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Pulsar reception at 22 MHZ Dewdney, P. 1970

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PULSAR RECEPTION AT 22 MHZ by P. DEWDNEY B.A.Sc. i n Engineering Physics, University of B.C., 1968 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE i n the Department of E l e c t r i c a l Engineering We accept t h i s t hesis as conforming to the required standard Research Supervisor i Members of the Committee Acting Head of the Department.. Members of the Department of E l e c t r i c a l Engineering THE UNIVERSITY OF BRITISH COLUMBIA January, 1970 In presenting t h i s thesis in p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library shall make i t f r e e l y available for reference and study. I further agree tha permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It is understood that copying or publication of t h i s thesis f o r f i n a n c i a l gain shall not be allowed without my written permission. Department of The University of B r i t i s h Columbia Vancouver 8, Canada Date A B S T R A C T An a t t e m p t i s made t o r e c e i v e p u l s a r s i g n a l s a t a f r e q u e n c y ( 2 2 MHz) l o w e r t h a n t h e y h a v e b e e n r e c e i v e d b e f o r e . T h e p r o b l e m o f h i g h g a l a c t i c b a c k g r o u n d r a d i a t i o n i s t h e d o m i n a n t o n e . T h e r e c e i v e r u s e d o p t i m i z e s t h e r a t i o o f s i g n a l t o s k y b a c k g r o u n d n o i s e * T h e t e c h n i q u e u s e s t h e p r o p e r t y t h a t p u l s a r s i g n a l s a r e d i s p e r s e d b y i n t e r v e n i n g e l e c t r o n s . . U s i n g t h e k n o w n d i s p e r s i o n r e l a t i o n i t i s p o s s i b l e t o p r e d i c t t h e p h a s e o f t h e p u l s a r s i g n a l a t o n e f r e q u e n c y ( .22 MHz) i f i t i s k n o w n a t a n o t h e r f r e q u e n c y ( 1 5 0 M H z ) . T h e r e c e i v e r t h e n t r a c k s t h e p u l s e i n f r e q u e n c y v s . t i m e w i t h a b a n d w i d t h w h i c h i s s m a l l e n o u g h t o m a t c h t h e i n s t a n -e o u s b a n d w i d t h o f t h e signal» A l t h o u g h i t was f o u n d t h a t p u l s a r s i g n a l s f r o m CP 1 9 1 9 a r e s t i l l t o o w e a k t o b e r e c e i v e d o n s u c h a s y s t e m , a n u p p e r l i m i t t o t h e i r s t r e n g t h was o b t a i n e d b y m e a s u r i n g i t s s e n s i t i v i t y . A t t h e t i m e o f o b s e r v a t i o n ( A u g u s t , 1 9 6 9 ) t h e s i g n a l s t r e n g t h a t 22 MHz, a v e r a g e d o v e r 2 6 0 0 p u l s e s , was l e s s t h a n 1,0 x 10 2 — j o u l e s p e r in o f c a p t u r e c r o s s - s e c t i o n p e r u n i t b a n d w i d t h . i i T A B L E O F C O N T E N T S page ABk) 1 R AC l r i > e 4 « e < * « » « « « » o e » « « « i ) t * < * * « 0 » * t « « . 1 1 T A B L E Ol1 C 0 iV 1 ilj JN7 P S « e « o o « * « 6 « « e « « e « o e « * * e e « 1 1 1 L I S T O F I L L U S T R A T I O N S i v 1 e J P I J L S A I I S e 6 0 * c « e * * a « e o * o * » « » c > t J » « 9 e e * * o 1 2. J U S T I F I C A T I O N O F T H E E X P E R I M E N T . . . . . 7 3 . -• S I G N A L - - T 0 ~N 0 1 S E P R O B L E M . 8 A- c T H E " A N T E N X A o o * » « © « o o * o » » « * » * « e o « « # « e 11 5 o T H E S Y S T L M • « ( « * « t « « « « « « o « » a * < « o « « o * * 13 6 © T H E E . \ 1 ? E R I JViEN T o o « » « « « * o e * o o * c e « « * « » o IT T • T H E E E S U E T S « * 6 « » « o 0 « « o s « « « c « « « « « « « o * 19 8 • G O N G L U S X O N S © < J « * » © * o « o o » o * « # « « « « o * * « * 21 TZEFEr^ENCES o « o * » « * o » * e » » * « » « o * * « « « > o « « « « e « » 24 i i i L I S T OF I L L U S T R A T I O N S F i g u r e p a g e 1-1 T h e F r e q u e n c y v s . T i m e S w e e p o f t h e P u l s a r S i g n a l • • 2 1-2 P u l s a r S i g n a l s R e c e i v e d a t 4 0 8 M H z • • » • « • « » • « » • • • • « » 3 1-3 P u l s a r S p e c t r a • • • • • • • • « . e « . « » . 6 w . « . . o * « . . . o « « o . . » . o 5 3-1 S w e e p P a t h F o l l o w e d by- t h e R e c e i v e r a t 22 M H z • » • • • • 9 5-1 S y s t e m D i a g r a m . . . . . . . . 14 5-2 P u l s a r S i g n a l R e c e i v e d on t h e 1 5 0 MHz S y s t e m «•••«•• 16 6-1 O u t p u t f r o m t h e 22 MHz 5^ y 3 i ^ c i l i ° 0 , > , > o c c * * ® * * c , , * o o o , > * 0 * 18 i v ACKNOWLEDGEMENT •I wish to thank Professor F .K. Bowers for his guidance and encouragement in carrying out this research. Furthermore, I would l ike to thank the staff of the Dominion Radio Astrophysical Observatory for their assistance and encouragement, and to acknowledge the use of f a c i l i t i e s there*, This research was conducted with the financial assistance from a National Research Council bursary and from National Research Council grant A-3295 given to the Department of Electr ica l Engineering at the University of Bri t i sh Columbia. v 1 1. PULSARS Since pulsars were discovered in 196B by hewish et a h at Cambridge, much work has been done to measure the properties of (1 J known pulsating sources, and to search for new ones. Pulsar: signals exhibit very unusual properties. At the source they arc-pulses of radio frequency noise which are emitted over a very broad range of frequencies (from 40 MHz to a few GHz)c The pulse repetition rate is exceedingly constant (to within at least 7 (2) one part in 10 per year) . Although i t has been presumed that the pulses are emitted simultaneously on a l l frequencies, i t has been found that pulsar signals observed on a low frequency are delayed with respect to those observed on a high frequency. Investigations of this phenomenon have shown, that there is disper-sion caused by intervening electrons along the line of-sight to the pulsar. The measured dispersion accurately f i ts the relation 41 = -•£.-<• " D T 7~ f p d L where f is the received frequency c is the velocity of light , --3 A f is the plasma frequency ( f - 8.98 x 10"" n^ 2 ) n e is the electron density in the intervening medium dL is an increment of distance along the line of sight, or £1 = 1.2048 x 10~4 f 3 •s e where f is in MHz >- n is in electrons-cm 3 . f ° -> n dL is known as the dispersion measure and is usually e —3 quoted in parsec-cm ( 1 parsec = 3,26 l ight- years ). An i l lus trat ion of the shape of the frequency vs. time curve of observed pulsar signal is shown in Figure 1-1. In this model the pulse width is independent of frequency. There has been some evidence that the duration of the observed pulse is longer at low frequencies than at high ones. The effect is small but requires a more complicated theoretical explanation (4) of the dispersion . Among the pulsars there is quite a wide z range of pulse repetition rates and pulse durations. The fastest repetition rate is about 30 per second; the slowest is about 1 every 4 seconds„ The pulse widths vary from 2 msec to 200 msec. The fastest repetition rates are usually accompanied by the longest pulse durations. Pulsar signals received at 408 MHz can be seen in Figure 1-2, Figure 1-1 The pulsar signal sweep (not to scale), t is the pulse width (constant with frequency). P is the pulsar period. B. is the instanteous bandwidth. 1 The radio frequency spectra of the pulsar signals are very d i f f i cu l t to obtain because of the large, random variations in the intensity of the pulses. Some histograms of intensity distributions have been measured for the pulsar CP 1919. ( For the designation 0 f pulsars see reference 4 ) A measured dis-tribution can be seen in reference 5. The distribution function 3 decays exponentially at intensities above the average intensity,, This type of distribution has been observed at several frequencies by Lovelace and Craft at Arecibo Ionospheric Observatory. Pulses outside a small frequency range do not seem to be correlated in intensity. For instance, at 430 MHz correlation was observed only in a range of about 3 MHz for CP 1919. — ^ J V ^ ^ — Figure. 1-2 Pulsar signals received at 408 MHz. The method of expressing pulsar signal intensity varies s l ight ly in the l i terature. The intensity wi l l be taken here to mean the total energy per average pulse striking a unit capture cross-section per unit bandwidth. The intensities quoted in the l i terature have been standardized to this form. Where necessary pulse widths for this standardization have been taken from ref-erence 4. Some spectra are plotted in Figure 1-3 for CP 1919. The wide range of intensities is due partly to the different averaging techniques used by the various observers, and partly due to the large, long-terra variations in pulsar signal strength. Each spectrum in Figure.1-3 is denoted by a letter and a number. The letter stands for the observatory where the spectrum was measured. The number is the number of pulse periods used to obtain the average. Spectra which have been drawn from selected large pulses or groups of such pulses are noted. Inevitably, there wi l l be some selection effect because weak pulses wi l l be lost in background noise. Note that although there are shifts of overall level , . the shapes of the spectra measured by different observatories are similar,, (with the exception of one point on the Parkes spectrum ). Periods quoted in the l iterature are usually referred to the earth-sun barycentre. Owing to the motion of an earth-4 bound o b s e r v e r w i t h r e s p e c t t o t h i s p o i n t , t h e p e r i o d w i l l be d o p p l e r - s h i f t e d . The e a r t h ' s o r b i t a l m o t i o n c o n t r i b u t e s most o f t h i s e f f e c t b u t t h e r e i s a l s o a d a i l y v a r i a t i o n due t o i t s a x i a l r o t a t i o n . 5 P u l s a r S i g n a l I n t e n s i t y ( j o u l e s - m ~ 2 - H z - 1 ) x 10" 10 MHz 100 MHz 1 GHz Fi g u r e 1-3 6 N o t e 1: T h e P a r k e s s p e c t r u m r e p r e s e n t s a 5 0 0 0 p u l s e a v e r a g e . A t 85 -•-'6 -2 ~] MHz i d e n t i f i c a t i o n o f p u l s e s l e s s t h a n .3 x 10 " j o u l e s - m - H z w a s r e p o r t e d t o b e v e r y d i f f i c u l t . H o w e v e r , t h e a v e r a g e w'as d e t e r - -- 2 6 -2 -1 m i n e d t o be .06 x 10 j o u l e s - m - H z . B e c a u s e many o f t h e low-e n e r g y p u l s e s may h a v e b e e n e x c l u d e d f r o m t h i s a v e r a g e , i t may be l o w e r t h a n o n e w h i c h i n c l u d e s t h e s e p u l s e s . N o t e 2 s T h e A r e c i b o s p e c t r u m i s a 6 1 4 4 p u l s e a v e r a g e a t 1 1 1 . 5 MHz, a n d a 4 0 9 6 p u l s e a v e r a g e a t 4 3 0 M H z . A t 1 1 1 . 5 MHz A r e c i b o r e p o r t s t h a t t h e r e i s a v a r i a t i o n i n 6 e q u a l s u b d i v i s i o n s o f t h e d a t a o f .85 t o 1.3 t i m e s t h e a v e r a g e . S i m i l a r l y , i n 4 e q u a l s u b d i v i s i o n s a t 4 3 0 MHz t h e r a n g e was f r o m .4 t o 1,6 t i m e s t h e ( 5 ) a v e r a g e . A t 4 0 MHz t h r e e o b s e r v a t i o n s o f 4 0 9 6 , 7 > 1 , a n d 3 9 4 0 p u l s e s w e r e a v e r a g e d . T h e r a n g e was f r o m .2 t o 1.6 t i m e s t h i s ( p r i v a t e l e t t e r ) n x . . . , . . . j , a v e r a g e . r B e c a u s e t h e s e i n t e n s i t i e s w e r e r e p o r t e d a s p e a k f l u x , a f a c t o r o f 1/ 2 was i n t r o d u c t e d t o d e t e r m i n e t h e mean p u l s e height« N o t e 3 s T h e J o d r e l l s p e c t r u m i s a n a v e r a g e o v e r a p p r o x i m a t e l y ( 1 5 ) 8 1 0 0 p u l s e s . I n c l u d e d i n t h i s s p e c t r u m a t 8 2 MHz i s a C a m o n d g e raeasureinenx. N o t e 4t A t 22 MHz t h e l e v e l o f t h r e e s t a n d a r d d e v i a t i o n s o f t h e n o i s e l e v e l h a s b e e n i n d i c a t e d f o r a 5 0 0 0 a n d 8 1 0 0 p u l s e a v e r a g e a s a n u p p e r l i m i t f o r c o m p a r i s o n w i t h t h e o t h e r s p e c t r a . 7 2. JUSTIFICATION OF THE EXPERIMENT There are no observations of any of the pulsar spectra at frequencies less than 40 MHz, and the work done at frequencies (3) less than 80 MHz is very limited. Although the data are very incomplete, i t appears that the pulsar intensity may be less than I O ' 2 0 joules -m" 2 -Hz™ 1 at 22 MHz.(see Figure 1-3). The avai labi l i ty of a large antenna at this frequency at the Dominion Radio Astro-physical Observatory near Penticton, B.C. makes detection in this flux density range feasible. The physical area of this antenna 2 is 23,693 ra . It is comparable to the collecting area of Arecibo Ionospheric Observatory's circular dish at this frequency. Even an upper l imit in this sensit ivity range would be sufficiently low to be of use in determining the slope of the spectra of pulsar signals at low frequencies, A spectral cistoff at low frequencies would be useful in restrict ing theories of the origin of pulsar signals. Also, since the dispersion is proportional to the inverse square of the frequency, a measurement of the delay from frequencies above 100 MHz would give a very accurate dispersion measure. Thirdly, as indicated in chapter 1, there is some evidence that the pulse gets systematically wider as the frequency decreases. Previously, i t had been thought that the pulse width vras indepen-dent of frequency. Detection - of-the pulsar signals at 22 MHz would be a sensitive measure of this trend* These considerations provide the basis of just i f icat ion of the experiment described herein. 8 3 . S I G N A L - T 0 - X 0 1 S E P R O B L E M A t f r e q u e n c i e s n e a r 2 2 MHz t h e b a c k g r o u n d n o i s e f r o m t h e s k y i s c o n s i d e r a b l e . T h e " c o l d e s t " p a r t o f t h e s k y i s a b o u t 4 0 , 0 0 0 d e g r e e s o f b r i g h t n e s s t e m p e r a t u r e . I n t h e p l a n e o f t h e G a l a x y t h e s k y b r i g h t n e s s t e m p e r a t u r e r i s e s a s h i g h a s 2 8 0 , 0 0 0 d e g r e e ' s . (7) T h i s r a d i a t i o n i s p r i m a r i l y n o n t h e r m a l i n o r i g i n . T h e b a c k -g r o u n d b r i g h t n e s s t e m p e r a t u r e i n t h e v i c i n i t y o f CP 1 9 1 9 i s a b o u t 5 5 . , 0 0 0 d e g r e e s . A s e x p l a i n e d i n c h a p t e r 1, t h e s p e c t r a l i n d e x f o r p u l s a r s i s n o t a c c u r a t e l y k n o w n . H o w e v e r , f r o m t h e d a t a a v a i l a b l e i t a p p e a r s t h a t t h e p o w e r s p e c t r u m o f t h e s k y b a c k g r o u n d r a d i a t i o n h a s a s t e e p e r s l o p e t h a n t h e p u l s a r p o w e r s p e c t r u m , T h e s l o p e o f t h e b a c k g r o u n d r a d i a t i o n i s i n d i c a t e d i n F i g u r e 1 - 3 f o r c o m p a r i s o n . T h e r e f o r e , a t l o w f r e q u e n c i e s t h e p a r a m o u n t p r o b l e m i s g e t t i n g a h i g h e n o u g h s i g n a l - t o - n o i s e r a t i o . A t y p i c a l i n s t a n -t a n e o u s b a n d w i d t h o f p u l s a r s i g n a l s i s 1 0 k H z a t 2 2 MHz ( s e e F i g u r e 3 - 1 ) . I n c r e a s i n g t h e b a n d w i d t h o f a r e c e i v e r t o m o r e t h a n t h e i n s t a n t a n e o u s b a n d w i d t h o f t h e p u l s a r s i g n a l i n c r e a s e s t h e n o i s e l e v e l w i t h o u t i n c r e a s i n g t h e s i g n a l l e v e l . On t h e o t h e r h a n d , t h e p u l s a r t a k e s a l o n g e r t i m e t o s w e e p t h r o u g h a l a r g e b a n d -w i d t h t h a n t h r o u g h a s m a l l o n e , t h e r e b y i n c r e a s i n g t h e a v a i l a b l e i n t e g r a t i o n t i m e p e r p u l s a r p e r i o d . I t i s s h o w n i n A p p e n d i x A t h a t t h e f i r s t e f f e c t i s o v e r w h e l m i n g , a n d t h a t t h e o p t i m u m b a n d w i d t h f o r r e c e i v i n g p u l s a r s i g n a l s i s t h e i n s t a n t a n e o u s b a n d -w i d t h o f t h e p u l s a r . N o w , i n o r d e r t o g a i n t h e maximum i n t e g r a t i o n t i m e p e r p u l s a r p e r i o d , t h e c e n t r e f r e q u e n c y o f t h e r e c e i v e r c a n b e s w e p t s o t h a t i t f o l l o w s t h e f r e q u e n c y v s , t i m e c u r v e o f t h e p u l s a r . T h e r e s u l t i s t h a t t h e s i g n a l - t o - n o i s e p o w e r r a t i o i s i n c r e a s e d b y a f a c t o r fsP •J B . i o v e r a n o p t i m u m f i x e d b a n d w i d t h . w h e r e 6 f i s t h e s w e e p b a n d w i d t h o f t h e r e c e i v e r 1J\ i s t h e i n s t a n t n a e o u s b a n d w i d t h o f t n e p u l s a r 9 Because of the l i k e l i h o o d 0 f g a i n f l u c t u a t i o n s i n the r e c e i v e r a comparison source i s used i n the same way as i n a Dicke r e c e i v e r . The comparison i s the background n o i s e . The r e c e i v e r bandwidth f o l l o w s the p u l s a r f o r o n e - h a l f of the p u l s a r p e r i o d , and r e p e a t s t h e sweep f o r the o t h e r h a l f . Of c o u r s e , the second time the r e c e i v e r sees o n l y background n o i s e (see F i g u r e 3-1). A l l the r e c e i v e r parameters a r e t h e same f o r the f i r s t h a l f as f o r the second h a l f . The background n o i s e i s a l s o the same f o r both sweeps. The r e c e i v e r t a k e s t h e d i f f e r e n c e between the outputs of two s u c c e s s i v e sweeps which i s p r o p o r t i o n a l t o the p u l s a r s i g n a l s t r e n g t h . Sweep Bandwidth F i g u r e 3-1 Arrows i n d i c a t e the p a t h f o l l o w e d by the r e c e i v e r when i t i s i n phase w i t h the p u l s a r s i g n a l . For CP1919 the sweep bandwidth i s 70.7 kHz and the. i n s t a n t a n e o u s bandwidth ( B ^ i s 4.2 kHz. CP1919's h e l i o c e n t r i c p e r i o d i s 1.3373011 seconds. One i m p o r t a n t assumption t h a t has been made i s t h a t the bandwidth sweep i s i n phase w i t h the p u l s a r . I f a s e a r c h of the phases i s r e q u i r e d , then the g a i n s i n s i g n a l - t o - n o i s e r a t i o made by a sweeping bandwidth are n u l l i f i e d . However, any i n f o r m a t i o n about the phase can r e s u l t i n a h i g h e r p r o b a b i l i t y of d e t e c t i o n w i t h a sweeping bandwidth. I f o b s e r v a t i o n s are made s i m u l t a n e o u s l y 10 on a h i g h f r e q u e n c y a s w e l l a s a t 2 2 MHz, t h e p h a s e o f t h e p u l s a r s i g n a l a t 2 2 MHz c a n be p r e d i c t e d , l i q u a t i o n 1-1 c a n be i n t e g r a t e d t o g i v e t h e t i m e d i f f e r e n c e b e t w e e n t h e e p o c h a t a h i g h f r e q u e n c y a n d t h e e p o c h a t 2 2 MHz. T h e t i m e d i f f e r e n c e i s d i r e c t l y p r o p o r -t i o n a l t o t h e d i s p e r s i o n m e a s u r e . T h i s p r o b l e m w i l l bo d i s c u s s e d a g a i n i n c h a p t e r 6. I f t h e p u l s a r s w e r e a s t e a d i l y p u l s a t i n g p h e n o m e n o n i n w h i c h o n e p u l s e a m p l i t u d e i s s t a t i s t i c a l l y i n d e p e n d e n t o f t h e o t h e r a m p l i t u d e s , t h e p r o b a b i l i t y o f d e t e c t i o n c o u l d be i n c r e a s e d b y a d d i n g o n e p u l s e p e r i o d t o t h e n e x t . H o w e v e r , i t a p p e a r s t h a t t h e y a r e n o t s t a t i s t i c a l l y i n d e p e n d e n t , b u t o d c u r i n t r a i n s w rhose l e n g t h ( 9 ) i s d e p e n d e n c u p o n t h e f r e q u e n c y . F o r CP 1 9 1 9 a t 8 5 MHz s t r o n g p u l s e t r a i n s s e e m t o l a s t f o r a b o u t o n e m i n u t e on t h e a v e r a g e . T h e r e f o r e , i t i s d i s a d v a n t a g e o u s t o i n t e g r a t e f o r i i n i e s l o n g e r t h a n t h e a v e r a g e t i m e t h a t a p u l s a r i s a c t i v e . A t l o w f r e q u e n c i e s t h e o p t i m u m i n t e g r a t i o n t i m e c a n o n l y b e g u e s s e d . T h e i n t e g r a t i o n t i m e u s e d f o r C P 1 9 1 9 w a s 30 s e c o n d s . Of c o u r s e , l o n g e r i n t e g r a t i o n c a n b e o b t a i n e d b y a f u r t h e r a v e r a g i n g o f t h e r e c o r d s . I t was d e c i d e d t o c o n c e n t r a t e e f f o r t s on t h e p u l s a r CP 1 9 1 9 . A t t h e t i m e o f y e a r t h a t o b s e r v a t i o n s w e r e b e i n g made CP 1 9 1 9 was t r a n s i t i n g t h e m e r i d i a n a t n i g h t . A t 2 2 MHz man-made i n t e r -f e r e n c e r e s t r i c t s o b s e r v a t i o n s t o b e t w e e n a b o u t 9:30 p.m. a n d 7 :00 a.m. A l s o , CP 1 9 1 9 h a s a n a c c u r a t e l y k n o w n d i s p e r s i o n m e a s u r e . T h i s a c c u r a c y i s r e q u i r e d i n o r d e r t o p r e d i c t t h e p h a s e o f t h e p u l s a r s i g n a l s a t 2 2 MHz. CP 1 9 1 9 h a s b e e n s e e n a t 4 0 MHz b y A r e c i b o I o n o s p h e r i c O b s e r v a t o r y , a n d t h e r e h a s b e e n c o n s i d e r a b l e w o r k d o n e a t h i g h e r f r e q u e n c i e s . 11 4. THE ANTENNA T h e a n t e n n a c o n s i s t s o f t h e N o r t h - S o u t h a r m o f t h e 22 MHz T - s h a p e d a r r a y u s e d a t D . R . A . O . ^ ^ I t i s a n a r r a y o f 24.0 f u l l -w a v e d i p o l e s p o l a r i z e d i n t h e E a s t - W e s t d i r e c t i o n a n d m o u n t e d a b o v e a r e f l e c t i n g s c r e e n , , T h e a r m i s 4X w i d e , a n d i t s c e n t r e f r e q u e n c y i s 2 2 . 2 5 MHz. I t h a s a h a l f - p o w e r b e a m i w i d t h o f 14 d e g r e e s i n R i g h t A s c e n s i o n a n d 2.1 d e g r e e s i n D e c l i n a t i o n a t t h e z e n i t h . P h a s i n g o f t h e i n d i v i d u a l r o w s o f d i p o l e s i n t h e N o r t h - S o u t h a r m b y s w i t c h i n g l e n g t h s o f c a b l e a s d e l a y s p r o v i d e s s t e e r i n g i n D e c l i n a t i o n , b u t n o t i n R i g h t A s c e n s i o n . C a l i b r a t i o n o f t h e s e n s i t i v i t y o f t h e a n t e n n a was d o n e b y t w o m e t h o d s . T h e f i r s t m e t h o d was t o p o i n t t h e a n t e n n a a t t h e well-known, b r i g h t s o u r c e Casseopeia A. O w i n g t o t h e b r o a d n e s s o f t h e beam ( a p p r o x i m a t e l y o n e h o u r ) i n R i g h t Ascension, s e v e r a l s c a n s a f e w d e g r e e s i n D e c l i n a t i o n o n e a c h s i d e o f C a s s . A h a d t o be made t o p r o v i d e a b a s e l i n e . C a s s . A i s l o c a t e d i n t h e p l a n e o f t h e G a l a x y , T h e r a d i a t i o n f r o m t h e G a l a x y m u s t b e s u b t r a c t e d f r o m t h e t o t a l i n o r d e r t o i s o l a t e t h e e f f e c t o f C a s s . Ac, O b s e r v a t i o n s o f Cass, A w e r e f u r t h e r c o m p l i c a t e d b y i o n o s p h e r i c s c i n t i l l a t i o n s w h i c h c a u s e d f l u c t u a t i o n s o f t h e o r d e r o f a f a c t o r o f t w o i n t h e r e c o r d . H o w e v e r , i t h a s b e e n s h o w n t h a t g o o d f l u x d e n ~ ( 1 1 ) s i t i e s c a n b e d e t e r m i n e d e v e n f r o m s c i n t i l l a t i n g r e c o r d s . C o m p a r i s o n a g a i n s t a n o i s e / s o u r c e s u b s t i t u t e d f o r t h e a n t e n n a was d o n e i n e a c h c a s e t o g e t a b s o l u t e v a l u e s o f n o i s e temperature* T h i s method g i v e s t h e o v e r a l l e f f e c t i v e - a r e a i n c l u d i n g t h e ohmic l o s s f a c t o r f o r t h e f e e d c a b l e s i n t h e a n t e n n a . C a s s . A a t 22 MHz h a s a f l u x d e n s i t y o f 5 1 , 4 0 0 f l u x u n i t s ( 1 f l u x u n i t = 1 Q ~ 2 6 w a t t s - r a ~ 2 - H z " " 1 ) ( 1 2 ) a n d i s r a n d o m l y p o l a r i z e d . T h e n o i s e t e m p e r a t u r e d u e t o . t h i s s o u r c e a t t h e i n p u t t o t h e a m p l i f i e r was c o m p a r e d w i t h a standard a n d f o u n d t o be 7 0 0 0 d e g r e e s . T h e r e s u l t i n g s e n s i t i v i t y o f t h e a n t e n n a i s t h e n 7.3 f l u x u n i t s p e r d e g r e e . T h e o v e r a l l e f f e c t i v e a r e a i s t h e n A ---~2-|- = 376 m 2 ° e o T h e r e i s a f a c t o r t w o b e c a u s e t h e a n t e n n a i s r e c e p t i v e t o o n l y o n e p o l a r i z a t i o n o T h e a p e r t u r e e f f i c i e n c y i s t h e n e f f e c t i v e a r e a = l»7f«. p h y s i c a 1 a r e a The o t h e r m e t h o d i s t o l o o k a t t h e b a c k g r o u n d s k y . T h e r e g i o n n e a r t h e N o r t h P o l e was c h o s e n . T h i s r e g i o n h a s a n a v e r a g e b r i g h t n e s s t e m p e r a t u r e o f 3 8 , 0 0 0 d e g r e e s a t 22 MHz. T h e v a l u e o f t h e b r i g h t n e s s t e m p e r a t u r e a t 22 MHz was o b t a i n e d f r o m t h e v a l u e a t 38 M H z ^ u l i n g a s p e c t r a l i n d e x o f 2 . 5 0 ^ ^ . i . e . T, , f„ 2.50 A b 2 Jl w h e r e i s t h e b r i g h t n e s s t e m p e r a t u r e o f t h e b a c k g r o u n d a t f r e q u e n c y f ^. * s ^ n e b r i g h t n e s s t e m p e r a t u r e o f t h e b a c k g r o u n d a t f r e q u e n c y f 0 . B e c a u s e t h e o u t p u t o f t h e a n t e n n a i s i n d e p e n d e n t o f t h e beam s h a p e (as l o n g a s t h e beam i s n o t s o b r o a d t h a t t h e b a c k g r o u n d s k y c a n b e c o n s i d e r e d t o be an e x t e n d e d s o u r c e o f c o n s t a n t b r i g h t n e s s t e m p e r a t u r e ) , t h i s m e a s u r e m e n t g i v e s t h e o h m i c l o s s f a c t o r , , T h e n o i s e t e m p e r a t u r e a t t h e i n p u t t o t h e p r e a m p l i f i e r was i n t h i s c a s e m e a s u r e d t o b e 1 1 8 0 d e g r e e s . O h m i c L o s s F a c t o r = 1 1 8 0 3 8 , 0 0 0 - 3.1# F i l l i n g F a c t o r = O v e r a l l A p e r a t u r e E f f i c i e n c y _ - A „ O h m i c L o s s F a c t o r ~ * T h e o h m i c l o s s f a c t o r d o e s n o t a f f e c t t h e s i g n a l - t o -n o i s e r a t i o s i g n i f i c a n t l y . S i n c e m o s t o f t h e n o i s e c o m e s b a c k g r o u n d s k y , c a b l e l o s s e s a t t e n u a t e t h e s i g n a l a n d t h e n o i s e e q u a l l y . V e r y l i t t l e e x t r a n o i s e i s a d d e d b y t h e c a b l i n g s y s t e m i n t h e a n t e n n a . T h e f i l l i n g f a c t o r , h o w e v e r , d o e s a f f e c t t h i s r a t i o b e c a u s e , i n t h i s c a s e , o n l y h a l f o f t h e g e o m e t r i c a l a r e a o f t h e a n t e n n a c a n a b s o r b s i g n a l p o w e r . B u t , a s m e n t i o n e d a b o v e , t h e o u t p u t o f n o i s e p o w e r f r o m t h e a n t e n n a i s m o r e o r l e s s i n d e p e n d e n t o f t h e s h a p e o f t h e a n t e n n a b e a m . T h e s m a l l e r a n t e n n a h a s t h e same n o i s e p o w e r b u t l e s s s i g n a l t h a n t h e l a r g e r o n e . 1 3 5. THE S Y S T E M A b l o c k d i a g r a m o f t h e s y s t e m i s s h o w n i n F i g u r e 5 - 1 . T h e 2 2 MHz a n t e n n a i s c o n n e c t e d t o a p r e a m p l i f i e r w h i c h i s f e d i n t o a n a m p l i f i e r w i t h a 3 d b b a n d p a s s o f 3 0 0 k H z c e n t r e d a t 2 2 . 2 5 0 MHz.It i s m i x e d w i t h a s w e p t l o c a l o s c i l l a t o r c e n t r e d a t 3 2 . 9 5 0 MHz, a n d f e d i n t o an I , F . p r e a m p l i f i e r a n d t h e n a c r y s t a l f i l t e r . I t i s f u r t h e r a m p l i f i e d , a t 1 0 a 7 MHz a n d t h e n d e t e c t e d . T h e d e t e c t e d o u t p u t i s a m p l i f i e d . The s i g n a l i s t h e n f e d i n t o a p h a s e d e t e c t o r made f r o m f i e l d - e f f e c t t r a n s i s t o r s , a n d t h e n i n t o an. i n t e g r a t o r w h i c h i s d i s c h a r g e d e v e r y 30 s e c o n d s . T h e o u t p u t i s d i s p l a y e d o n a c h a r t r e c o r d e r . T h e t i m i n g i s d o n e ' b y d i v i d i n g d o wn a 1 MHz c r y s t a l o s c i l l a t o r i n t h e f r e q u e n c y s y n t h e s i z e r . 1'he f r e q u e n c y o f t h i s was c a l i b r a t e d w i t h t h e W.W.V.B. 60 k H z c a r r i e r . The t i m i n g c o n t r o l p r o v i d e s v a r i o u s t r i g g e r i n g l e v e l s a t t h e pulsar,- a s w e l l a s a 1 7 - p o s i t i o n , a c c u r a t e d i g i t a l d e l a y * T h e d e l a y can b e u s e d t o t r i g g e r a f r e q u e n c y c o u n t e r s o t h a t t h e o u t p u t o f t h e l o c a l o s c i l l a t o r can b e c h e c k e d a t v a r i o u s p o i n t s a l o n g t h e sweep* F o r t u n a t e l y , t h e p u l s a r sweep i s l i n e a r t o a g o o d a p p r o x i m a -t i o n a t 22 MHz. I n a s w e e p c o r r e s p o n d i n g t o h a l f a p u l s a r p e r i o d CP 1 9 1 9 d e v i a t e s a b o u t 42 Hz f r o m l i n e a r i t y a t 2 2 . 2 5 MHz. T h i s i s o n l y a s m a l l f r a c t i o n o f t h e 4.2 k H z i n s t a n t a n e o u s bandwidth of CP 1 9 1 9 . T h e r e f o r e , a v o l t a g e r a m p was u s e d t o c o n t r o l t h e o u t p u t o f t h e H e w l e t t - P a c k a r d f r e q u e n c y s y n t h e s i z e r a s a l o c a l o s c i l l a t o r f o r the s y s t e m . T h e n o n - l i n e a r i t y o f t h e f r e q u e n c y s y n t h e s i z e r was m o r e t h a n t h a t o f t h e p u l s a r . I t was m e a s u r e d , t o be a m aximum o f 4 0 0 H z . T h i s d e v i a t i o n i s s t i l l o n l y o n e t e n t h of t h e i n s t a n e o u s b a n d w i d t h o f t h e p u l s a r . I n a c c o r d a n c e w i t h t h e m e t h o d d e s c r i b e d i n c h a p t e r 3, t h e r e f e r e n c e ( s k y b a c k g r o u n d ) m u s t b e s u b t r a c t e d f r o m t h e s o u r c e p l u s r r e f e r e n c e ( s i g n a l p l u s s k y b a c k g r o u n d ) . T h i s s u b t r a c t i o n i s d o n e b y a l t e r n a t e l y s w i t c h i n g t h e o u t p u t i n t o o p p o s i t e i n p u t s o f a d i f f e r e n t i a l a m p l i f i e r e a c h j j u l s a r h a l f - p e r i o d . S i n c e a l l s w i t c h i n g i s d o n e at t w i c e t h e p u l s a r r e p e t i t i o n r a t e , t h e r e i s no p o s s i b i l i t y t h a t a s p u r i o u s t i m i n g s i g n a l c a n b u i l d u p i n t h e i n t e g r a t o r . T h e o f f - p e r i o d s o f t h e p h a s e d e t e c t o r a r e o v e r -l a p p e d s l i g h t l y s o t h a t d u r i n g t h e p e r i o d o f p o s s i b l e t r a n s i e n t s , 16.6 db Coupler 22 MHz 22 MHz 10.7MHz C r y s t a l 10.7MHz Preamp. Cable R.F. Mixer I.F. F i l t e r Amp. Amp. Preamp. B.W.= 4kHz. N o i s e G e n e r a t o r Switch 25.6 m P a r a b o l o i d with 151 MHz feed V Frequency Counter Timing C o n t r o l Master Clock 150 Mi iz Preamp. Cable Receiver Frequency Synthesi zer Sweep Time O s c i l l a t o r Synchronous Averager Chart Recorder Detector High-Pass Amp. Phase Detector I n t e g r a t o r Reset Timer i n t e g r a t o r Chart iecorder Figure 5 - i 15 S y s t e m D i a g r a m : T h i c k l i n e s r e p r e s e n t R. F„ p a t h s a n d m a i n s i g n a l p a t h s , , T h i n l i n e s r e p r e s e n t p a t h s o f c o n t r o l s i g n a l s . S i g n a l s f r o m T i m i n g C o n t r o l t o * f r e q u e n c y S y n t h e s i z e r V o 1 1 a g e Ramp c o n t r o 1 s o u t p u t f r e q u e n c y F r e q u e n c y C o u n t e r P h a s e D e t e c t o r S v n c h r o n o u s A v e r a c e r N o i s e G e n e r a t o r S w i t c h •* On e P u i s a r P e r i o d -L e a d i n g e d g e t r i g g e r s c c u a t e r . P h a s e c a n be a d j u s t e d i n s t e p s o f 4 0 m s e c . S o l i d s q u a r e w a v e c o n t r o l s o n e h a l f . o f p h a s e detector; d o 1 1 e a s q u a r e w a v e , t h e o t h e r h a l f . L e a d i n g edge t r i g g e r s sweep o f F a b r i t e k A v e r a g e r . T h e p h a s e o f t h i s s i g n a l can a l s o bo a d j u s t e d , T h i s s i g n a I a d A s n o i s e t o t h e i n p u t o f t h e s y s t e i n e v e r y h a. I f p u i s a r p e r i o d f o: c a l i b r a t i o n . 5-1 t h e r e i s no s i g n a l c o m i n g o u t o f t h e p h a s e d e t e c t o r . . T h e s e n s i t i v i t y o f t h e s y s t e m c a n be c a l i b r a t e d b y s w i t c h i n g a. n o i s e o n a n d o f f i n t o t h e p r e a m p l i f i e r s y n c h r o n o u s l y w i t h t h e s w e e p on. a l t e r n a t e h a l f - p e r i o d s . T h e o v e r a l l s e n s i t i v i t y w i l l be d i s c u s s e d i n c h a p t e r 7, T h e 150 MHz a n t e n n a i s t h e 2 6 . 5 m p a r a b o l i o d . l o c a t e d a t D . l t . A . O . A d i p o l e f e e d i s u s e d w i t h a S i n y t h e p r e a m p l i f i e r . T h e d e t e c t e d o u t p u t i s f e d i n t o a 2 5 6 - c h a n n e l - s i g n a l a v e r a g e r a r r a n g e d s o t h a t o n e s w e e p o f t h e 2 3 6 c h a n n e l s i s o n e p u l s a r p e r i o d , A t y p i c a l o u t p u t f r o m t h i s s y s t e m i s s h o w n i n F i g u r e 5-2. T h e s w e e p i s t r i g g e r e d f r o m t h e t i m i n g c o n t r o l i n t h e 22 MHz s y s t e m . T h e p h a s e o f t h e l o c a l o s c i l l a t o r s w e e p i n t h e 22 MHz s y s t e m c a n b o a d j u s t e d r e l a t i v e t o t h i s t r i g g e r p u l s e . One P u l s a r P e r i o d - -C=» 1 P u l s a r S i g n a 1 F i g u r e 5-2 A t y p i c a l 1 0 m i n u t e a v e r a g e f r o m t h e 1 5 0 MHz s y s t e m , 17 6, THE E X P E R I M E N T As was p o i n t e d o u t i n c h a p t e r 3, t h e b a n d w i d t h s w e e p m u s t b e i n p h a s e w i t h t h e p u l s a r s i g n a l i n o r d e r t o g e t t h e f u l l b e n e f i t o f t h e s y s t e m * I t i s p o s s i b l e t o g e t t h e pvO.se e p r c h a t a h i g h f r e q u e n c y a n d u s e i t t o p r e d i c t t h e p h a s e a t 22 MHz, T h e h i g h f r e q u e n c y c h o s e n was 1 5 1 . 5 MHz, T h e l i t e r a t u r e i n d i c a t e s t h a t t h e p u l s a r s i g n a l s a r e l i k e l y t o be s t r o n g e s t n e a r 1 5 0 Mlizo I f o b s e r v a t i o n s a r e made s i m u l t a n e o u s l y o n t h e t w o f r e q u e n c i e s t h e n e q u a t i o n 1 c a n be i n t e g r a t e d t o g i v e t f 2 " t f l where f 2 = 1 5 1 . 5 MHz f1 = 2 2 . 2 5 0 MHz. F o r C P 1 9 1 9 t = 1 0 9 . 9 1 s e c o n d s . T h e r e q u i r e d p h a s e c h a n g e c a n e a s i l y b e c a l c u l a t e d . O b s e r v a t i o n s w e r e made i n t h i s w a y o n t e n o c c a s i o n s a T h e p h a s e o f t h e l o c a l o s c i l l a t o r s w e e p was s c a n n e d d u r i x i g t h e o b s e r v a t i o n s t o c o v e r a r a n g e o f a b o u t 1 0 0 m s e c o n e a c h o f t h e p r e d i c t e d p h a s e . T h i s m e t h o d was u s e d a s a c o m p r o m i s e b e t w e e n s e a r c h i n g a l l p o s s i b l e p h a s e s a n d s p e n d i n g t h e e n t i r e o b s e r v i n g t i m e a t t h e p r e d i c t e d p h a s e . E a r l i e r o b s e r v a t i o n s w e r e made d u r i n g w h i c h a l l o f t h e p o s s i b l e p h a s e s w e r e s c a n n e d , b u t , a s e x p l a i n e d b e f o r e , t h i s m e t h o d h a s no a d v a n t a g e o v e r o b s e r v i n g w i t h a f i x e d b a n d w i d t h . F i g u r e 6-1 s h o w s a t y p i c a l p i e c e o f c h a r t r e c o r d made d u r i n g t h e o b s e r v a t i o n s . T h e s t r a i g h t l i n e s o c c u r a t t h e p o i n t s w h e n t h e i n t e g r a t o r was r e s e t ( e v e r y 30 s e c o n d s ) . T h e r e f o r e , t h e v a l u e o f t h e o u t p u t j u s t b e f o r e t h i s p o i n t r e p r e s e n t s a 30 s e c o n d a v e r a g e . T h e f i r s t s e c t i o n c o n t a i n s o n l y b a c k g r o u n d n o i s e a n d t h e s e c o n d s e c t i o n c o n t a i n s a s i m u l a t e d p u l s a r s i g n a l ( s e e F i g u r e 5 -1) s l i g h t l y l a r g e r t h a n o n e s t a n d a r d d e v i a t i o n . 1 Q J. O . F i g u r e 6-1 O u t p u t f r o m the 22 MHz p u l s a r system. T h e u p p e r t r a c e i s b a c k g r o u n d n o i s e ; the l o w e r o n e i s b a c k g r o u n d n o i s e p l u s a s i m u l a t e d p u l s a r s i g n a l . 19 7. THE R E S U L T S T h e m i n i m u m d e t e c t a b l e b r i g h t n e s s t e m p e r a t u r e w i l l be d e t e r m i n e d b y t h e b a c k g r o u n d s k y t e m p e r a t u r e a n d t h e r e c e i v e r n o i s e t e m p e r a t u r e . T h e n o i s e t e m p e r a t u r e o f t h e r e c e i v e r was m e a s u r e d t o be 2 0 0 d e g r e e s , a b o v e a m b i e n t t e m p e r a t u r e . T h e b a c k g r o u n d b r i g h t n e s s t e m p e r a t u r e c o n t r i b u t e s 5 5 , 0 0 0 d e c r e e s + 1 0 , 0 0 0 d e g r e e s i n t h e r e g i o n ' n e a r CP 1 9 1 9 , ^ 1 3 ' T h e e f f i c i e n c y w i t h w h i c h t h i s i s t r a n s m i t t e d t o t h e p r e a m p l i f i e r i s 3 . 1 $ . T h e r e f o r e , T = G 'T. + ( l - e ) T +. 1' S A o R = 2 1 8 6 d e g r e e s w h e r e r i \ i s t h e o v e r a l l s y s t e m t e m p e r a t u r e s T . , i s t h e a n t e h r i a t e m p e r a t u r e A T q i s a m b i e n t t e m p e r a t u r e T i s t h e r e c e i v e r t e m p e r a t u r e . R T h e s t a n d a r d d e v i a t i o n of t h e n o i s e o u t p u t i s •a = 2 T w h e r e B i s t h e I . F . b a n d w i d t h o f t h e s y s t e m x i s t h e i n t e g r a t i o n t i m e ° F o r a s i n g l e p o i n t o n t h e r e c o r d t h e r e i s a 30 s e c o n d i n t e g r a t i o n t i m e , a n d t i i e b a n d w i d t h ( c h o s e n a s t h e i n s t a n t a n e o u s b a n d w i d t h o f CP 1 9 1 9 ) i s 4.0 k H z . T h e n , a = 1 2 . 6 d e g r e e s « T h i s f i g u r e i s s o m e w h a t l o w e r t h a n t h e m e a s u r e d s t a n d a r d d e v i a t i o n o f 1 3 . 8 d e g r e e s ( t a k e n f r o m 1 2 0 s a m p l e s ) . T h e d i f f e r e n c e o f a p p r o x i m a t e l y 1 0 $ i s w i t h i n t h e l i m i t s o f a c c u r a c y o f t h e c a l c u l a t i o n s b u t c o u l d a l s o b e a t t r i b u t e d t o l o w - l e v e l , man-made i n t e r f e r e n c e . S i n c e t h e s e n s i t i v i t y o f t h e a n t e n n a i s 6.9 f l u x u n i t s p e r d e g r e e a n d t h e p u l s e l e n g t h o f CP 1 9 1 9 i s 4 0 m s e c . , t h e n t h e s t a n d a r d d e v i a t i o n o f t h e n o i s e i s p : «7 x 10 ^ j o u l e s - m "-Hz ( e x p r e s s e d i n t h e same u n i t s a s p u l s a r i n t e n s i t y ) . 20 As e x p l a i n e d i n c h a p t e r 3 t h e i n t e g r a t i o n can be done f o r l o n g e r p e r i o d s than 30 se c o n d s by a v e r a g i n g t h e r e c o r d s . The t o t a l t i m e t h a t t h e s o u r c e i s i n t h e a n t e n n a beam i s about one h o u r . I f , as i n a t y p i c a l n i g h t ' s o b s e r v a t i o n s , 120 p o i n t s a r e u s e d , t h e n a = 1.2 d e g r e e s . T h i s s t a n d a r d d e v i a t i o n o f n o i s e c o r r e s p o n d s t o a a p u l s a r s i g n a l i n t e n s i t y o f .34x10"^ j o u l e s - m ^_Hz" . 23 APPENDIX T h e optimura f i x e d b a n d w i d t h f o r r e c e i v i n g p u l s a r s i g n a l s c a n be d e d u c e d a s f o l l o w s t T h e r e c e i v e r i n t e g r a t e s p u l s a r s i g n a l p o w e r p l u s n o i s e p o w e r o v e r t i m e T j , a n d i n t e g r a t e s t h e n o i s e power o v e r t i m e T Q ( a s s u m i n g t h a t t h e r e c e i v e r h a s a s q u a r e l a w d e t e c t o r ) . The o u t p u t v o l t a g e of the receiver, v, is t h e d i f f e r e n c e b e t w e e n t h e s e t w o i n t e g r a t i o n s , i . e . < v > = ^ l V ^ T 0 If 6B<B = p 6 B + nB ( n e g l e c t i n g c o n s t a n t s o f Y Q ~ n B p r o p o r t i o n a l i t y ) w h e r e p i s p u l s a r s i g n a l p o w e r p e r u n i t b a n d w i d t h 6 B i s t h e i n s t a n t a n e o u s b a n d w i d t h o f t h e p u l s a r B i s t h e b a n d w i d t h o f t h e r e c e i v e r T h e b e s t s i g n a l - t o - n o i s e r a t i o i s o b t a i n e d w h e n var v is m a x i m i z e d , , S i n c e t h e t w o p r o c e s s e s a n d v ^ a r e i n d e p e n d e n t , var $> = var<v^>T+ v a r ^ If a n d V Q a r e a s s u m e d t o b e G a u s s i a n , t h e n var v,, = v a r v , = "2(p£B+nB; a n d v a r(v^) v a r v„ = n B 2 I f p « n v a r / ? , ) « v a r v. = n B ' 2HT7" 1 B u t T , --^ . B a n d T„ = T - T . 1 d f T d t " 0 1 where d f / d t i s t h e p u l s a r s w e e p r a t e T is t h e p u l s a r p e r i o d S u b s t i t u t i n g v a r < V > ~ n d f / d t (^ ) 2 2 S i n c e <v) ~ i s p " 6 B t h e b a n d w i d t h B s h o u l d be a s s m a l l a s p o s s i b l e as l o n g as 6B<B. If 6B>B, t h e n v± =(p + n) B var v n -j T, T h e r e f o r e , o r B s h o u l d b e made a s l a r g e a s p o s s i b l e a s l o n g a s B < 5 B . T a k i n g i n t o a c c o u n t t h e a b o v e r e s u l t , t h e o p t i m u m r e c e i v e r b a n d w i d t h m u s t b e B — 5B« 24 REFERENCES 1. Hewish, B e l l , P i l k i n g t o n , S c o t t , C o l l i n s , " O b s e r v a t i o n s of a R a p i d l y P u l s a t i n g Radio Source", N a t u r e , V o l . 217, Feb. 24, 1968, p. 709. 2. D a v i e s , S m i t h , Hunt, "Changing P e r i o d i c i t i e s i n t h e P u l s a r s " , N a t u r e , V o l . 221, J a n . 4, 1969, ;;p„ 27. 3. Drake, Gunderraann, J a n c e y , C o m a e l l a , Z e i s s i g , C r a f t , "The R a p i d l y V a r r y i n g E a d i o Source i n V u l p e c u l a " , S c i e n c e , V o l . 160, May 3, 1968, p. 503. . 4. T a y l o r , J.H., "Ca t a l o g u e of 37 P u l s a r s " , A s t r o p h y s i c a l L e t t e r s , Vol.::3, 1969, p. 205. 5. L o v e l a c e , C r a f t , " I n t e n s i t y V a r i a t i o n s of the P u l s a r CP 1919", N a t u r e , V o l . 220, Nov. 30, 1968, p. 875. 6. B r i d l e , A.H., "The Spectrum of t h e Radio Background Between 13 and 404 MHz", Monthly N o t i c e s of the R.A.S.,Vol» 136, 1967, p. 219. 7. K r a u s , John D., "Radio Astronomy", M c G r a w - H i l l , 1966, p.236, p. 310. 8. W i l l i a m s , K e n d e r d i n e , B a l d w i n , "A Survey of Radio Sources and Background R a d i a t i o n a t 38 MHz",Memoirs Royal A s t r o n o m i c a l S o c i e t y , V o l . 70, 1966, p. 53. 9» Robinson, Cooper, G a r d i n e r , W i e l e b i n s k i , Landecker, "Measure-ments of the P u l s e d Radio Source CP 1919 between 85 and 2700 MHz", N a t u r e , V o l . 218, June 22, 1968, p, 1143. 10. C o s t a i n , Lacey, Roger, "A Large 22 MHz A r r a y f o r Radio Astronomy", I.E.E.E. T r a n s a c t i o n s on Antennas and P r o p a g a t i o n , V o l . AP-17, No. 2, March, 1969, p. 162. 11. P u r t o n , C.R., "The S p e c t r a of Radio Sources and Background R a d i a t i o n " y t D o c t o r a l T h e s i s , U n i v e r s i t y of Cambridge, June, 1966. 12. Roger, C o s t a i n , Lacey, " S p e c t r a l F l u x D e n s i t i e s of Radio Sources at 22,25 MHz", A s t r o p h y s i c a l J o u r n a l , V o l . 174, No, 3, A p r i l 1969, p. 366. 2 5 13. G a i t , C o s t a i n , "Low-frequency Radio Astronomy", T r a n s a c t i o n s of t h e Royal S o c i e t y o f Canada, Fourth S e r i e s , V o l . 3, Sec, 3, 1965. 14. W i e l e b i n s k i , R . , " S t a t i s t i c a l P r o p e r t i e s of P u l s a r CP 1919", Nature, V o l . 219, Sept., 1968, p. 1135. 15. Lyne, R i c k e t t , "Measurements of the Pulse Shape and Spe c t r a of the P u l s a t i n g Radio Sources", Nature, V o l . 218, A p r i l , 1968, p. 329. 

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