3.k CHANNEL CAPACITY The bandwidth e f f i c i e n c y can be measured as the i n f o r m a -t i o n c a p a c i t y o f the system compared w i t h the t h e o r e t i c a l l i m i t f o r a c h a n n e l of the same bandwidth and power. The c a p a c i t y o f an i d e a l system i s g i v e n by 1 p C = W l o g 2 (1 + ^r) b i n a r y d i g i t s / s e c o n d where W = bandwid t h P —. average s i g n a l power N = w h i t e n o i s e power The c a p a c i t y o f a b i n a r y PCM system o p e r a t i n g a t a s i g -n a l - t o - n o i s e r a t i o o f 2 0 db so t h a t the frequ e n c y o f e r r o r s i s n e g l i g i b l e i s C = 2W Qn = 2W For a s y m m e t r i c a l b i n a r y system ( t h a t i s a system u s i n g p o s i -t i v e and n e g a t i v e p u l s e s o f e q u a l s i z e r a t h e r than p u l s e s and spaces) w i t h p u l s e s o f a m p l i t u d e X° the peak power i s the 2 2 same as the average power and i s J_2 . T h e r e f o r e , f o r V 2 k 12. = 2 0 db ^ N V 2 N 1|N k The t h e o r e t i c a l c a p a c i t y o f a system w i t h t h i s s i g n a l - t c -noi.se r a t i o i s C = W l o g P (1 + 2 5 ) = i+.7W i 6 . T h e r e f o r e , PCM has o n l y about 0.1+25 o f the t h e o r e t i c a l m a x i -mum c h a n n e l c a p a c i t y . I n o r d e r to approach c l o s e r to the t h e o r e t i c a l l i m i t , i t i s n e c e s s a r y to use e r r o r c o r r e c t i n g codes. These are n e c e s -s a r i l y complex, r e q u i r i n g complex c o d i n g equipment. They a l s o produce a dela y i n t r a n s m i s s i o n s i n c e the r e c e i v e r must w a i t f o r the e r r o r s to be c o r r e c t e d b e f o r e the o u t p u t i s g i v e n . C o n s i d e r i n g the s i m p l i c i t y o f a PCM system and the f a c t t h a t there i s no d e l a y , the c h a n n e l c a p a c i t y a c h i e v e d i s v e r y good. 17o If. PULSE CODE MODULATION EQUIPMENT The job o f a PCM coder i s to t a k e an a m p l i t u d e sample, q u a n t i z e i t , and r e p r e s e n t t h i s q u a n t i z e d v a l u e as a p a t t e r n o f p u l s e s ( u s u a l l y b i n a r y p u l s e s ) . The decoder must take these p u l s e p a t t e r n s and from them r e c o n s t r u c t the sample a m p l i t u d e s . The samples can be d e r i v e d f r o m a s i n g l e s i g -n a l o r from s e v e r a l s i g n a l s each sampled i n t u r n . I f sev-e r a l s i g n a l s a r e b e i n g s e n t over the same c h a n n e l , the coder i s preceded by a c o l l e c t i o n s w i t c h and the decoder i s f o l -lowed by a d i s t r i b u t i o n s w i t c h w h i c h i s s y n c h r o n i z e d w i t h the c o l l e c t i o n s w i t c h . The a c t u a l coder and decoder would be t h e same as f o r a system t r a n s m i t t i n g o n l y one s i g n a l . 1+..1 CODERS PCM coders c an b e grouped i n t o t h e f o l l o w i n g t h r e e c a t e -g o r i e s depending on t h e i r method o f measuring the sample am-p l i t u d e . A. The a m p l i t u d e o f t h e sample i s measured by count-i n g the number o f u n i t s c o n t a i n e d i n i t one by one u n t i l t h e 3 remainder i s l e s s than a quantum. This type o f coder i s of c o m p a r a t i v e l y s i m p l e c o n s t r u c t i o n and i s o f t e n used i n p u l s e h e i g h t a n a l y z e r s . These c o d e r s are the s l o w e s t o f the t h r e e groups. B. The sample a m p l i t u d e i s measured i n a s i n g l e op-e r a t i o n by c o m p a r i s o n w i t h a s e t o f s c a l e d v a l u e s . ^ This system i s e x t r e m e l y f a s t , the whole c o d i n g o p e r a t i o n can be done i n a b o u t one m i c r o s e c o n d . The c i r c u i t r y , however, i s 18. complex and u s u a l l y i n v o l v e s s p e c i a l beam c o d i n g t u b e s . ^ These coders become e c o n o m i c a l o n l y when used i n systems w i t h e i t h e r many ch a n n e l s o r wide bandwidth s i g n a l s where t h e i r speed can be p u t to f u l l u s e . G. A m p l i t u d e i s measured by comparing the sample w i t h one d i g i t v a l u e a f t e r a n o t h e r p r o c e e d i n g from the most s i g n i f i c a n t d i g i t to the l e a s t . There are two ways o f do-i n g t h i s . A s e t o f v o l t a g e c o m parators, each c o r r e s p o n d i n g to a d i g i t v a l u e , can be used, the s i g n a l b e i n g a p p l i e d to each i n t u r n , o r a s i n g l e comparator can be used the s i g n a l b e i n g c i r c u l a t e d around a l o o p , a m p l i f i e d a p p r o p r i a t e l y and a p p l i e d to the same co m p a r a t o r f o r each l e v e l . Both these types o f coders a re o f i n t e r m e d i a t e speed, the f i r s t b e i n g o f i n t e r m e d i a t e c o m p l e x i t y and the second o f r e l a t i v e s i m -p l i c i t y . They l e n d themselves to t h e c o d i n g o f a few s i g -n a l s o f moderate bandwidth, s u c h as speech. The c o d e r d i s -cussed i n the f o l l o w i n g c h a p t e r s i s o f the l o o p e d s i g n a l t y p e . 1|.2 DECODERS PCM decoders a r e a l s o o f many t y p e s , two o f w h i c h w i l l be mentioned very b r i e f l y h e r e . The f i r s t type uses a d e l a y l i n e to o b t a i n a l l the s i g -n a l p u l s e s at one time adding t h e i r w e i g h t e d v a l u e s to ac h i e v e the sample a m p l i t u d e . They l e n d themselves to f a s t o p e r a t i o n where the d e l a y l i n e s i n v o l v e d are s h o r t but o f course s u f f e r from the d i f f i c u l t i e s accompanying a l l d e l a y l i n e s . 19-The second uses a l o o p p r i n c i p l e a n a l o g o u s to the l o o p e d s i g n a l coder and w o u l d be the t y p e most l o g i c a l l y a s s o c i a t e d w i t h the coder w i t h w h i c h t h i s t h e s i s i s concerned,, 2 0 o 5 . OPERATION AND CONSTRUCTION OP THE CODER We w i l l now d i s c u s s the o p e r a t i o n and c o n s t r u c t i o n o f the coder w h i c h was designed and b u i l t as the s u b j e c t o f t h i s t h e s i s . A sampling r a t e o f 8 kc and a 6 - d i g i t code were chosen as they a re s u f f i c i e n t to t r a n s m i t telephone q u a l i t y speech and can be a c h i e v e d w i t h r e a d i l y a v a i l a b l e t r a n s i s t o r s o The c i r c u i t was developed s t a r t i n g from t h e si m p l e b l o c k diagram ( f i g u r e 5<>l)o I t was d e c i d e d t h a t d e l a y l i n e was to be a v o i d e d I n the s i g n a l p a t h s i n c e the r e q u i r e d d e l a y s were l o n g f o r c o n t i n u -ous l i n e s and i t was f e l t t h a t b e t t e r l i n e a r i t y and a c c u r a c y c o u l d be a c h i e v e d w i t h the use o f h o l d i n g c a p a c i t o r s . The h o l d i n g system used a l s o a l l o w s g r e a t f l e x i b i l i t y I n the speed o f o p e r a t i o n . To change the s a m p l i n g r a t e and number o f d i g i t s , o n l y the t i m i n g c i r c u i t s need to be a d j u s t e d . The c i r c u i t i n t h i s f i n a l form i s o n l y a l a b o r a t o r y model* No g r e a t e f f o r t was expended t r y i n g to reduce the number o f components to a minimum a l t h o u g h s i m p l i c i t y o f the o v e r a l l c i r c u i t was a major c r i t e r i o n . The number o f s u p p l y v o l t -ages c o u l d a l s o be reduced w i t h o u t undue d i f f i c u l t y . Only a s i n g l e c h a n n e l system was b u i l t b u t the speed o f o p e r a t i o n Is s u f f i c i e n t to a l l o w about 3 c h a n n e l s t o be coded i n the a v a i l a b l e time w i t h o n l y a r e - d e s i g n o f the t i m -i n g system. U s i n g the e x i s t i n g components, the speed o f the p r e s e n t c i r c u i t c o u l d p r o b a b l y be i n c r e a s e d to handle about 5> c h a n n e l s . 2 1 . 5 . 1 OPERATION OF THE CODER The o p e r a t i o n o f the coder w i l l be e x p l a i n e d on the b a s i s o f the s i m p l i f i e d b l o c k diagram ( f i g u r e 5*1 )• Diode g a t e No. 1 i s o p e r a t e d by p u l s e s o c c u r r i n g a t the 8 kc sampling f r e q u e n c y . When a g a t i n g p u l s e a r r i v e s , gate No. 1 a l l o w s a sample o f the i n p u t s i g n a l to be a p p l i e d to the v o l t a g e comparator. I f the sample p u l s e a m p l i t u d e i s g r e a t e r t h a n V, the v o l t a g e c o r r e s p o n d i n g to 3 2 u n i t s , the p u l s e g e n e r a t o r g i v e s an o u t p u t . The a c t i o n o f the p u l s e g e n e r a t o r when t h i s o u t p u t o c c u r s a l s o s u b t r a c t s V v o l t s from the sample p u l s e i n the s u b t r a c t i o n c i r c u i t . The r e m a i n i n g s i g n a l p u l s e i s then a m p l i f i e d by 2 and s t o r e d i n the h o l d -i n g c i r c u i t u n t i l diode gate No. 2 , w h i c h i s o p e r a t e d by p u l s e s o f the same w i d t h as gate No. 1 b u t o f 6 times the fr e q u e n c y , a l l o w s i t to be a p p l i e d to the comparator. I f t h i s a m p l i f i e d p u l s e i s g r e a t e r than V, an out p u t p u l s e I s g e n e r a t e d . I f no output p u l s e o c c u r s , the s i g n a l p u l s e has n o t h i n g s u b t r a c t e d from i t and i s a g a i n doubled b e f o r e b e i n g s t o r e d and then compared w i t h V a g a i n . I n t h i s way, the 6 b i n a r y p u l s e s o f w e i g h t s 3 2 , 1 6 , 8 , If, 2 , and 1 a r e g e n e r a -ted i n t h a t o r d e r . A f t e r the s i g n a l has been a p p l i e d to the comparator f o r the s i x t h t i m e , g a t e No. 2 i s i n h i b i t e d and the charge r e -ma i n i n g i n the h o l d i n g c i r c u i t i s removed. A t the same t i m e , gate No. 1 a g a i n samples the i n p u t s i g n a l and the c o d i n g op-e r a t i o n begins a g a i n . G a t i n g P u l s e s I S i g n a l i n G a t i n g P u l s e s | H o l d i n g C i r c u i t Comparatpr P u l s e Generator Code out S u b t r a c t o r A m p l i f i e r F i g u r e 5«1 S i m p l i f i e d B l o c k Diagram o f Coder ro ro GP.3+! S i g n a l I n 3s I J Diode Gate No. 1 ( F i g u r e 5 ° 8 ) GP1+ GP3 = Diode Gate No. 2 ( F i g u r e 5<>9) H o l d i n g C i r c u i t ( F i g u r e 5 . 1 2 ) GP1-B u f f e r A m p l i f i e r ( F i g u r e 5 = 1 0 ) E m i t t e r -F o l l o w e r Gate ( F i g u r e 5 o i l ) Compara t o r P u l s e G e n e r a t o r ( F i g u r e 5.10) [ Code out V S u b t r a c t o r ( F i g u r e 5 . 1 0 ) A m p l i f i e r ( F i g u r e 5«10) I GP2- |GPld F i g u r e 5 * 2 Complete B l o c k Diagram o f Coder V 2k* The e x t r a u n i t s i n the complete b l o c k diagram ( f i g u r e 5 » 2 ) do n o t a l t e r the mode o f o p e r a t i o n o f the c i r c u i t . The e x t r a gate removes s p i k e s from l e a d i n g and t r a i l i n g edges o f the s i g n a l p u l s e w h i c h r e s u l t from i m p e r f e c t t i m i n g o f the sub-t r a c t i o n o p e r a t i o n . The b u f f e r s match impedance l e v e l s and i s o l a t e c e r t a i n c i r c u i t s p r e v e n t i n g unwanted fe e d b a c k e f -f e c t s , , 5 . 2 TIMING CIRCUITS The. c i r c u i t ' s used to g e n e r a t e the n e c e s s a r y g a t i n g p u l s e s are s t a n d a r d m u l t i v i b r a t o r s . The master c l o c k ( f i g u r e S »k) > w h i c h s e t s the f r e q u e n c y f o r the whole c i r c u i t s i s an o r d i n -ary a s t a b l e m u l t i v i b r a t o r w i t h a p e r i o d of 2 0 . 8 m i c r o s e c o n d s f o r a 6 - d i g i t code and an 8 kc sampling r a t e . The p o t e n t i o -meters R|^  and R^ p r o v i d e a d j u s t m e n t o f the p e r i o d o f the c l o c k from about 1 5 microseconds to a b o u t 2\ m i c r o s e c o n d s . The f r e q u e n c y d i v i d e r ( f i g u r e B»k) I s a n o t h e r a s t a b l e m u l t i v i b r a t o r w h i c h i s s y n c h r o n i z e d to the m a s t e r c l o c k by p u l s e s f e d to the e m i t t e r s o f T3 and T^ t h r o u g h C3 and C^. The p o t e n t i o m e t e r s R-j_2 and R]_J^  p r o v i d e adjustment o f the p e r i o d and t h e r e f o r e the r a t i o o f f r e q u e n c i e s . Use o f t h i s adjustment i n c o n j u n c t i o n w i t h adjustment o f the master c l o c k f r e q u e n c y a l l o w s the s a m p l i n g r a t e and number o f p u l s e s per code group to be chosen o v e r a f a i r l y wide r a n g e . The g a t i n g p u l s e g e n e r a t o r s ( f i g u r e 5 - 5 ) are o r d i n a r y monostable m u l t i v i b r a t o r s t r i g g e r e d by t i m i n g p i p s d e r i v e d from the l e a d i n g edges o f the waveforms o f the master c l o c k Delayed P u l s e A m p l i f i e r ( F i g u r e 5.6) G P l d j Master C l o c k ( F i g u r e 5.4) Delay Delay G a t i n g P u l se Genera t o r No. 1 ( F i g u r e 5.5) GP1+~J GP1-| Frequency D i v i d e r ( F i g u r e S«h) Ga t i n g P u l s e Genera t o r No. 2 ( F i g u r e 5.5) GP2+| GP2- | G a t i n g P u l s e G e n e r a t o r No. 3 ( F i g u r e 5.5) GP3+~J GP3-| F i g u r e 5.3 B l o c k Diagram o f Timing C i r c u i t MASTER CLOCK FREQUENCY DIVIDER =13o5v—k I"l s T2 S R 1 = 1^70 R 2 = 1.5K R 3 = 33K R^ = 5 0 K R 5 = 33K L 3> xh T. = 2N2I+7 R 6 = 5 0 K R ? = 1|70 Eq = 1.5K R 9 = 2.2K. R 1 0 = IK R l l = 1 ^ 0 K 1 2 u 1 3 R i 5 R 16 100K 1 5 0 K IOOK 2.2K IK C 2 ° 3 % C 5 c 6 = = I f 7 0 uuf = 1+70 uuf = 3 3 0 uuf = 0 . 0 0 2 u f = 0 . 0 0 2 u f = 3 3 0 uuf F i g u r e 5.K C i r c u i t Diagram o f Master C l o c k and Frequency D i v i d e r IV) o - I 3 » 5 v R R. 1 8 Z C Q 1 I d — ~ 6 v T 6 = 2N21+7 D l s D 2 s D 3 = 1N^97 R 1 7 = 1.5K R L 8 = 33K R l 9 = I . 5 K R20 = 10K c 8 R 2 1 = 10K °9 R22 = 608K 200 uuf 50 uuf 100 uuf F i g u r e 5 ° 5 C i r c u i t Diagram o f G a t i n g P u l s e G e n e r a t o r -13.5v GPl-Ov T GPl d c l 6 I \ J 5 R 3 7 ? R 11 3 5 2N21+7 1.8K R 3 6 = 5.6K = 7 0 uuf R^y = 6 8 0 Delay L i n e = 1 . 0 usee. HH2500 F i g u r e 5 « 6 C i r c u i t Diagram o f Delayed P u l s e A m p l i f i e r 28. Master C l o c k GP3 + F i g u r e 5 ° 7 Timing Waveforms 29o and frequency d i v i d e r . The o u t p u t p u l s e s GP1- and GP2- must be o f e x a c t l y 6 v o l t s a m p l i t u d e . The d i o d e s on the c o l l e c -t o r s a re used to s e t t h i s l e v e l . Diodes a r e used on a l l the c o l l e c t o r s to s e t the am p l i t u d e o f the v o l t a g e swing and hence the p u l s e w i d t h a t a w e l l c o n t r o l l e d v a l u e . The p u l s e g e n e r -a t o r s g i v e output p u l s e s o f 6 v o l t s a m p l i t u d e and 2.5 m i c r o -seconds d u r a t i o n . G a t i n g p u l s e s GP1+ and GP3- must be a c c u r a t e l y s y n c h r o n -i z e d s i n c e every s i x t h p u l s e o f GP1+ i s i n h i b i t e d by GP3- i n diod e g a t e No. 1. The O.I4. microsecond d e l a y l i n e between the master c l o c k and p u l s e g e n e r a t o r No. 1 overcomes the d i s -c r e p a n c i e s i n t h i s t i m i n g due t o f i n i t e r i s e times and d e l a y i n s y n c h r o n i z a t i o n . T h i s d e l a y l i n e Is not p r o p e r l y t e r m i n a -ted but the r e f l e c t e d p u l s e s are s m a l l and the diode D^ i s so b i a s e d t h a t o n l y the top o f the i n i t i a l p u l s e i s a p p l i e d to the p u l s e g e n e r a t o r . The d e l a y e d p u l s e a m p l i f i e r ( f i g u r e 5.7) p r o v i d e s a one micr o s e c o n d wide p u l s e d e l a y e d 1„0 m i c r o s e c o n d w i t h r e s p e c t to GPl+o This p u l s e c o n t r o l s the e m i t t e r - f o l l o w e r g a t e . The d e l a y l i n e i s c o r r e c t l y t e r m i n a t e d a t the r e c e i v i n g end and so p r o v i d e s a c l e a n p u l s e to the a m p l i f i e r , , 5.3 DIODE GATES The diode g a t e s must be l i n e a r to about 1% a c c u r a c y o v e r the range o f a few m i l l i v o l t s to about 3.5 v o l t s and have e i t h e r no p e d e s t a l o r e l s e one o f s m a l l c o n s t a n t a m p l i t u d e . The g a t e s must a l s o f e e d a common l o a d w i t h o u t i n t e r f e r i n g 30-w i t h each other's operation., U n i d i r e c t i o n a l Lewis g a t e s 7 were chosen s i n c e they s a t i s f y the above r e q u i r e m e n t s and a r e o f s i m p l e d e s i g n . Diode gate No. 1 ( f i g u r e 5 - 8 ) i s a s i m p l e u n i d i r e c t i o n a l Lewis ga t e w h i c h Is o p e r a t e d by g a t i n g p u l s e GP3+ w h i c h i s a p p l i e d through c a p a c i t o r C]_yo T h i s p o s i t i v e p u l s e t u r n s o f f the n o r m a l l y c o n d u c t i n g d i o d e D-^ Q thereby a l l o w i n g the j u n c t i o n p o i n t o f the t h r e e d i o d e s to r i s e from - 6 v o l t s toward the + 6 v o l t s u p p l y . This p o i n t , however, i s caught a t the s i g n a l v o l t a g e when D-^ s t a r t s to c o n d u c t . The low impedance s i g n a l s o u r c e t h e r e f o r e s e t s the l e v e l to whi c h the j u n c t i o n p o i n t and hence the h i g h impedance l o a d p o i n t r i s e s . I n t h i s way, the gat e a l l o w s a 2 . 5 m i c r o s e c o n d wide sample o f the i n p u t s i g n a l to e n t e r the c o d i n g c i r c u i t ev-ery 1 2 5 microseconds,, Diode gate No. 2 ( f i g u r e 5 ° 9 ) i s i d e n t i c a l to No. 1 ex-cep t f o r the I n h i b i t o r . The g a t i n g p u l s e s GP1+ a p p l i e d through c a p a c i t o r C 2£ o p e r a t e the g a t e . C o i n c i d e n t w i t h ev-ery s i x t h p u l s e o f GP1+ t h e r e i s a p u l s e o f GP3- a p p l i e d through c a p a c i t o r C2~,° T h i s n e g a t i v e p u l s e p r e v e n t s the j u n c t i o n p o i n t o f the g a t e from r i s i n g to the s i g n a l l e v e l and hence i n h i b i t s the a c t i o n o f the g a t e . T h e r e f o r e , the base o f t r a n s i s t o r T^2 3 G & S the s i g n a l f r om gate No. 1 and then f o r the n e x t f i v e g a t i n g p e r i o d s i t sees the s i g n a l from g a t e No. 2 , the s i x t h s i g n a l once more coming from g a t e 31. •I3c5v GP3+ S i g n a l i n Ov -+6v T l 2 = 2N2lf7 R38 = 2.2K D 1 Q P D l l t D 1 2, D 1 3 = lNi+97 R 3 9 = 3.3K %0 C 1 7 3.3K .01 u f F i g u r e 5°8 C i r c u i t Diagram o f Diode Gate No. 1 •13.5v T 1 2 * T 2 i = 2N2if7 D 1 3, D 2 l s D 2 2 s D 2 3, D 2^ = lNl+97 \o = 33K R 6 3 = klO R 6'5 = 2o2K R 66 = 10K R 6 7 = 3.3K c 26 = .01 u f c 2 ? = .01 u f F i g u r e 5<>9 C i r c u i t Diagram o f Diode Gate No. 2 3 2 o No. - I . The l o a d f o r these g a t e s i s the i n p u t impedance o f the b u f f e r a m p l i f i e r T 1 2» R e s i s t o r R^o, and d i o d e D-^ b i a s T l 2 a t the c o r r e c t l e v e l . When a p o s i t i v e p u l s e i s a p p l i e d to the base o f T^ 2» I s G u t o f f . When the g a t e s i g n a l d r o p s , D 1 2 ^ s c u ^ a n d ^ e base o f T]_2 decays toward - 1 3 ° 5 v o l t s t hrough R^o, b e i n g caught a t - i f . 5 v o l t s by D-^. I f i n s t e a d o f D]_3 a r e s i s t o r to ground was used, the base o f T-j_2 would decay toward - i f . 5 v o l t s i n s t e a d o f - 1 3 ° 5 v o l t s and so the t r a i l i n g edge o f t h e . s i g n a l p u l s e w o u l d l a s t much l o n g e r . 5 . i f COMPARATOR. PULSE GENERATOR The comparator has to meet s t r i n g e n t r e q u i r e m e n t s . I t d e c i d e s whether o r not the s i g n a l coming to the p u l s e gen-e r a t o r exceeds a f i x e d r e f e r e n c e v o l t a g e ; i f i t does, t h e n the p u l s e g e n e r a t o r i s to send out a s t a n d a r d p u l s e . This d e c i s i o n s h o u l d be made unambiguously and t h e r e s h o u l d be no d r i f t i n the r e f e r e n c e l e v e l . I n p r a c t i c e , w i t h a quan-tum s i z e o f about 5 0 mv, a d r i f t o f 2 0 mv and a "maybe-re-g i o n " o f 1 mv are j u s t about p e r m i s s i b l e . The comparator and p u l s e g e n e r a t o r ( f i g u r e 5 » 1 0 ) are combined i n a s i n g l e u n i t w h i c h i s a c o n v e n t i o n a l S c h m i t t c i r c u i t . T r a n s i s t o r T-^ i s n o r m a l l y c o n d u c t i n g a c u r r e n t o f 7 ma. w h i l e t r a n s i s t o r T-^ i s n o r m a l l y o f f . A p o s i t i v e p u l s e o f a m p l i t u d e g r e a t e r t h a n o r e q u a l to 1 . 0 v o l t a p p l i e d through d i o d e D]_^ to the base o f T-^ -j s w i t c h e s t h i s c u r r e n t to T]_[|_. The c u r r e n t i s s w i t c h e d back t o T-^ when the s i g n a l •13.5v T 1 2 s T 1 3, T x i | = 2N21+7 = 2N585 A l l Diodes = lNl+97 R 4 o = 33K R 4 1 ^ 7 0 R 4 2 Ri+3 R 4 4 %s %6 10K 4.7K I . 5K 10K 680 R 4 7 " 4-7K R 4 8 = IK \ 9 ~ 4 7 0 R 5 o ~ IK c l 8 = 50=100 uuf F i g u r e 5«10 C i r c u i t Diagram o f Comparator P u l s e G e n e r a t o r and Sub t r a c t o r 2>h* v o l t a g e f a l l s below a n o t h e r l e v e l V v o l t s . The o u t p u t p u l s e Is t a k e n from the c o l l e c t o r o f T-^« Diode D ^ i s i n c l u d e d i n the c i r c u i t t o unhook the p u l s e g e n e r a t o r when i t s w i t c h e s s t a t e s and so p r e v e n t i t from a f -f e c t i n g the s i g n a l o u t p u t o f T-^ • Diode D-^ c a t c h e s the base o f T-^ a t -3 v o l t s and p r o -duces t h e r e a f l a t - t o p p e d waveform. T h i s ensures a c o n s t a n t c u r r e n t i n T-^ d u r i n g the s u b t r a c t i o n p e r i o d . The d-c c o u p l e d S c h m i t t c i r c u i t was chosen r a t h e r than the a-c c o u p l e d f o r two r e a s o n s . F i r s t , by a d j u s t i n g G-^ Q, the c i r c u i t can be made r e l a t i v e l y independent o f the r i s e time o f the t r i g g e r i n g p u l s e . This i s n e c e s s a r y s i n c e the i n i t i a l sample p u l s e has a c o n s i d e r a b l y f a s t e r r i s e time than the p u l s e s w h i c h have c i r c l e d the l o o p . The second r e a s o n f o r c h o o s i n g the d-c c o u p l e d c i r c u i t i s t h a t the r e c o v e r y time o f a monostable e m i t t e r - c o u p l e d m u l t i v i b r a t o r i s l o n g compared to the ou t p u t p u l s e w i d t h . T h i s would r e q u i r e a l o n g dead time between d i g i t p u l s e s w h i c h would s e r i o u s l y l i m i t the speed of the system. 5.5 SUBTRACTING CIRCUIT When t h e s i g n a l p u l s e a p p l i e d to the comparator i s g r e a -t e r than V, the c u r r e n t s w i t c h e d to T ^ i s s u b t r a c t e d from the s i g n a l c u r r e n t i n T]_2 i n the common l o a d r e s i s t o r R^g ( f i g u r e 5 o l 0 ) . The p o t e n t i o m e t e r RJ^ Q i s a d j u s t e d so t h a t when the s i g n a l i s j u s t l a r g e enough to t r i g g e r the S c h m i t t c i r c u i t , t h e s u b t r a c t i o n p r o c e s s l e a v e s zero s i g n a l a t the 35. base o f T-^. T r a n s i s t o r s T-^ and T^^, as w e l l as f o r m i n g the s u b t r a c -t e r and a b u f f e r , a l s o p r o v i d e the r e q u i r e d l o o p g a i n . Tran-s i s t o r T-L^  i s NPN to e n a b l e d-c c o u p l i n g w h i c h g i v e s f a s t e r p o s s i b l e o p e r a t i o n than a-c c o u p l i n g w i t h d-c r e s t o r a t i o n . However, t h i s i s p a i d f o r w i t h the r e s u l t i n g s u s c e p t i b i l i t y to d r i f t i n g o f d-c l e v e l s and temperature i n s t a b i l i t y . S i n c e the s i g n a l p u l s e has f i n i t e r i s e and f a l l times and the p u l s e g e n e r a t o r o n l y g i v e s an ou t p u t when the s i g n a l p u l s e a m p l i t u d e i s g r e a t e r than V, the r e i s a p o r t i o n a t the b e g i n n i n g and end o f each s i g n a l p u l s e where the s u b t r a c t o r has no e f f e c t . F o r some s i g n a l l e v e l s , t h i s causes s p i k e s a t each edge o f the s i g n a l p u l s e a f t e r s u b t r a c t i o n . To r e -move these s p i k e s , the o u t p u t o f the s u b t r a c t o r must be r e -gated b e f o r e i t r e a c h e s the h o l d i n g c i r c u i t . 5.6 EMITTER-FOLLOWER GATE The e m i t t e r - f o l l o w e r g a t e ( f i g u r e 5 « H ) removes the l e a d -i n g and t r a i l i n g edges f r o m the s i g n a l p u l s e a f t e r the sub-t r a c t i o n . A dio d e gate c o u l d n o t be used here s i n c e c u r r e n t g a i n was needed i n o r d e r to be a b l e t o charge the f i r s t h o l d -i n g c a p a c i t o r to the maximum s i g n a l l e v e l i n the one m i c r o s e -cond a v a i l a b l e . The common e m i t t e r p o i n t i s clamped a t the v o l t a g e o f whi c h e v e r base i s most n e g a t i v e , n o r m a l l y t h a t o f t r a n s i s t o r T]_y. When a l a r g e p o s i t i v e g a t i n g p u l s e GPld i s a p p l i e d to T-j_y t h rough c a p a c i t o r C ^ , T-, y i s c u t o f f and the v o l t a g e on 36, •13o5v 2N585 T l 5 T l 6 D l 6 = 1N^97 T 1 ? = 2Nl+0L\ R ^ 0 = 1 K = 1.2K R 5 2 = 2 7 K R 53 ! 2 0 27K .01 uf 680 uuf F i g u r e 5 « H C i r c u i t Diagram o f E m i t t e r - F o l l o w e r Gate the e m i t t e r r i s e s toward +6 v o l t s . However, when t h e e m i t t e r v o l t a g e reaches the l e v e l o f the base o f T-^, i t i s clamped by the s i g n a l on the base o f T^. T h e r e f o r e , the f i r s t h o l d -i n g c a p a c i t o r C 2Q i s charged up to the s i g n a l l e v e l t h r o u g h diode D]_^. 5.7 HOLDING CIRCUIT The h o l d i n g c i r c u i t ( f i g u r e 5 .12) s t o r e s the s i g n a l p u l s e a m p l i t u d e , a f t e r s u b t r a c t i o n and d o u b l i n g , u n t i l the c o d e r i s ready t o a p p l y 11 to the comparator to produce the nex t d i g i t . Two stages are n e c e s s a r y i n the h o l d i n g c i r c u i t s i n c e i t must be g i v i n g an o u t p u t s i g n a l a t t h e same time a s the n e x t d i g i t s i g n a l i s e n t e r i n g i t . The t h i r d stage o f the c i r c u i t , capa-c i t o r C 2 2.9 i s merely a means o f d i s p l a y i n g the s i g n a l f o r L3o5v T l 8 s T 1 9 = 2Nif0if D l 6 > V D l 8 > D l 9 = * 9 7 R5k R 5 5 R 5 6 R 5 8 68 100 39K 10K ^20 " °21 = C22 ~ °23 =  C2lf = 680 uuf 680 u u f 680-750 uuf .01 u f .01 u f F i g u r e 5<>12 C i r c u i t Diagram o f H o l d i n g C i r c u i t 38. diode gate No, 2. I t c o u l d be e l i m i n a t e d at the expense of p r o v i d i n g some, new t i m i n g p u l s e s . The h o l d i n g i s done by c h a r g i n g the f i r s t c a p a c i t o r C20 to the s i g n a l v o l t a g e t h r o u g h the d i o d e . When the s i g -n a l p u l s e on the e m i t t e r o f the e m i t t e r - f o l l o w e r g a t e f a l l s , d i o d e D-]^  i s c u t o f f , and the v o l t a g e on G^Q remains c o n s t a n t s i n c e t r a n s i s t o r T-^ g i s a l s o o f f . G a t i n g p u l s e GP2-, w h i c h i s o f a c c u r a t e l y c o n t r o l l e d c o n s t a n t a m p l i t u d e , p u l l s the base and hence the e m i t t e r o f T-^Q down to -6 v o l t s , thereby t r a n s f e r r i n g the charge r e s p o n s i b l e f o r the v o l t a g e b e i n g g r e a t e r than -6 v o l t s onto c a p a c i t o r C2]_« The charge i s h e l d on C2^ u n t i l GP1- once more t r a n s f e r s i t to C22 from where i t i s a p p l i e d t h r o u g h the double e m i t t e r - f o l l o w e r and d i o d e g a t e No. 2 to the c omparator. GP2-, a p p l i e d t h r o u g h C23 to t h e j u n c t i o n o f D]_y and D]_Q, d i s c h a r g e s C22 i n p r e p a r a t i o n f o r the next c y c l e o f o p e r a t i o n . C22 i s v a r i a b l e as a c o n v e n i e n t means o f h a v i n g a f i n e a d justment on the l o o p g a i n o f the c i r c u i t . T h i s i s p o s s i b l e s i n c e the charge on C20 i s B e ^ by the v o l t a g e o u t p u t from the e m i t t e r - f o l l o w e r gate and the v o l t a g e on C22 i s t h e n s e t by t h i s charge. T h e r e f o r e , the v o l t a g e on C22 i s dependent on the r e l a t i v e s i z e s o f C20 and G^ but does not depend on G^" One f a u l t o f the c i r c u i t was t h a t a l t h o u g h t h e p u l s e s GP2- and GP1- b r o u g h t the b a s e s o f T^Q and T]_g to t h e i r c o r -r e c t l e v e l s , the e m i t t e r s d i d not always f o l l o w depending on the charge on the r e s p e c t i v e c a p a c i t o r s . This was found to 39. be due to the v a r i a t i o n o f the t r a n s i s t o r d i f f u s i o n c a p a c i -tances due to the heavy c u r r e n t s i n v o l v e d i n s w i t c h i n g the char g e s . R e s i s t o r s R ^ and R ^ are i n c l u d e d t o l i m i t the se c u r r e n t s and e f fee t i v e l y r educ e the above e f f e c t . R e s i s t o r R ^ squares up the round l e a d i n g edge of the p u l s e g i v e n out o f the h o l d i n g c i r c u i t . I n t h i s way, the t i m i n g o f the o u t p u t o f the p u l s e g e n e r a t o r comparator i s improved, and ( s i n c e the p u l s e now resembles more c l o s e l y the i n i t i a l sample p u l s e ) , the r i s e time dependence o f the comparator i s o f l e s s i m p o r t a n c e . S i n c e t h e g a i n o f the c i r c u i t i s a f f e c t e d by the r e l a -t i v e s i z e s o f C 2Q and 0^2? the o p e r a t i o n i s s e n s i t i v e to temperature v a r i a t i o n s of these components. As l o n g as they have e q u a l temperature c o e f f i c i e n t s and a r e p l a c e d c l o s e t o -g e t h e r so t h a t t h e i r t e m p e r a t u r e s are e q u a l , no e f f e c t on the g a i n o f the c i r c u i t w i l l be o b s e r v e d . I n the c o d e r c o n -s t r u c t e d , to i n s u r e temperature s t a b i l i t y , temperature com-pen s a t e d c a p a c i t o r s were used f o r C^Q and 0^2* 5.8 DOUBLE EMITTER-FOLLOWER The v o l t a g e on the t h i r d h o l d i n g c a p a c i t o r C22 must be a p p l i e d to the d i o d e gate No. 2. This r e q u i r e s a b u f f e r stage w i t h an i n p u t impedance h i g h enough so t h a t the droop on' the r e s u l t i n g waveform i s h e l d to a s m a l l v a l u e . The o u t -p u t impedance must a t the same time be low enough to d r i v e the r e l a t i v e l y low = impedanc e g a t e . The b u f f e r s tage must 1+0. •13.5v-R 59 < : | R 6 i | R 6 4 • C 25 > > T 2 Q , T 2 1 = 2N21+7 D 2 Q S D 2 1 = 1NJ+97 R^9 = 680 R 6 O = IK R 6 3 = 470 R 6 l = 330K R61^ = 470 R^2 = 8.2K C 2^ = .002 u f F i g u r e 5*13 C i r c u i t Diagram of Double E m i t t e r - F o l l o w e r t h e r e f o r e y i e l d c o n s i d e r a b l e impedance t r a n s f o r m a t i o n as w e l l as changing the d=c l e v e l from -13.5 v o l t s to -4*5 v o l t s j thus r e q u i r i n g a-c c o u p l i n g . I n o r d e r t o a c h i e v e the n e c e s s a r y i n p u t and o u t p u t i m-pedances, two cascaded e m i t t e r - f o l l o w e r s were n e c e s s a r y ( f i -gure 5«13). The i n p u t impedance o b t a i n e d i s about 120k g i v -i n g an i n p u t time c o n s t a n t o f about 55 m i c r o s e c o n d s . The diode D20 and d i v i d e r c h a i n R£o,» R^ Q P r o v i d e d-c r e s t o r a -t i o n a f t e r the a-c c o u p l i n g t h r o u g h C2e;. The r e s t o r a t i o n time o f about 6 m i c r o s e c o n d s l i m i t s the speed o f o p e r a t i o n o f the p r e s e n t c i r c u i t . R e s i s t o r R^ i n the c o l l e c t o r c i r c u i t o f T21 merely keeps the power d i s s i p a t e d i n T 2 i to a t o l e r a b l e v a l u e . The p o t e n t i o m e t e r R^Q i s used to a d j u s t the d-c output l e v e l o f the e m i t t e r - f o l l o w e r to match e x a c t l y t h e i n p u t l e v e l o f diode g a t e No. 2. 1+2, 6. ADJUSTMENTS Before p u t t i n g the c i r c u i t i n t o o p e r a t i o n , i t must be a d j u s t e d c o r r e c t l y . These ad j u s t m e n t s need not be made i n the o r d e r g i v e n b u t t h i s p r o c e d u r e i s the f a s t e s t method o f a c h i e v i n g c o r r e c t o p e r a t i o n . I n o r d e r to a d j u s t t h e c o d e r , we must be a b l e to p r e s e n t a t i t s i n p u t an a d j u s t a b l e d-c s i g n a l . T r a n s i s t o r T 2 2 ( f i g u r e 6.1) was added to the c i r -c u i t to a c h i e v e t h i s . The o u t p u t l e v e l of T 2 2 i s c o n t r o l l e d by the m i c r o - p o t R^o,. A l l t e s t s on the c o d e r were done u s -i n g t h i s d-c i n p u t c o n t r o l . 13.5v-R 68 R 69 ® T 22 R 70 R 71 Ov F i g u r e 6.1 C i r c u i t Diagram o f d-c I n p u t C i r c u i t The f i r s t a d j u s t m e n t i s to the d-c l e v e l at the output o f the d o u b l e e m i t t e r - f o l l o w e r . P o t e n t i o m e t e r R^ Q must be a d j u s t e d so t h a t when t h e r e i s no s i g n a l i n p u t to the coder 14-3. t h e r e i s no b u i l d - u p o f the zero s i g n a l p u l s e s . These p u l s e s are o f about 0.1 v o l t s a m p l i t u d e a t the e m i t t e r o f Tj2 • To make the adjustment a complete s e t o f s i g n a l p u l s e s s h o u l d be examined on the e m i t t e r o f T-^ 2 a t zero i n p u t s i g n a l and R^ Q a d j u s t e d u n t i l they are a l l o f a c o n s t a n t s m a l l a m p l i -t u d e . I f R^ Q i s s e t so t h a t the d-c l e v e l o f the double e m i t t e r - f o l l o w e r o u t p u t i s too p o s i t i v e , the zero s i g n a l p u l -ses w i l l be b u i l t up by the l o o p g a i n o f the c i r c u i t . There-f o r e , the ex c e s s s i g n a l w i l l be e a s i l y v i s i b l e i n the s i x t h d i g i t . I f the l e v e l i s s e t too n e g a t i v e , the zero s i g n a l p u l s e s w i l l a l l be o f the same s i z e or n o n - e x i s t e n t . I f they are o f the same s i z e , t h e a d j u s t m e n t i s not n e c e s s a r i l y c o r -r e c t . T h e r e f o r e , the adjustment s h o u l d be made w o r k i n g from a too p o s i t i v e l e v e l toward the c o r r e c t v a l u e . I t i s t h i s adjustment t h a t i s most o f t e n i n need of a t -t e n t i o n s i n c e i t c o r r e c t s f o r any d-c d r i f t i n the c i r c u i t . Once the d-c l e v e l i s a d j u s t e d , t h e s u b t r a c t o r can be set up. T h i s i s done by s e t t i n g the i n p u t to the coder so that the p u l s e g e n e r a t o r i s j u s t t r i g g e r e d by the sample p u l s e . P o t e n t i o m e t e r R[^ Q i s then a d j u s t e d so t h a t the s i g n a l a t the c o l l e c t o r o f T-j_2 i s z e r o . The b e s t way to determine the c o r r e c t adjustment i s by e x a m i n i n g a complete p u l s e group at the e m i t t e r o f T]_ 2. When RJ^ Q i s s e t c o r r e c t l y , the p u l s e group should c o n s i s t o f one p u l s e , the sample, f o l l o w e d by f i v e zero l e v e l p u l s e s . A s l i g h t p o s i t i v e s i g n a l l e f t o v e r a f t e r s u b t r a c t i o n w i l l produce an e a s i l y v i s i b l e s i g n a l i n the s i x t h p u l s e p o s i t i o n . On the o t h e r hand, i f too much i s s u b t r a c t e d no e f f e c t w i l l be v i s i b l e . T h e r e f o r e , the a d j u s t -ment s h o u l d be made by a p p r o a c h i n g the c o r r e c t v a l u e from the d i r e c t i o n o f too s m a l l a s u b t r a c t i o n . T h i s adjustment o n l y needs a t t e n t i o n i f the r e f e r e n c e l e -v e l o f the comparator d r i f t s . As t h i s l e v e l was ve r y s t a b l e , the s u b t r a c t o r , once s e t up, d i d not r e q u i r e r e a d j u s t m e n t d u r -i n g the f o l l o w i n g t e s t s . The f i n a l a d j u s t ment to be made i s t h a t o f the l o o p g a i n o f the system. C a p a c i t o r C22 must be s e t so a s to make the lo o p g a i n e x a c t l y two. The b e s t way o f a c c o m p l i s h i n g t h i s i s by making sure the quantum s i z e f o r t h e l e v e l s 31 and 32 are the same as f o r the o t h e r v a l u e s . Any v a r i a t i o n from two i n the l o o p g a i n w i l l appear most r e a d i l y i n the s i z e s o f these two q u a n t a . 4 5 . PERFORMANCE OF THE CODER The o p e r a t i o n o f t h e coder i n v o l v e s many s i g n a l p u l s e s o f u n u s u a l l y a c c u r a t e a m p l i t u d e s and o f v a r i o u s time r e l a -t i o n s h i p s and i s b e s t s t u d i e d w i t h the a i d o f the d-c i n p u t c i r c u i t d e s c r i b e d i n Chapter 6 and a f a s t p u l s e o s c i l l o s c o p e . Many weeks were s p e n t s t u d y i n g the c i r c u i t o p e r a t i o n and r e -d e s i g n i n g v a r i o u s p a r t s i n o r d e r to a c h i e v e b e t t e r p u l s e -shapes, s y n c h r o n i z a t i o n and s t a b i l i t y . I n the co u r s e o f t h i s work, many S m a l l b u t s i g n i f i c a n t i r r e g u l a r i t i e s o f t r a n s -i s t o r b e h a v i o u r had to be overcome. As i t s t a n d s now, the c i r c u i t r y i s v e r y s a t i s f a c t o r y . None o f the r e q u i r e m e n t s on components, r i s e - t i m e s or p u l s e shapes are e x c e e d i n g l y c r i t i c a l and a l l components o p e r a t e w e l l w i t h i n t h e i r r a t i n g s . The most d i f f i c u l t p r o b lem i n the d e s i g n o f the c o d e r , and the main i m p e r f e c t i o n s t i l l r e -m a i n i n g , i s the e x i s t e n c e o f s m a l l d r i f t s i n some o f the d-c l e v e l s . Some o f the t e s t s t o be d e s c r i b e d d e a l w i t h the e f -f e c t s o f thes e d r i f t s . F i g u r e 7.1 shows the i n p u t - o u t p u t c h a r a c t e r i s t i c o f t h e coder u s i n g the d-c i n p u t c i r c u i t . Of c o u r s e , t h e a c t u a l o u t p u t of the c o d e r i s a s e r i e s o f b i n a r y p u l s e s , b u t thes e have been g i v e n t h e i r d e c i m a l v a l u e s f o r the purposes of p l o t t i n g . There a r e two i m p e r f e c t i o n s i n t h i s c u r v e . The st e p s i z e s are n o t u n i f o r m and the c e n t e r s o f the s t e p s do n o t a l l l i e on a s t r a i g h t l i n e . B o t h e f f e c t s may be due to a s l i g h t v a r i a t i o n i n the 2.0 3.0 if.O 5.0 Input V o l t s Figure 7«1 Input-Output C h a r a c t e r i s t i c o f the Coder 47. l o o p g a i n w i t h p u l s e h e i g h t . The v a r i a t i o n i n s t e p s i z e i n -c r e a s e s t h e q u a n t i z i n g n o i s e by about 10%, w h i c h i s not too s e r i o u s . The n o n - l i n e a r i t y o f the curve o c c u r s m a i n l y at t h e ends. W i t h a s i g n a l c e n t e r e d a t the m i d - p o i n t , i t would a f -f e c t o n l y the l a r g e s i g n a l v a l u e s and then o n l y to the e x t e n t of about \\% w h i c h i s p r o b a b l y t o l e r a b l e . The s t a b i l i t y o f the c o d e r was t e s t e d by s e t t i n g the d-c i n p u t l e v e l i n the m i d d l e o f a code s t e p and m e a s u r i n g the time w h i c h e l a s p e d b e f o r e the o u t p u t code changed. When f i r s t c o n s t r u c t e d , t h i s s t a b i l i t y was v e r y poor, f l u c t u a t -i n g by s e v e r a l s t e p s i n a few m i n u t e s . W i t h improved d e s i g n , the d r i f t a t normal room temperature i s now l e s s t h a n one d i g i t i n two h o u r s . The coder i s , however, s t i l l f a i r l y s e n s i t i v e to temper-a t u r e changes and to d r i f t s i n b a t t e r y v o l t a g e . Most of t h e r e m a i n i n g d r i f t comes from the d-c c o u p l e d s u b t r a c t o r and a m p l i f i e r T]_2 and T-j_^  e s p e c i a l l y from the NPN t r a n s i s t o r T]_c; w h i c h g i v e s most o f the l o o p g a i n . I t i s f e l t t h a t t h i s d r i f t and temperature s e n s i t i v i t y can b e a p p r e c i a b l y r e d u c e d by r e p l a c i n g t h i s a m p l i f i e r by a s i l i c o n t r a n s i s t o r , s uch as the 2 N 3 3 2 , used i n a c u r r e n t a m p l i f y i n g c o n f i g u r a t i o n r a t h e r than the p r e s e n t v o l t a g e a m p l i f i e r s e t u p . The major power r e q u i r e m e n t s o f the c i r c u i t a r e imposed on the -13.5 v o l t and +6 v o l t s u p p l i e s w h i c h d e l i v e r about 110 ma. and 30 ma. r e s p e c t i v e l y . The d r a i n on the o t h e r s u p p l i e s i s n o m i n a l e x c e p t f o r the -6 v o l t s w h i c h d e l i v e r s about 9 ma. The t o t a l power consumption o f the c i r c u i t is therefore about 1 .8 watts . There was no p a r t i c u l a r e f f o r t made to keep the power requirements to a minimum and they could probably be reduced i f d e s i r e d . 1+9. CONCLUSIONS The coder w h i c h was b u i l t was v e r y s u c c e s s f u l . A 6 - d i g i t code a t an 8 kc sa m p l i n g r a t e was a c h i e v e d w i t h q u i t e good s t a b i l i t y . As i t s t a n d s , the coder uses o n l y r e a d i l y a v a i l -a b l e components and si m p l e c i r c u i t s . S i n c e the c r i t i c a l d-c v o l t a g e l e v e l s had to be h e l d to an a c c u r a c y o f a few tens o f m i l l i v o l t s , s t a b i l i t y was a ma-j o r problem. Most o f the d i f f i c u l t y a r o s e i n the tendency o f the d-c c o u p l e d a m p l i f y i n g s t a g e s to d r i f t . The use o f s i l -i c o n t r a n s i s t o r s o r some means o f temperature s t a b i l i z a t i o n o f the p r e s e n t c i r c u i t would be w o r t h w h i l e i n v e s t i g a t i n g . A l t e r n a t i v e l y , use o f a-c c o u p l i n g and d-c r e s t o r a t i o n would r e l a x some o f the r e q u i r e m e n t s on d-c d r i f t . However, t h i s would a l s o reduce the upper l i m i t on speed o f o p e r a t i o n . B e t t e r s t a b i l i t y , b e s i d e s i n c r e a s i n g t h e r e l i a b i l i t y o f the c o d e r , would enable the use o f a 7 - d i g i t code. The e x i s t i n g c i r c u i t c o u l d be made to handle up to 5 ch a n n e l s w i t h o u t changing to f a s t e r t r a n s i s t o r s . The d-c r e s t o r a t i o n time o f the double e m i t t e r - f o l l o w e r and the charge-up time o f the s t o r a g e c a p a c i t o r s are a l l t h a t would r e q u i r e improvement. The power r e q u i r e m e n t s o f the system and the number o f su p p l y v o l t a g e s c o u l d c e r t a i n l y be r e d u c e d . A t the same t i m e , the number o f components c o u l d p r o b a b l y be reduced s l i g h t l y . The s u c c e s s f u l c o n s t r u c t i o n o f t h i s coder has demonstra-50. ted the s u i t a b i l i t y o f the l o o p e d p u l s e type o f c o d e r f o r ap-p l i c a t i o n s i n v o l v i n g one o r a few s i g n a l s a t a u d i o f r e q u e n c i e s . A l t h o u g h o n l y a coder was b u i l t , t h e r e s h o u l d be no g r e a t d i f f i c u l t y i n d e s i g n i n g and b u i l d i n g a complete system i n c l u d -i n g r e p e a t e r s and a decoder. 5 1 . REFERENCES 1. Shannon, C.E., "A M a t h e m a t i c a l Theory o f Communication", B e l l , System T e c h n i c a l J o u r n a l , v o l . 27, J u l y , O c t . 1948. 2 . O l i v e r , B.M., P i e r c e , J.R., Shannon, C.E., "The P h i l o s -ophy o f PCM", P r o c e e d i n g s o f t h e I.R.E., v o l . 3 6 , 19^ 8. 3 . B l o c k , H.S., Edson, J.O., " P u l s e Code M o d u l a t i o n " , T r a n s a c t i o n s o f the A . I . E . f i . , v o l . 6 6 , 191+7. 1+. Meacham, L.A., P e t e r s o n , E. , " E x p e r i m e n t a l M u l t i c h a n n e l P u l s e Code M o d u l a t i o n System o f T o l l Q u a l i t y " , B e l l System T e c h n i c a l J o u r n a l , v o l . 2 7 , J u l y 191+8. 5. S e a r s , R.W., " E l e c t r o n Beam D e f l e c t i o n Tube f o r P u l s e Code M o d u l a t i o n " , B e l l System T e c h n i c a l J o u r n a l , v o l . 27, J u l y 191+8. 6 . G o o d a l , W.M., "Telephony by P u l s e Code M o d u l a t i o n " , B e l l System T e c h n i c a l J o u r n a l , v o l . 2 6 , J u l y 191+7 • 7 . M i l l m a n , J . and. Taub, H., P u l s e and D i g i t a l C i r c u i t s , New Y o r k , M c G r a w - H i l l , 1956. 

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