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A microwave frequency standard Scovil, Henry Evelyn Derek 1949

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I A HICROY/AVE FREQUENCY STANDARD by Henry Evelyn Derek S c o v i l A t h e s i s submitted 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 the degree of MASTER OF ARTS i n the Department of PHYSICS THE UNIVERSITY OF BRITISH COLUMBIA September, 1949 A MICROWAVE FREQUENCY STANDARD by Henry Evelyn Derek S c o v i l ABSTRACT OF a t h e s i s submitted 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 the degree of MASTER OF ARTS i n the Department A Microwave Frequency Standard -£T The various stages i n the design and c o n s t r u c t i o n of a frequency standard f o r use i n microwave spectrocopy. i s d e s c r i b e d . Procedures are l a i d down f o r the use of the instrument. The standard i s of the v a r i a b l e frequency type which g i v e s i t s e v e r a l advantages over the f i x e d frequency machines. I t i s capable of p r o v i d i n g complete coverage a t microwave frequencies and i s capable of determining the microwave harmonic generated without recourse to a wave-meter. Furthermore i t can conveniently be used i n c o n j u n c t i o n w i t h a c a l i b r a t e d communications r e c e i v e r t o measure frequency separations to a high degree of accuracy. A s t a b l e 200 kc c r y s t a l c o n t r o l l e d o s c i l l a t o r i s used as the base frequency. This frequency i s m u l t i p l i e d by harmonic generators to 4800 kc and s u p p l i e d to the o s c i l l a t o r g r i d s of a balanced mixer. The v a r i a b l e 125 to 250 kc output from a BC221 s i g n a l generator i s s u p p l i e d to the s i g n a l g r i d s of t h i s same mixer. The mixer output i s tuned w i t h a band pass c i r c u i t passing 4925 to 5050 kc and consequently the two input frequencies are summed. The balanced mixer serves t o e l i m i n a t e s e v e r a l troublesome frequency components. The mixer s i g n a l i s m u l t i p l i e d s u c c e s s i v e l y to 25 mc , 75 mc and 200 mc - . An output from one of these VHF stages i s chosen and impressed upon a 1N26 s i l i c o n c r y s t a l r e c t i f i e r . T h i s r e c t i f i e r a c t s as a harmonic generator producing frequency markers i n the microwave r e g i o n . Since both the 200 kc o s c i l l a t o r and BC221 s i g n a l s have a h i g h degree of s t a b i l i t y and can conveniently be c a l i b r a t e d r e l a t i v e t o standard frequency broadcasts from the U.S. Bureau of Standards, these microwave frequency markers are known to a high degree of accuracy. This microwave standard s i g n a l i s fed i n t o the spectrometer waveguide and picked up by a c r y s t a l mixer. The spectrometer k l y s t r o n s i g n a l i s a l s o detected by t h i s c r y s t a l and consequently beats are produced equal to the d i f f e r e n c e between the standard harmonics and the k l y s t r o n f r e q u e n c i e s . This beat note i s supplied to a tuned a m p l i f i e r whose, output i s a p p l i e d to the Y p l a t e s of the spectrometer o s c i l l o g r a p h . The time base of the l a t t e r i s s u p p l i e d by the sawtooth wave which sweeps the k l y s t r o n frequency. When the beat note frequency i s equal to the frequency to which the a m p l i f i e r i s tuned a marker pip w i l l occur on the screen. Since the frequency of the standard s i g n a l and the tuning of the a m p l i f i e r are known the frequency of the k l y s t r o n at t h i s p o i n t of the time base can be determined. By tuning the v a r i a b l e frequency o s c i l l a t o r , the p i p can be moved across the h o r i z o n t a l axis u n t i l i t i s coincident with a spectrum line. Con-sequently the frequency of this line can be measured with an accuracy /of 1 part i n 10^. 7 Acknowledgements I should p a r t i c u l a r l y l i k e to express my a p p r e c i a t i o n f o r the guidance and help given by Dr. A. Van der Z i e l who supervised the p r o j e c t . I owe thanks to T. L. C o l l i n s and T. R;Hartz, both of whom made many h e l p f u l suggestions and gave valuable a s s i s t a n c e . I am indebted to R. T. Tomlinson who constructed the power supply and gave other a i d . This p r o j e c t could not have been undertaken without the use of the f a c i l i t i e s of the Department of Physics and a grant f o r equipment given by the Defense Research Board. The work was conducted w i t h the a s s i s t a n c e of a N a t i o n a l Research C o u n c i l Bursary. TABLE OF CONTENTS TEXT Page I n t r o d u c t i o n " I 1. D e s c r i p t i o n of Frequency Standard 1 1.1 Requirements 1 1.2 Microwave Frequency Generation 2 1.3 Prototype Frequency Standard 3 1.4- Present Standard, General D e s c r i p t i o n 4 1.5 Design Considerations 5 1 .6 Power Supply 7 1.7 Master O s c i l l a t o r 8 1.8 Low Frequency M u l t i p l i e r s • 13 1.9 Tunable O s c i l l a t o r s 17 1.10 V a r i a b l e Frequency Generation 21 1.11 High Frequency M u l t i p l i e r s 25 1.12 Matching U n i t 27 1.13 Microwave Harmonic Generation 29 1.14 Reception of Standard Frequency Broadcasts 30 2. P r e s e n t a t i o n of Ab s o r p t i o n Lines 32 3. Production of Marker Pips 35 4. Measurement of Spectrum Line Frequency 38 4.1 P r e l i m i n a r y Adjustments 38 4.2 Absolute Frequency Determination 39 4.3 Measurement of Frequency D i f f e r e n c e s 39 Page 5. D i f f i c u l t i e s Encountered 40 6. Conclusion 41 7. Acknowledgements 43 8. B i b l i o g r a p h y 44 FIGURES 1. Block Diag. of Prototype Standard 9 2. Block Diag. of Present Standard 10 3. Master O s c i l l a t o r 11 4. Low Frequency M u l t i p l i e r 14 5. Tunable O s c i l l a t o r 18 6. BC221 F i l t e r & A m p l i f i e r 19 7. Balanced Mixer 22 8. High Frequency M u l t i p l i e r 26 9 . Matching U n i t 28 10. Microwave Spectrometer 33 PLATES 1. Front View of Instrument F a c i n g Page 4 2. Rear View of Instrument Facing Page 5 3. Matching Unit Facing Page 28 INTRODUCTION I I n microwave spectroscopy the measurement of absorp-t i o n l i n e frequencies i s necessary to determine moments of i n e r t i a , bond d i s t a n c e s , quadruple c o u p l i n g coeff-. i c i e n t s , o r i e n t a t i o n of the atoms and other constants of the molecules under i n v e s t i g a t i o n . Wavelength i s the most convenient q u a n t i t y to measure. However measurement of wavelength Involves the measure-ment of l e n g t h which, cannot be done w i t h a h i g h degree of accuracy. Furthermore wavemeter c a l i b r a t i o n s are subject to temperature and humidity v a r i a t i o n s . With the most c a r e f u l measurements an accuracy no b e t t e r than one p a r t 4 i n 10 can be obtained. When one measures hyperfine s t r u c t u r e due e i t h e r to nuclear moments or to the presence of d i f f e r e n t i s otopes of the same element i t i s found that the frequency d i f f e r e n c e s are o f t e n i n the order of 100 kc. I n t h i s case more accurate instruments than a wavemeter must be employed. • The measurement of frequency i n v o l v e s a standard of time.. F o r t u n a t e l y standard time s i g n a l s and standard frequencies whose submultiples agree w i t h standard time are a v a i l a b l e from s t a t i o n WW a t the U. S. Bureau of Standards. During the past three years s e v e r a l microwave 1-6 frequency standards " have been described. These a l l depend upon a c r y s t a l c o n t r o l l e d o s c i l l a t o r whose Ref. 4, Page 34-3 harmonics a r e m u l t i p l i e d t o t h e microwave r e g i o n . The o s c i l l a t o r f r e q u e n c y c a n be c a l i b r a t e d a g a i n s t s t a n d a r d f r e q u e n c y s i g n a l s . These machines have had the d i s a d v a n -tage of p r o d u c i n g f i x e d f r e q u e n c y markers making i n t e r -p o l a t i o n n e c e s s a r y . The s t a n d a r d d e s c r i b e d here p r o v i d e s c o n t i n u o u s f r e q u e n c y c o v e r a g e . A f t e r t h i s was d e s i g n e d 4 and a l m o s t completed MIT produced a v e r y s i m i l a r machine. 7 D a i l e y and o t h e r s have used the method of p r o d u c i n g images of the spectrum l i n e by f r e q u e n c y m o d u l a t i o n of the k l y s t r o n o s c i l l a t o r , w i t h a t u n a b l e l o w f r e q u e n c y o s c i l l a t o r . T h i s has been employed t o measure h y p e r f i n e g s e p a r a t i o n s . C a r t e r and S m i t h have suggested the use of a secondary o s c i l l a t o r o p e r a t i n g i n the range o f , t h e source o s c i l l a t o r . M o d u l a t i o n of t h i s secondary o s c i l l -a t o r produces sidebands w h i c h i n t e r f e r e w i t h the source o s c i l l a t o r s i g n a l , r e s u l t i n g i n a s e r i e s o f markers of known and a d j u s t a b l e s e p a r a t i o n . These methods a r e c o n v e n i e n t f o r measuring s m a l l f r e q u e n c y d i f f e r e n c e s b u t cannot be used t o d e t e r m i n e a b s o l u t e f r e q u e n c y . R e f . 4 , Page 3 4 3 1 D e s c r i p t i o n of Frequency Standard 1 1.1 Requirements Primary frequency standards such as the .one at the Bureau of Standards are c a l i b r a t e d a g a i n s t astronomical observations and o b t a i n an accuracy of greater than 1 p a r t 7 i n 10 over long periods of time. However because of l i m i t a t i o n s imposed by e x i s t i n g measurement techniques., microwave frequency standards r e q u i r e an accuracy of about one p a r t i n 10^. Consequently, i t i s s u f f i c i e n t to use a simple stable c r y s t a l o s c i l l a t o r as a b a s i c frequency source. This o s c i l l a t o r can be monitored r e l a t i v e t o standard frequency broadcasts. In the event t h a t standard broadcasts are i n t e r r u p t e d i t i s d e s i r a b l e to have an o s c i l l a t o r having long term s t a b i l i t y . The mnly e s s e n t i a l requirement, however, i s h i g h s t a b i l i t y during the p e r i o d of measurement which i s i n the order of a minute. Although a standard operating on a f i x e d frequency can be used^ i n conduction w i t h a h i g h frequency communication r e c e i v e r i t i s p r e f e r a b l e to provide continuous frequency coverage as t h i s i ncreases v e r s a t i l i t y . A v a r i a b l e frequency standard has the a d d i t i o n a l advantage that i t can d i s c r i m i n a t e between i t s own harmonics, whereas a wave-meter must be employed i n the case of a f i x e d frequency machine. The instrument should provide continuous frequency coverage at the lowest frequency and s u f f i c i e n t power a t the highest frequency to be measured. 1.2 Microwave Frequency Generation 2. A h i g h l y s t a b l e v a r i a b l e source of low frequency i s m u l t i p l i e d to the VHF r e g i o n by conventional vacuum tubes. The VHF output i s f e d i n t o a s i l i c o n c r y s t a l r e c t i f i e r which i s used as a harmonic generator. These harmonics provide a s e r i e s of standard microwave frequencies separated by the VHF output frequency. The turning range of the source o s c i l l a t o r i s so chosen t h a t the ranges of the microwave harmonics overlap and hence provide complete coverage a t the d e s i r e d frequency. I f i t i s d e s i r e d to extend the high frequency l i m i t of the instrument to the m i l l i m e t e r r e g i o n i t w i l l be necessary to m u l t i p l y the VHF output by means of a k l y s t r o n a m p l i f i e r before feeding i t i n t o the s i l i c o n . r e c t i f i e r . 1 .3 Prototype Frequency Standard ( E i g . l ) A. 200 kc. c r y s t a l c o n t r o l l e d o s c i l l a t o r provided the b a s i c frequency. This was m u l t i p l i e d s u c c e s s i v e l y to 400, 800, and 4,000kc. P a r t of the output of the 800 kc m u l t i p l i e r was fed t o the o s c i l l a t o r g r i d of a pentagrid mixer tube. The v a r i a b l e 12? to 250 kc output of a BC221 frequency meter was fed t o the s i g n a l g r i d of the same tubej. The sum of the two frequencies was taken from t h i s stage by means of a band pass c i r c u i t . The 4,000 kc m u l t i p l i e r output was supplied to the o s c i l l a t o r grid of a second pentagrid mixer. The variable 925 to 1050 kc output from the f i r s t mixer was fed to the si g n a l g r i d of the second. The l a t t e r was tuned with a band pass c i r c u i t passing 4925 to 5050 kc and hence the two frequencies were summed.1 This signal was then multi-p l i e d successively to 25 mc - 1.25$, 7 5 mc* 1.25$, and 150 mc - 1.25$. The VHF output was impressed upon a 1N.26 s i l i c o n c r y s t a l r e c t i f i e r where the microwave . harmonics were produced.. When t h i s model was tested a • number of f a u l t s were discovered. Cross-modulation existed i n both low and high frequency, m u l t i p l i e r stages. This produced numerous spurious signals i n the VHF output. This f a u l t could have been remedied by replacing the g r i d r e s i s t o r s i n the m u l t i p l i e r tubes by radio frequency chokes. Another f a u l t was discovered i n the mixer stages. A small sig n a l frequency of the order of a tenth of a v o l t had to be used so that signal harmonics would be n e g l i g i b l e . This and the fa c t that the mixer had quite a low conversion transconductance resulted i n a small output sig n a l . The o s c i l l a t o r signal, however, was ten v o l t s , and the transconductance of the mixer was s u f f i c i e n t to give a signal of o s c i l l a t o r frequency whose magnitude was comparable to that of the desired output si g n a l . Because o s c i l l a t o r and output.frequencies were f a i r l y close, and.since the band width was r e l a t i v e l y wide t h i s spurious s i g n a l could not be suppressed by the band pass c i r c u i t . This phenomenon occured a t r b o t h mixer stages and consequently c o n t r i b u t e d considerable spurious s i g n a l a t the output. I t was considered impossible to improve t h i s machine, hence a new standard was designed on the b a s i s of experience gained. 1.4 Present Standard-General D e s c r i p t i o n (see f i g . 2 ) A 200 kc c r y s t a l c o n t r o l l e d o s c i l l a t o r i s used as the base frequency. The 24th harmonic of t h i s source i s s u p p l i e d to the o s c i l l a t o r g r i d s o£ a balanced mixer. The 125 kc to 250 kc output of a BC221 frequency meter i s s u p p l i e d to the s i g n a l g r i d s . These two frequencies are summed i n the mixer and the output i s m u l t i p l i e d to the VHF r e g i o n where i t i s impressed upon a s i l i c o n r e c t i f i e r which produces the microwave s i g n a l by harmonic generation. The c r y s t a l o s c i l l a t o r frequency can be added to the mixer i n place of that from the BC221. I n t h i s case the mixer has an output frequency of 5 mc. and hence can be r e a d i l y compared w i t h one of the standard broadcast frequency s i g n a l s by means of a communications r e c e i v e r . Harmonics of the 200 kc o s c i l l a t o r are f e d to a mixer stage. The output from the tunable o s c i l l a t o r i s a l s o supplied to t h i s mixer. The r e s u l t i n g beat note i s d i s p l a y e d on a comparison o s c i l l o g r a p h , and consequently the tunable o s c i l l a t o r can be c a l i b r a t e d r e l a t i v e to the master oscillator. 1.5 Design Considerations (Fig.2) The basic crystal controlled frequency f 0 i s multi-plied to a frequency *Sf0 which i s mixed with the signal * of frequency f^ obtained from the tunable oscillator to produce a signal of frequency nfg f^. This variable signal i s multiplied by vacuum tubes to provide a VHF signal of frequency n]_(nf0+ f j). This signal i s i n turn multiplied by sil i c o n crystal redtifiers acting as harmonic generators to provide a microwave output frequency, ^n-^nf 0+ fj_). The usable upper limit of microwave frequencies i s determined by the signal to noise ratio i n the crystal. The lower limit i s governed by the frequency at which complete coverage occurs which in turn i s determined by both the output frequency and range of variation of the VHF signal. In practice i n order to obtain a wide band of usable microwave frequencies i t i s necessary to u t i l i z e several output frequencies i n the VHF region. Having decided upon the lowest order harmonic n 2 m i n # to be utilized the range of the variable frequency can be calculated. In order for the lowest and next to lowest harmonics to produce overlapping coverage, n l ( n 2 m i n ) ( n f o * » n i C n a ^ D Cnf0* ' i W which reduces t o . (nf +f ) n 2 . + 1 - u 1 m a x ^ ^min JL Although the range of the v a r i a b l e frequency and hence that of the tunable o s c i l l a t o r i s determined by the above, the a c t u a l frequency f ^ of the tunable o s c i l l a t o r i s governed by other c o n s i d e r a t i o n s . A balanced mixer e l i m i n a t e s the component n f 0 i n the output. Other components must be e l i m i n a t e d by means of a band pass c i r c u i t passing nfo+f-^. The most t r o u b l e -some frequency components to e l i m i n a t e w i l l be nfg f ^ and nf0-*-2f2 and hence f± must be chosen so that n f 0 / f ] _ K where K i s determined by the band pass f i l t e r . The r a t i o n f 0 / f i .determines the r a t i o of. the e r r o r s introduced by the frequency v a r i a t i o n s i n f 0 and f j _ . As a r e s u l t of t h i s c a l c u l a t i o n i t i s found that the tunable o s c i l l a t o r w i l l introduce the greater e r r o r . The accuracy of t h i s o s c i l l a t o r i s determined by both i t s s t a b i l i t y and accuracy of c a l i b r a t i o n . Since the time occupied by the measurement i s of the order of a minute frequency d r i f t can be reduced to a n e g l i g i b l e a mount and hence c a l i b r a t i o n e r r o r s w i l l be the main f a c t o r . The tunable o s c i l l a t o r can be c a l i b r a t e d a t a number of check p o i n t s provided by i n t e r f e r e n c e between i t s harmonics and p r e v i o u s l y c a l i b r a t e d frequency n f 0 . I t i s d e s i r a b l e to use a h i g h order of these harmonics since then f o r a given e r r o r i n the beat note determination the percentage e r r o r i n f-j_ w i l l be s m a l l . The beat note can be checked r e l a t i v e to a known frequency by means of the comparison o s c i l l o g r a p h . This known frequency can be e i t h e r f i x e d or v a r i a b l e depending on whether or not d i s c r e t e or continuous c a l i b r a t i o n of the tunable o s c i l l a t o r i s d e s i r e d . I f the l a t t e r i s r e q u i r e d a beat frequency audio i n t e r p o l a t i o n o s c i l l a t o r should be used. This o s c i l l a t o r should be c a l i b r a t e d r e l a t i v e to sub-m u l t i p l e s F 0 / N 5 obtained from the master o s c i l l a t o r by frequency d i v i s i o n . 1.6 Power Supply As power i s to be s u p p l i e d to two o s c i l l a t o r s both r e q u i r i n g a h i g h order of s t a b i l i t y i t was decided to use a.regulated supply. The p l a t e transformer, r e c t i f i e r and f i l t e r s e c t i o n s are capable of d e l i v e r i n g 400 v o l t s a t 300 m i l l i a m p e r e s . Three outputs are taken from t h i s , each one going to a conventional vacuum tube r e g u l a t i n g • c i r c u i t . These three r e g u l a t o r s produce 210, 150, and 100 v o l t s r e s p e c t i v e l y . Three 6.3 v o l t f i l a m e n t s u p p l i e s are provided. The main f i l a m e n t transformer i s capable of d e l i v e r i n g 16 amps, and feeds a l l of the machine w i t h the exception of the two o s c i l l a t o r s . I t was considered d e s i r a b l e to supply • f i l a m e n t power to the two o s c i l l a t o r s independently i n order to minimize voltage f l u c t u a t i o n s and to prevent RF feedback through the f i l a m e n t c i r c u i t . These o s c i l l a t o r transformers are each capable of d e l i v e r i n g two amperes. The voltages and the c u r r e n t d r a i n s a t the three outputs are measured by meters. The three supply voltages can be adjusted by means of potentiometers. Seven 6 prong amphinol sockets are provided a t the back of the c h a s s i s as power o u t l e t s . Corresponding sockets are provided a t the back of the v a r i o u s units., 1.7 Master O s c i l l a t o r ( F i g . 3) In t h i s a p p l i c a t i o n i t i s d e s i r e d to have a short time s t a b i l i t y of about 1 p a r t i n 10?'. An e l e c t r o n coupled c r y s t a l c o n t r o l l e d o s c i l l a t o r was chosen to f u l f i l t h i s f u n c t i o n . The theory of these o s c i l l a t o r s Q i s adequately' t r e a t e d i n Terman's Radio Engineering'. Two f a c t o r s are of prime importance i n c r y s t a l o s c i l l a t o r s t a b i l i t y , , these are v a r i a t i o n s i n c r y s t a l temperature and load impedence. Since the c r y s t a l — E -c 4 £ t 1 -3 9. o 4-> 3 o "0 o o r 0 0 o A . « • J a 3 h -U J O o o Q_ 0 Q In H 3 r 5 I •* — 3 » j I- o « o 10. o O CVJ c C £ o o s o o ct > £ v> c u: .2-5 c CO r 4-> • -o I S -CO Li_ O Z < O < o o CQ 1 A. 1 «) c CL o Com l A . J o O 0 resonant frequency depends upon mechanical dimensions, i t i s d e s i r a b l e to design the c r y s t a l so t h a t i t has zero temperature c o e f f i c i e n t a t a temperature moderately above room temperature. A 200 kc Monitor quartz, c r y s t a l i s used. This i s mounted i n a t h e r m o s t a t i c a l l y c o n t r o l l e d oven maintained a t a temperature of 60° C. Any reactance that the load couples i n t o the resonant c i r c u i t a f f e c t s the resonant, frequency;' i n a d d i t i o n , r e s i s t a n c e from the load coupled i n t o t h i s c i r c u i t modifies the phase r e l a t i o n s i n the o s c i l l a t o r and hence a f f e c t s the frequency. Thuss i t i s d e s i r a b l e to separate the load from the o s c i l l a t o r . This i s accomplished by using an e l e c t r o n coupled o s c i l l a t o r . I n the c i r c u i t used here the c r y s t a l provides feedback coupling between the screen g r i d and the c o n t r o l g r i d of a 6SJ7, i n which cathode and f i r s t and second g r i d s a c t as a t r i o d e , thereby c o n t r o l l i n g the frequency through i t s e f f e c t on the magnitude and phase of the feed-back. Only a few of the e l e c t r o n s are i n t e r c e p t e d by the screen to maintain o s c i l l a t i o n s . The remainder go to the p l a t e and produce power output i n the l o a d . The v a r i a b l e c a p a c i t a t o r s a f f e c t the resonant frequency of the c r y s t a l s l i g h t l y . I n t h i s manner i t i s p o s s i b l e to a d j u s t the o s c i l l a t o r frequency to e x a c t l y 200 kc. As the amplitude of the o s c i l l a t i o n s i s kept s m a l l i n order to promote s t a b i l i t y , i t i s necessary to a m p l i f y the output. This i s done w i t h a 6SJ7 whose p l a t e i s tuned to 200 kc w i t h a h i g h Q c i r c u i t . This c i r c u i t suppresses the o s c i l l a t o r harmonics. This output i s s u p p l i e d to the g r i d s of. two 6SN.7's which a c t as cathode f o l l o w e r s i n order to provide an impedence match to three c o a x i a l outputs. The f o u r t h cathode f o l l o w e r i s used as p a r t of a vacuum tube voltmeter to monitor the o s c i l l a t o r . . The 210 v o l t supply i s f i l t e r e d to prevent RF leakage. The 6.3 v o l t f i l a m e n t supply i s from one transformer. The 6.3 c r y s t a l oven heater supply i s from another. 1.8 Low Frequency M u l t i p l i e r s (Fig.4) The theory of vacuum tube m u l t i p l i e r s i s t r e a t e d i n Terman's Radio Engineering^. I n a c l a s s C a m p l i f i e r the pulses of p l a t e c u r r e n t have appreciable harmonic content. Consequently i t i s merely necessary to tune the tank c i r c u i t t o the harmonic d e s i r e d and to adjust the angle of f l o w of p l a t e current to a value t h a t i s f a v o r a b l e f o r generating t h i s harmonic. M u l t i p l i c a t i o n i s performed by f a c t o r s up t o 5 before the harmonic content of the s i g n a l i s so low t h a t the tune p l a t e c i r c u i t can no longer s u c c e s s f u l l y d i s c r i m i n a t e between the harmonics. I t i s extremely important to e l i m i n a t e these unwanted harmonics since they produce cross-modulation i n the f o l l o w i n g m u l t i p l i e r and thus spurious s i g n a l s i n the microwave output. S e l e c t i v i t y i s most important i n the e a r l y stages of m u l t i p l i c a t i o n . F o r t u n a t e l y the e a r l y m u l t i p l i e r s operate i n such a freq-; uency range that conventional a m p l i f i e r s of hig h s e l e c t i v i t y may be used to d i s c r i m i n a t e a g a i n s t the unwanted harmonics. The c o n s i d e r a t i o n s i n v o l v e d i n the design of harmonic generators are e s s e n t i a l l y the same as i n the case of a c l a s s C a m p l i f i e r , except f o r the f a c t t h a t , since the harmonic output depends upon the angle of f l o w of p l a t e c u r r e n t , i t i s necessary to choose t h i s angle i n r e l a t i o n to the harmonic d e s i r e d . The a v a i l a b l e harmonic power output decreases almost i n v e r s e l y w i t h the order of the harmonic i f the peak space curren t remains constant. The load impedence increa s e s w i t h the order of the harmonic since i t i s i n v e r s e l y propor-t i o n a l to the output power. The e x c i t i n g power in c r e a s e s r a p i d l y w i t h the order of the harmonic since the angle of cur r e n t f l o w decreases. The maximum instantaneous g r i d voltage E m a y and the peak space current I m are determined by the same consider-a t i o n s as i n c l a s s C a m p l i f i e r s (see r e f . 9 ) . The angle of cur r e n t f l o w 0 i s s e l e c t e d i n accordance w i t h the P Ref. 9 Page 374-393 f o l l o w i n g t a b l e . 16. Harmonic 2 3 4 5 Optimum l e n g t h of pulse i n e l e c t -r i c a l degrees a t the fundamental frequency. 90-120 80-120 70-90 60-72 Approx. power output w i t h c l a s s C output taken as 1.0. 0.65 0.,40 0.30 0.25 R e l a t i v e > l e a d impedence w i t h c l a s s C as 1.0. 1.5 2.5 3.3 4.0 The g r i d b i a s r e q u i r e d to produce a given angle of f l o w i s given by the f o l l o w i n g E f e (l-cos§£) + E m i n cos *f ne E c " 'max cos 2 u (1-cos 1) 2 1-cos e where n i s the order of the harmonic i n v o l v e d where u i s the a m p l i f i c a t i o n f a c t o r where e i s the angle determined by the above t a b l e where the other f a c t o r s are given by the f o l l o w i n g diagram p l a t e voltage g r i d voltage > t-^ ^' ^ M i H 17. Having evaluated these constants the parameters of the m u l t i p l i e r may be c a l c u l a t e d . The low frequency m u l t i p l i e r c o n s i s t s of a 200 kc a m p l i f i e r which serves both to amplify the o s c i l l a t o r s i g n a l and to d i s c r i m i n a t e against any harmonics which may have been generated i n the cathode f o l l o w e r output. This i s followed by three m u l t i p l i c a t i o n stages, as shown, which m u l t i p l y the input frequency to 4800 kc. This output i s supplied to a cathode f o l l o w e r which a c t s as an. impedence match to the mixer c h a s s i s . A second cathode f o l l o w e r i s fed from the f i r s t i n order to supply the c a l i b r a t i n g s i g n a l f o r the tunable o s c i l l a t o r . Since t h i s stage i s f r e q u e n t l y switched on and o f f i t i s not coupled d i r e c t l y to the l a s t m u l t i p l i e r i n order to av o i d any dynamic e f f e c t s . As shown i n the diagram when t h i s stage i s switched o f f i t s output i s grounded to avoid pickup which w i l l cause undesirable beats i n the tunable o s c i l l a t o r output. 1.9 Tunable O s c i l l a t o r ( F i g . 5 & 6) This o s c i l l a t o r f u r n i s h e s a frequency v a r i a b l e from 125 to 250 kc. This frequency i s added to the 4800 kc output of the low frequency m u l t i p l i e r to give the v a r i a b l e frequency 4925 to 5050 kc which i s m u l t i p l i e d to the microwave r e g i o n . +1 » J 3 ri c 3 6 O O O /6. -onrfirBv-s i r it lof -4 3t-1 r***-*'"^  K \ he-O o CO O L L J C D < H '9. O O T <c L J b ' Lu OJ OJ o DO 4 20. z As has p r e v i o u s l y been shown.(1.5) t h i s o s c i l l a t o r c o n t r i b u t e s to the r e s u l t a n t e r r o r i n the microwave output and therefore must have a high degree of s t a b i l i t y . For t h i s reason S i g n a l Corps BC221 frequency meter was chosen. This instrument e x h i b i t s a h i g h degree of s t a b i l i t y over short periods and by means of a v e r n i e r scale the frequency can be read w i t h an accuracy of 3cps. This machine i s f u r n i s h e d w i t h a mixer stage i n t o which the c a l i b r a t i n g s i g n a l can be i n j e c t e d . The mixer i s f o l l o w e d by an audio stage f o r a m p l i f y i n g the r e s u l t i n g beat frequency. T h i s instrument could not be used d i r e c t l y , as i t was s u p p l i e d , f o r two reasons. The output v a r i e s approximately i n v e r s e l y as the frequency and since i t was designed as a wide range frequency 1 meter t h i s output i s n e c e s s a r i l y r i c h i n harmonics. Consequently ah a m p l i f i e r was b u i l t i n c o r p o r -a t i n g f i l t e r s and automatic g a i n c o n t r o l . As shown i n F i g . 6 BC221 output i s suppl i e d to a cathode f o l l o w e r whose cathode impedence i s made up from a low pass f i l t e r of two intermediate T s e c t i o n s . • .This- f i l t e r was designed i n accordance w i t h i n f o r m a t i o n supplied i n Ternari's Radio Engineering handbook 1 0 and e x h i b i t s sharp c u t o f f c h a r a c t e r i s t i c s . Using Ternan's n o t a t i o n f 2 • 235 kc and f^= 250 kc. The s i g n a l a t 250 kc i s down 20 db. from t h a t i n the pass band. This f i l t e r i s followed by a frequency Ref. 10, page 227 compensating network, c o n s i s t i n g of a 10 pf condenser and a 30 mh choke, which serves to c o r r e c t output v a r i a t i o n s so t h a t they are.down to about 20%. A remote c u t o f f pentode a m p l i f i e s the s i g n a l and feeds i t d i r e c t l y to the output connector. This pentode a l s o feeds a sharp c u t o f f pentode w i t h r e s i s t i v e l o a d i n g a c t i n g as an auto-matic g a i n c o n t r o l a m p l i f i e r . The s i g n a l then goes to a 6H6 double diode one h a l f of which s u p p l i e s the r e c t i f i e d voltage f o r AVG a c t i o n . The other h a l f acts as p a r t of a vacuum tube voltmeter to monitor the output. The s i g n a l which i s fed t o the mixer c h a s s i s has an amplitude v a r i a t i o n of l e s s than 5% over the e n t i r e frequency range of the BC221. This i s s u f f i c i e n t l y s m a l l t o give a n e g l i g i b l e e f f e c t on microwave s i g n a l . 1.10 V a r i a b l e Frequency Generation ( F i g . 7) Rating taken i n t o c o n s i d e r a t i o n the d i f f i c u l t i e s encountered i n the prototype standard i t was decided to use a balance mixer to generate the v a r i a b l e frequency that i s to be m u l t i p l i e d to the microwave r e g i o n . Since the 4 ,800 kc s i g n a l from the low frequency m u l t i p l i e r contains harmonics of 200 kc other than 4 ,800 kc a s e l e c t i v e a m p l i f i e r precedes the mixer stage and provides a pure 4,800 kc s i g n a l of 12 v o l t s rms. This s i g n a l i s coupled to the mixer o s c i l l a t o r g r i d s by means of a 6SN7 . 23. a c t i n g as a cathode f o l l o w e r which d e l i v e r s 7 v o l t s rms. Th i s stage i s used so that the a m p l i f i e r may be h i g h l y s e l e c t i v e while the g r i d r e s i s t a n c e i n the mixer can be low to avoid cross-modulation e f f e c t s . The 125 to 250 kc' output from the tunable' o s c i l l a t o r u n i t i s coupled i n t o a 6c5 t r i o d e a c t i n g as a'cathode r e s i s t o r phase i n v e r t e r to provide an output, balanced to ground, f o r the f o l l o w i n g mixer s i g n a l g r i d s . This s i g n a l i s small (.lvolts rms.) i n order to reduce harmonic generation to a minimum. The balance mixer c o n s i s t s of two P t i i l i p s ECH 21 t r i o d e heptode tubes which were chosen because of t h e i r h i g h conversion transconductance of 750 micro mhos. These tubes are coupled i n push p i i l l to a band pass out-put c i r c u i t having a pass band from 4925 to 5050 kc. Thfe-4800 kc input s i g n a l does not tend to appear i n the -push p u l l output since t h i s s i g n a l i s a p p l i e d to g r i d s that are d r i v e n i n phase. The tunable o s c i l l a t o r s i g n a l does tend to appear since i t i s a p p l i e d to g r i d s that are d r i v e n i n push p u l l by a phase i n v e r t e r , however t h i s s i g n a l i s suppressed by the p l a t e c i r c u i t s e l e c t i v i t y . I t i s a l s o necessary to suppress n f 0 - f j and n f Q + 2 f i (see 1.5) by the p l a t e c i r c u i t s e l e c t i v i t y and i t i s p o s s i b l e that f o r reasons given i n the Conclusion t h i s may not have been done to the d e s i r e d extent. I f t h i s i s found to be the case i t might be necessary to choose 2 4 . *1> given by n f Q / f ^ K , such that K i s smaller than i t i s a t present. The mixer may be adjusted f o r balance by var y i n g the screen voltages on the ECH 2 1 's by means of a p o t e n t i o -meter. The.variable frequency from the balance mixer i s increas e d i n amplitude by an a m p l i f i e r w i t h band pass c h a r a c t e r i s t i c s . This s i g n a l i s then f e d to a 6SN7 double t r i o d e a c t i n g as two cathode f o l l o w e r s . One cathode f o l l o w e r s u p p l i e s the output s i g n a l to the high frequency m u l t i p l i e r . The other i s connected to a double throw switch by means of which the s i g n a l can e i t h e r be sent to a voltmeter f o r monitoring the c i r c u i t or t o an output connector. When the 200 kc s i g n a l from the master o s c i l l a t o r i s s u b s t i t u t e d f o r the 125 to 250 kc s i g n a l from the tunable o s c i l l a t o r t h i s connector s u p p l i e s a 5 mc s i g n a l . This s i g n a l can be monitored r e l a t i v e to standard frequency broadcasts i n order to adjust the master o s c i l l a t o r . When b u i l d i n g t h i s c i r c u i t i t was necessary to pay p a c t i c u l a r a t t e n t i o n to p h y s i c a l l a y o u t , s h i e l d i n g of the vari o u s components, and adequate f i l t e r i n g i n order to prevent spurious s i g n a l s from reaching the mixer g r i d s . 25. 1.11 High Frequency M u l t i p l i e r s (Big.8) The c o n s i d e r a t i o n s i n v o l v e d i n the design of the high frequency m u l t i p l i e r s are e s s e n t i a l l y those l a i d down i n 1.8. 6AK5 HF pentodes were chosen as the m u l t i p l i e r and a m p l i f i e r tubes. 6j6 HF t r i o d e s were used as the cathode f o l l o w e r s . The v a r i a b l e frequency from the mixer i s s u p p l i e d to the f i r s t - m u l t i p l i e r by a tuned a m p l i f i e r w i t h a low Q c i r c u i t . The s i g n a l i s s u c c e s s i v e l y m u l t i p l i e d and a m p l i f i e d t o 25 mc , 75 mc- , 200 mc as shown. Three outputs are taken at 25, 75 and 200 mc. The cathode f o l l o w e r s are used i n the 25 and 75 mc output stages i n order to provide an impedence match to a c o a x i a l l i n e . As i t i s d i f f i c u l t to use cathode f o l l o w e r s s u c c e s s f u l l y a t 200 mc a 6AK5 tuned a m p l i f i e r i s used. The inductance c o n s i s t s of four turns of no. 12 wi r e . The c o a x i a l l i n e i s coupled to t h i s w i t h a loop c o n s i s t i n g of one t u r n . A voltmeter consistingobf a m i l l i a m e t e r and 1N34 germanium c r y s t a l s can be connected t o the three outputs by means of a r o t a r y s w i t c h i n order to monitor the output-and t o f a c i l i t a t e tuning. The 25, 75 and 200 mc outputs supply 2, 1.5? and 1 v o l t r e s p e c t i v e l y across the 70 ohm c o a x i a l l i n e impedence. This i s more than s u f f i c i e n t to d r i v e the c r y s t a l . In order to accomodate the v a r i a b l e s i g n a l t h i s u n i t has a pass band of 2^% which i s accomplished by-stagger tuning of the m u l t i p l i e r and a m p l i f i e r , stages. The voltage s u p p l i e s to the c h a s s i s are f i l t e r e d and a l l these stages are w e l l decoupled and s h i e l d e d to prevent feedback. 1.12 Matching U n i t (Fig.9) The v a r i a b l e frequency VHF output i s taken from the high frequency m u l t i p l i e r outputs by a 70 ohm c o a x i a l c a b l e . As the 1N26 s i l i c o n c r y s t a l r e c t i f i e r which a c t s as the harmonic generator has an impedence of the order of 1,000'ohms i t i s necessary to provide an impedence match between the c o a x i a l l i n e and the c r y s t a l . This i s accomplished by means of the matching u n i t . A s i n g l e loop i s connected across the cable and i s i n d u c t i v e l y coupled to an inductance c o n s i s t i n g of f o u r t u r n s which forms par t of a resonant c i r c u i t ' tuned to the output frequency. The c r y s t a l r e c t i f i e r takes i t s power from across t h i s c i r c u i t . The r e c t i f i e d c u r r e n t i s f i l t e r e d and measured w i t h a 0 to 1 ma. meter. The beat frequency caused by i n t e r f e r e n c e between the c r y s t a l harmonics and the k l y s t r o n s i g n a l i s taken from t h i s same u n i t as described i n 3. r -z D O z I u < z * £ I ' 1.13 Microwave Harmonic Generators The v o l t a g e - c u r r e n t c h a r a c t e r i s t i c of the s i l i c o n c r y s t a l i s approximately square-law i n the forward d i r e c t i o n . A s i l i c o n c r y s t a l r e c t i f i e r may be d r i v e n by a VHF s i g n a l to produce microwave frequencies by a c t i n g as a harmonic generator. I n t h i s r e g i o n the harmonic amplitude i s i n v e r s e l y p r o p o r t i o n a l to the order of the harmonic. The harmonic power can be increased by i n c r e a s i n g the current through the c r y s t a l . However c r y s t a l s are l i m i t e d i n the amount of current which they can handle. This c u r r e n t can be as much as 40 milliampe'res i n the case of a 1N21 or 1N22 c r y s t a l to as l i t t l e as 1 m i l l i -ampere f o r a 1N26 c r y s t a l . Since the power i n the c r y s t a l . i s l i m i t e d the order of the harmonic which can be detected above the noise i s l i k e w i s e l i m i t e d . Thus as higher frequency s i g n a l s are req u i r e d i t becomes necessary to incr e a s e the VHF frequency. Experience gained a t the MIT R a d i a t i o n Laboratory 4" by Talpey and Goldberg^ "and others has shown t h a t harmonics at l e a s t as h i g h as the 50th can be used. I t appears therefore t h a t t h i s machine i n i t s present form can be used to frequencies up to a t l e a s t 10,000 mc. I f i t i s d e s i r e d to make measurements a t higher frequencies i t may be necessary t o increase the frequency of the VHF output. This may be done e i t h e r Ref. 4, page 374 30. by the use of lighthouse tubes or, i f the frequencies are d e s i r e d i n the m i l l i m e t e r r e g i o n , by means of a k l y s t r o n m u l t i p l i e r . In the present arrangement a 1N26 r e c t i f i e r i s used i n a 1 cm. waveguide c r y s t a l c a v i t y . This c r y s t a l should have a back to f r o n t r e s i s t a n c e r a t i o of a t l e a s t 100;1. Here the c r y s t a l i s placed across a resonant guide s e c t i o n i n an o r i e n t a t i o n p a r a l l e l to the l i n e s of e l e c t i i c i n t e n s i t y i n the guide corresponding to the d e s i r e d mode. An adju s t a b l e s h o r t i n g plunger i s provided i n the guide s e c t i o n f o r tuning purposes. Harmonic c u r r e n t s f l o w i n g i n the c r y s t a l e x c i t e the waveguide. These s i g n a l s t r a v e l down the waveguide to the mixer c r y s t a l . Here they are met by s i g n a l s coming from the microwave o s c i l l a t o r by way of the magic, tee. The harmonic generator and the mixer c r y s t a l may be combined i n t o one c r y s t a l . The theory of c r y s t a l r e c t i f i e r s i s tr e a t e d very- comprehensively i n C r y s t a l R e c t i f i e r s 1 ! by Torrey and Whitmer. 1.14 Reception of Standard Frequency Broadcasts I t i s necessary to monitor the master o s c i l l a t o r r e l a t i v e to some frequency standard. F o r t u n a t e l y s t a t i o n WWV of the US Bureau of Standards broadcasts standard 31. f r e q u e n c i e s a t 2 . 5 , 5, 10, 15, 20, 25, 30 and 35 mc. At the U n i v e r s i t y of B r i t i s h Columbia the r e c e p t i o n of the 10, 15, and 20 mc frequencies i s quite good and i t was decided to use them f o r check purposes. A good communi-c a t i o n s r e c e i v e r has s u f f i c i e n t s e n s i t i v i t y to produce a reasonable s i g n a l a t these f r e q u e n c i e s . I t i s necessary, however, to c o n s t r u c t a tuned a e r i a l a r r a y f o r adequate r e c e p t i o n . The a e r i a l should feed a cathode f o l l o w e r to give an impedence match so that the s i g n a l can be f e d to the r e c e i v e r by a c o a x i a l cable. This p r o j e c t has not been undertaken as i t was considered t o be a depart-mental commitment. By u t i l i z i n g the beat frequency o s c i l l a t o r i n the r e c e i v e r the master o s c i l l a t o r can be adjusted t o zero beat with-WW. This i s done by l i s t e n i n g to the waxing and waning of the BF0 s i g n a l as the harmonics of the 5 mc m u l t i p l e i n t e r f e r e w i t h the standard frequency. 2 P r e s e n t a t i o n of Absorption Lines ( F i g . 10) 32 Since the frequency standard was designed to be used i n c o n j u n c t i o n w i t h the UBC one centimeter microwave spectrometer a b r i e f d e s c r i p t i o n f o l l o w s . As i s shown i n F i g . 10 a microwave k l y s t r o n i s frequency modulated by a sawtooth wave of la r g e amplitude and low frequency. This sawtooth wave a l s o provides the time base f o r the o s c i l l o g r a p h . I n order to improve the s i g n a l to noise r a t i o the k l y s t r o n i s a l s o frequency modulated by a RF s i g n a l of small amplitude. The micro-wave s i g n a l i s fed by means of a h y b r i d tee s e c t i o n i n t o the waveguide assembly which a c t s as the a b s o r p t i o n c e l l . The waveguide i s sealed by mica windows and i s pumped down to a hig h vacuum. The molecule to be i n v e s t i g a t e d , i s f e d i n t o the abs o r p t i o n c e l l u n t i l a pressure of approximately 1 micron i s reached. At the end of the waveguide i s a 1N26 s i l i c o n c r y s t a l d e t e c t o r . This i s fo l l o w e d by a f i l t e r , an RF a m p l i f i e r , a phase s e n s i t i v e d e t e c t o r , and an audio frequency a m p l i f i e r . The l a t t e r i s connected to the Y p l a t e s of the o s c i l l o g r a p h . As the microwave frequency i s slowly swept by the sawtooth wave the r e c t i f i e d c r y s t a l c u r r e n t i n d i c a t e d the power received from the k l y s t r o n . I f the range of the source frequency happens to i n c l u d e a frequency a t which the molecule i s resonant a sharp d i p i n power w i l l occur at the d e t e c t i n g c r y s t a l due to abs o r p t i o n . The f i l t e r removes the low frequency 34. components of the c r y s t a l s i g n a l which i s then a m p l i f i e d . Since the o s c i l l o g r a p h gives e s s e n t i a l l y a graph of frequency vs. s i g n a l i n t e n s i t y t h i s a b s o r p t i o n l i n e w i l l be shown as a sharp pip on the h o r i z o n t a l time base. The frequency of t h i s a b s o r p t i o n l i n e may now be measured e i t h e r w i t h a wavemeter or, more a c c u r a t e l y , by means of the frequency standard. 35.. 3 P r o d u c t i o n of Marker P i p s A s i l i c o n c r y s t a l r e c t i f i e r since i t has square-law c h a r a c t e r i s t i c s can be used as a mixer i n the same manner as a diode. I t has the advantage t h a t i t can be used a t microwave frequencies because of low t r a n s i t time e f f e c t s . The theory of microwave mixers i s discussed i n r e f . 1 2 & 13. A. 1 N 2 6 c r y s t a l . s e r v e s the d u a l purpose of a harmonic generator and mixer. The microwave harmonics are generated i n t h i s c r y s t a l . Here they are met by the s i g n a l s coming from the k l y s t r o n o s c i l l a t o r by way of a d i r e c t i o n a l c o u p l e r . Since the o s c i l l a t o r i s swept by a sawtooth wave i t s frequency i s a f u n c t i o n of time. The source s i g n a l mixes w i t h the standard harmonics, and together they produce a beat note equal to the di f f e r e n c e , between the -two. The beat frequency from the mixer i s s u p p l i e d to an a m p l i f i e r whose output i s f e d to the Y p l a t e s of the o s c i l l o g r a p h . I f the beat frequency i s equal to the frequency to which the a m p l i f i e r i s tuned the mixer s i g n a l w i l l appear on the screen as a p i p . Using the n o t a t i o n of 1 . 5 , the l o c a t i o n of the p i p on the h o r i z o n t a l a x i s w i l l correspond to a frequency of n 2n- L(nf 0+f j J t R mc. where R i s the frequency i n mc. t o which the a m p l i f i e r i s tuned. The production of marker pips may be b e t t e r understood w i t h reference to the f o l l o v / i n g f i g u r e : 5wt.op L'tmi't Swatp limit Intensity The sawtooth a p p l i e d to the k l y s t r o n sweeps the frequency from A to R. P o i n t B represents the frequency of the k l y s t r o n without a sweep. This p o i n t may be v a r i e d by mechanical tuning of the klystron's resonant c a v i t y . The standard frequency s i g n a l and the o s c i l l a t o r s i g n a l are always mixing a t the c r y s t a l but u n t i l the frequency d i f f e r e n c e of these two s i g n a l s i s equal to the a m p l i f i e r tuning R as a t p o i n t D there w i l l be no marker p i p on the screen.. At p o i n t D, which corresponds to a c e r t a i n time on the h o r i z o n t a l sweep and hence frequency, the s i g n a l i s a m p l i f i e d . This s i g n a l on the o s c i l l o g r a p h represents a d e f i n i t e frequency. A c t u a l l y two marker pi p s w i l l appear separated by 2 Rmc. This occurs because a beat note of frequency R mc. i s produced 37. when the k l y s t r o n frequency i s both below and above that of the standard by Rmc. These pi p s are shown (B) as they appear on the screen. The frequency and hence- the p o s i t i o n of these p i p s can be moved by changing the tunable o s c i l l a t o r s e t t i n g and hence the standard frequency. An appreciable index e r r o r may occur as a r e s u l t of d i f f e r e n c e s i n the amount of time delay between the marker and a b s o r p t i o n l i n e a m p l i f i e r channels. This e r r o r v a r i e s w i t h the sawtooth wave frequency. For t h i s reason i t i s d e s i r a b l e to use e i t h e r a common a m p l i f i e r or e l s e to d e s i g n the two channels to have i d e n t i c a l response. A l t e r n a t i v e l y , i t i s necessary to c a l i b r a t e the standard w i t h each spectrometer a g a i n s t a c c u r a t e l y known a b s o r p t i o n l i n e s . This c a l i b r a t i o n should be done f o r a l l sweep frequencies used. 4 Measurement of Spectrum Line Frequency 38. 4.1 P r e l i m i n a r y Adjustments Having l o c a t e d an a b s o r p t i o n l i n e and adjusted the pressure f o r minimum l i n e width i t i s necessary to c a l i b r a t e the standard. Having allowed the machine to warm up completely and checked the instrument to. see that i t i s f u n c t i o n i n g p r o p e r l y , the 200 kc o s c i l l a t o r s i g n a l i s s u b s t i t u t e d f o r that from the BC221 i n the mixer.. The 5 mc output i s l o o s e l y coupled to a communications r e c e i v e r i n p u t . The r e c e i v e r i s tuned to one of the WW frequencies which gives a good s i g n a l . The BFO i s switched on. A waxing and v/aning of the audio note should occur. T h i s i s a r e s u l t of modulation of the BFO by the beat note produced by mixing of the standard and WWV s i g n a l s . The coarse and f i n a l l y the f i n e c o n t r o l s on the master o s c i l l a t o r are adjusted u n t i l qero beat i s heard. The o s c i l l a t o r i s then tuned to e x a c t l y 200 kc. The c a l -i b r a t i n g s i g n a l from the low frequency m u l t i p l i e r i s i n j e c t e d i n t o the BC221 which i s then adjusted f o r zero beat a t one of i t s check p o i n t s . This serves only as a p r e l i m i n a r y c a l i b r a t i o n of the tunable o s c i l l a t o r . The 200 k c . s i g n a l to the mixer must now be replaced by that from the tunable o s c i l l a t o r . The c a l i b r a t i n g s i g n a l f o r the BC221 must be switched o f f to prevent beats o c c u r r i n g i n i t s output. t 4.2 Absolute.Frequency Determination 39. The BC221 i s tuned u n t i l a marker pip appears on the screen. This pip i s brought i n t o coincidence w i t h the spectrum l i n e . The BC221 i s now c a l i b r a t e d a t i t s nearest check p o i n t . The pip i s again superimposed on the l i n e and the BC221 frequency i s noted. The BC221 i s tuned U n t i l a second"pip appears and the procedure i s repeated. The mean of the two frequencies represented by the p i p s (See 3) i s the l i n e frequency. I t i s now necessary to determine the order ( n 2 ) of the c r y s t a l harmonic. This can be done by e i t h e r making a rough frequency check w i t h the wavemeter or by t a k i n g a second p a i r of readings w i t h the BC221. . 4 . 3 Measurement of Frequency D i f f e r e n c e s ' I f a c a l i b r a t e d communications r e c e i v e r i s s u b s t i t u t e d f o r the f i x e d frequency a m p l i f i e r , the frequency d i f f e r e n c e s - between the two marker pips can be v a r i e d . These pi p s then represent an a d j u s t a b l e frequency s c a l e . Small frequency separations can then be determined to a higher degree of accuracy than i s p o s s i b l e by making two absolute frequency measurements and then t a k i n g t h e i r d i f f e r e n c e . 5 D i f f i c u l t i e s Encountered Considerable t r o u b l e was experienced i n e l i m i n a t i n g unwanted frequencies i n the mixer output. This may have been the r e s u l t of a s l i g h t unbalance i n the push p u l l transformer. A more l i k e l y cause i s the choice of rather a n ^ o p t i m i s t i c value f o r K (see 1.5). In t h i s connection 4 i t i s to be noted that experience w i t h the MIT standard has shown that response should be down 40 db. at the d i f f e r e n c e frequency n f 0 - f T h i s has been achieved by u s i n g a K value of 5 which i s one e i g h t h of the value chosen here. A decrease i n K value would n e c e s s i t a t e the use of a higher tunable o s c i l l a t o r frequency. This i s r e a d i l y a v a i l a b l e from the BC221. . I f i t i s found t h a t the magnitude of small spurious s i g n a l s i n the output i s s u f f i c i e n t to cause trou b l e a f t e r a m p l i f i c a t i o n of the marker p i p , i t i s suggested that the IC value be r e v i s e d . Work on the machine was hampered con s i d e r a b l y by the l a c k of adequate t e s t i n g and measuring instruments. D i f f i c u l t y was experienced i n o b t a i n i n g the r e q u i r e d components without considerable delay. This m a t e r i a l l y hampered progress. Ref. 4 page 346 6 Conclusion 4J-' The microwave frequency standard has been completed and t e s t e d . The output of the standard has met the requirements of accuracy and frequency. The s i g n a l i s more than s u f f i c i e n t to d r i v e the c r y s t a l harmonic generators. The output has s u f f i c i e n t v a r i a t i o n to provide complete frequency coverage beginning a t 1000, 3000 and 8000 mc. by use of the 25, 75 and 200 mc outputs r e s p e c t i v e l y . Thus complete coverage i s obtained at a frequency s u f f i c i e n t l y low to f u l f i l l • the requirements of microwave spectroscopy. Since t h i s standard generates a v a r i a b l e frequency, . i t has s e v e r a l advantages over the e x i s t i n g f i x e d frequency' types. I t i s capable of p r o v i d i n g complete coverage at microwave frequencies and i s capable of determining the microwave harmonic generated without recourse to a wavemeter. Furthermore, i t can con-v e n i e n t l y be employed i n c o n j u n c t i o n w i t h a c a l i b r a t e d communications r e c e i v e r to measure frequency separations to a h i g h degree of accuracy. A small spurious s i g n a l of the order of m i l l i v o l t s • e x i s t s i n the output. I t was not p o s s i b l e to see whether or not t h i s i s of s u f f i c i e n t magnitude to cause appreciable disturbance on the o s c i l l o g r a p h screen as the instrument could not be t e s t e d i n c o n j u n c t i o n w i t h the spectrometer. This was because the f i l t e r f o r the spectrometer could not be b u i l t due to a l a c k of components. I f trou b l e i s found to occur i t i s expected t h a t the f a u l t can be removed by c a r r y i n g out the recommendations given i n s e c t i o n 5 . 8 B i b l i o g r a p h y 4A, 8. C a r t e r , R.L. & Smith, W.V., P h y s i c a l Review 72,1265 (1947) 7. D a i l e y , B. P., K y h l , R. L., Strandberg, M. W. P. Van Vleck, J . H. & Wilson, E. B., . P h y s i c a l Review 70, 984 (1946) 13.Edwards, C. F., Proceedings of the I n s t i t u t e of Radio Engineers 35, 1181 (1947) 1. Gardner, H. A., F i x e d Frequency Standard, MIT R a d i a t i o n Laboratory, Report M207, 1945 2. Gordy, W., Review of Modern Physics 20, 669 (1948) 3. Hunt, L. E., Proceedings of the I n s t i t u t e of Radio Engineers 35, 979 (.1947) 4. Montgomery, C. G., Technique of Microwave Measurements, McGraw-Hill 12.Pound, Microwave Mixers, McGraw-Hill 5. Talpey, R. G. & Goldberg, H., Proceedings of the I n s t i t u t e of Radio Engineers 35, 965 (1947) 9. Terman, F. E., Radio Engineering, McGraw-Hill _ . 10. Terman, F. E., Radio Engineer 1s'Handbook, McGraw-Hill 11. Torrey, H. C. & Whitmer, C. A., C r y s t a l R e c t i f i e r s , McGraw-Hill 6. Unterberger, R. R. & Smith, W. V., The Review of S c i e n t i f i c Instruments 19,580 (1948) 

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