Precise Measurement of Repeater Transmission

T H A D D E U S S L O N C Z E W S K I M E M B E R A I E E

I O N G T O L L t r a n s -_ j miss ion l ines pass ing

w i d e f r equency b a n d s must m e e t severe r e q u i r e ­ment s of equa l i za t i on of loss wi th f requency . A n e w wide ­b a n d coaxia l system u n d e r d e v e l o p m e n t will h a v e a l a rge n u m b e r of r e p e a t e r sect ions in t a n d e m . T h e over-al l e q u a l i z a d o n r e q u i r e d is such t h a t sys temat ic va r i a t ions in the t ransmiss ion cha rac te r i s t i c from 0.2 t o 8 megacyc les of one sect ion m u s t be k n o w n w i t h i n 1 /1 ,000 of a dec ibe l . O n t h e o t h e r h a n d , a r a n g e of on ly ± 0 . 0 5 dec ibe l needs to b e cove red .

T h e e x p e r i m e n t a l w o r k is c a r r i e d o n in a t e m p e r a t u r e -cont ro l led l a b o r a t o r y h o u s i n g t h e r e p e a t e r , t o g e t h e r w i t h a sect ion of c a b l e w o u n d u p on reels . T h u s t h e t r a n s ­mission of a typ ica l sect ion c a n b e m e a s u r e d b y c o n n e c t i n g its i n p u t a n d o u t p u t to a t r a n s m i s s i o n - m e a s u r i n g in s t ru ­m e n t in t h e l a b o r a t o r y .

T o fill t h e n e e d for m e a s u r e m e n t of t r ansmiss ion to t he r e q u i r e d precis ion t h e c i rcui t s h o w n on F i g u r e 1 was deve loped . A n osci l la tor a n d d e t e c t o r whose i m p e d a n c e s a r e equa l i zed t o m a t c h t h e coax ia l c a b l e i m p e d a n c e a r e a l t e rna te ly s t r a p p e d t o g e t h e r o r c o n n e c t e d t h r o u g h t h e r e p e a t e r sect ion. T h e v a r i a t i o n in t h e d e t e c t o r o u t p u t vol tage is a m e a s u r e of t h e inser t ion loss, o r ga in , of t h e u n k n o w n .

T h e difficulty t h a t has to be o v e r c o m e is t h a t in t h e t i m e it takes to swi tch b e t w e e n t h e u n k n o w n a n d t h e s t r a p a n d

X s

T h e transmiss ion of a r e p e a t e r sect ion cons i s t ing of 4 mi le s of coaxial c a b l e a n d a r e p e a t e r i n c l u d ­i n g e q u a l i z i n g n e t w o r k s w a s m e a s u r e d i n t h e laboratory . T h e m e a s u r i n g c ircuit w a s s w i t c h e d b e t w e e n t h e u n k n o w n a n d a short r e f e r e n c e s trap at a r a p i d rate . T h i s r e d u c e d t h e eflFects of l e v e l drifts i n t h e m e a s u r i n g osci l­lator ampli f ier a n d d e t e c t o r to a p o i n t w h e r e a n a c c u r a c y of =*= 0 .001 d e c i b e l o v e r a r a n g e of =t0.05 d e c i b e l w a s o b t a i n e d . T h e m e a s u r e ­m e n t s w e r e automat ica l ly r e c o r d e d i n p e n a n d ink o v e r a f r e q u e n c y r a n g e o f 0 .1 to 8.3 m e g a ­

cyc les .

to observe t he vo l t age v a r i a ­t ion , b o t h t h e osci l la tor level a n d t h e de t ec to r sensi t ivi ty m a y drift b y m o r e t h a n 0.001 dec ibe l .

T h e solut ion a d o p t e d is to swi tch a t t h e r a t e of 60 cycles. I t is found t h a t w i t h i n 1/60 of a second t h e to t a l

12 OF C O A X I A L

O S C I L L A T O R O S C I L L A T O R 0 TOO.1 db Ί

R E C O R D I N G P E N

drift in t h e c i rcui t is less t h a n 0.0002 dec ibe l .

I n a d d i t i o n t o be ing fast-ac t ing , t h e swi tch m u s t be a good t r ansmiss ion e l e m e n t . T h e m e r c u r y swi tch , see

F i g u r e 2, w h i c h is a modi f i ca t ion of a s t a n d a r d switch,^ m e e t s b o t h r e q u i r e m e n t s . T h e swi tch is enc losed in a cy l indr ica l shie ld s e p a r a t i n g it f rom t h e d r iv ing w i n d i n g . T h e leads a r e b r o u g h t o u t to coax ia l j a c k s in cy l indr ica l cavi t ies . T h e d i a m e t e r s of t h e cavi t ies a r e p r o p o r t i o n e d t o p r o v i d e a 7 5 - o h m i m p e d a n c e m a t c h i n g t h a t of t h e coaxia l cab le . N o t e t h e b u l g i n g ou t of t h e cav i ty a t t h e p o i n t w h e r e t h e m e r c u r y pool increases t h e d i a m e t e r of t h e i n n e r c o n d u c t o r . T h e c o m p l e t e r e l ay inc ludes t h r e e j a c k s i n t o w h i c h t h e u n k n o w n a n d t h e s t r a p m a y b e p lugged .

T h e ne t effect of p r o p e r l y d i s t r i b u t i n g t h e c a p a c i t a n c e a n d i n d u c t a n c e of t h e r e l ay is t h a t it p resen t s a negl ig ib le i m p e d a n c e i r r e g u l a r i t y a t all f requencies u p t o 80 m e g a ­cycles. I t i n t r o d u c e s i n t o a c i rcu i t a t r ansmiss ion loss caused by t h e res i s tance in t h e leads , w h i c h a r e m a d e of p e r m a l l o y . T h e loss rises f rom a b o u t 0.01 dec ibe l a t 1 m e g a c y c l e to 0.02 dec ibe l a t 10 megacyc le s . H o w e v e r , t h e losses for t h e t w o swi tch posi t ions a r e m a t c h e d t o b e t t e r t h a n

0.0002 dec ibe l u p t o 8 megacyc les . W h e n t h e swi tch is o p e n , its g a p

is b r i d g e d b y a d i r ec t c a p a c i t a n c e of a b o u t 0.3 m i c r o m i c r o f a r a d . T h e eflfect of th is c o u p l i n g is r e d u c e d t o a neg l ig ib le v a l u e b y us ing for a s t r a p 1 foot of coax ia l c a b l e . T h e c a p a c i t a n c e of t h e c a b l e forms t h e s h u n t a r m of a Τ n e t w o r k w h i c h a t t e n u a t e s t h e p i c k u p signal t h a t is fed t h r o u g h t h e c a p a c i t a n c e in t h e o p e n pos i t ion . As seen on F i g u r e 1,

AINDUCTION M O T O R

R E P E A T E R U N D E R T E S T

Figure 1, Block diagram of the repeater loop and measuring circuit

Full text of a conference paper presented at the AIEE Summer General Meeting, Atlantic City, N. J., June 15-19, 1953, and recommended for publica­tion by the AIEE Committee on Instruments and Measurements

Thaddeus Slonczewski is with Bell Telephone Labo­ratories, Inc., Murray Hill, N. J

346 Slonczewski—Precise Measurement of Repeater Transmission ELECTRICAL E N G I N E E R I N G

+ 0.03

Figure 2. Complete coaxial switch

t he de tec to r is followed b y e i the r a fixed or a v a r i a b l e a t ­t e n u a t o r w h i c h is a l t e r n a t e l y swi tched in a n d o u t as t h e u n k n o w n or t h e s t r a p is swi t ched . T h e rectif ied s ignal , w h i c h is of t h e form of a 60-cycle s q u a r e w a v e , is n u l l e d b y adjus t ing t h e v a r i a b l e a t t e n u a t o r . W h e n a nu l l o b t a i n s , t h e r e p e a t e r loop ga in equa l s t h e se t t ing of t h e a t t e n u a t o r .

T h e t i m i n g of t he swi tches is so a r r a n g e d t h a t wh i l e t h e swi tch ing t r ans ien t s a r e b e i n g d iss ipa ted , t h e ampl i f ie r t h a t follows t h e a t t e n u a t o r is d i s connec t ed f rom t h e c i r cu i t a n d its i n p u t vo l t age is he ld c o n s t a n t b y t h e c h a r g e on a capac i to r .

W i t h such a n a r r a n g e m e n t , n o n e of t h e c i rcu i t c o n ­s tan ts is cr i t ica l . Because a s q u a r e l aw rectif ier is used , a n add i t i ona l t w o to one a d v a n t a g e is o b t a i n e d so t h e y w o u l d h a v e to v a r y by 2 pe r cen t to p r o d u c e a n e r r o r of 0.001 decibel .

I n o r d e r to o b t a i n t h e results r a p i d l y in t h e fo rm of a p e n a n d ink r ecord , t he a t t e n u a t o r , w h i c h is of t h e fo rm of a po ten t i a l d iv ider , is m o t o r d r i ven , wh i l e t h e m o t o r is energ ized b y t h e ampl i f ier fol lowing t h e a t t e n u a t o r . T h u s t h e c i rcui t nul ls itself a u t o m a t i c a l l y . T h e p a p e r d r i v e a n d t h e oscil lator a r e l inked b y a s e r v o m e c h a n i s m w h i c h insures t h a t t h e f r equency scale fits a p r e p r i n t e d l i nea r gr id .2 A s a m p l e of t h e resul t o b t a i n e d is s h o w n on F i g u r e 3.

T h e a c c u r a c y of t h e i n s t r u m e n t was c h e c k e d by m e a s u r -

o I ζ

1.6 1.8 FREQUENCY - MEGACYCLES

Figure 3. Portion of a record

ing k n o w n l e n g t h s of coax ia l c a b l e whose a t t e n u a t i o n for l ong s ample s h a d b e e n m e a s u r e d b y o t h e r m e t h o d s . T h e la rges t d i s c r e p a n c y w a s f o u n d to b e 0 .0009 dec ibe l .

W h i l e d e v e l o p e d p r i m a r i l y for r e p e a t e r t ransmiss ion m e a s u r e m e n t , t h e c i r cu i t m a y b e used for i n t e r c o m p a r i s o n of a n y t w o 7 5 - o h m n e t w o r k s whose t ransmiss ions diff'er f rom o n e a n o t h e r b y less t h a n t h e r a n g e of t h e i n s t r u m e n t , ± 0 . 0 5 dec ibe l , a n d var ies b y less t h a n 15 decibels w h i c h is t h e a u t o m a t i c - v o l u m e - c o n t r o l r a n g e of t h e de t ec to r . T h u s m a n u f a c t u r i n g v a r i a t i o n s in a n e t w o r k m a y b e d e t e c t e d aga ins t a n e t w o r k s t a n d a r d .

R E F E R E N C E S

1. Mercury Contact Relays, J . T. L. Brown, C. E. Pollard. Electrical Engineering, volume 66, November 1947, pages 1106-09.

2. A Servo Drive for Heterodyne Oscillators, T. Slonczewski. Electrical Engineering volume 70, August 1951, page 683.

New Betameter Design for Coating Measurement

O r i g i n a l l y des igned b y I s o t o p e P r o d u c t s L t d . , to serve a specific n e e d in t h e p a p e r i n d u s t r y , t h e b e t a m e t e r evo lved in to t h e b e t a m a t i c to p r o v i d e c o n t i n u o u s p r o d u c t i o n con ­t rol . N o w a b e t a m e t e r i n s t r u m e n t h a s b e e n d e v e l o p e d to m e a s u r e t h e mass of adhes ives o n t a p e , th ickness of c a r b o n b lack on c a r b o n tissue, o r o t h e r c o a t i n g a p p l i c a t i o n s .

F o r c o a t i n g app l i ca t i ons , t w o m e a s u r e m e n t s of shee t thickness m u s t b e m a d e , o n e of t h e shee t m a s s before coa t ing , o n e after. T h e diflference is t h e mass of c o a t i n g app l i ed . I so tope P r o d u c t s ' n e w c o a t i n g g a u g e eflfectively p rov ides diflferential m e a s u r e m e n t of c o a t i n g a p p l i c a t i o n s w i t h o n e i n s t r u m e n t . W h e r e t h e b e t a m e t e r is b a l a n c e d b y a n i n t e r n a l set of d u p l i c a t e source a n d d e t e c t o r , t h e c o a t i n g g a u g e b a l a n c e s o n e m e a s u r i n g h e a d aga ins t t h e

o t h e r . B o t h d e t e c t o r h e a d s a r e c o n n e c t e d w i t h a n a m p l i ­fier w h i c h r eco rds t h e diflference in s ignals a n d re la tes t h e c o a t i n g m e a s u r e m e n t t o a r e c o r d e r o r con t ro l system.

I n t h e b e t a m e t e r t h e c u r r e n t flowing f rom t h e ion c h a m ­b e r d e t e c t o r is b a l a n c e d aga in s t a second c i rcu i t in t h e i n t e r n a l d e t e c t o r . B o t h c i rcu i t s a r e wel l i n su la t ed w i th in t h e i n s t r u m e n t a g a i n s t e x t e r n a l in te r fe rence . B u t t h e c o a t i n g g a u g e e m p l o y s t w o d e t e c t o r h e a d s w h i c h c a n b e u p to 20 feet a p a r t . T h e t w o h e a d s feed i n t o a n ampl i f ie r a t a c e n t r a l p o i n t . T o p r o v i d e s t ab l e o p e r a t i o n free from v i b r a t i o n a n d e lec t r ica l in t e r fe rence . I s o t o p e eng ineers e m p l o y e d a flexible low-noise c a b l e w h i c h car r ies t h e t iny c i rcui t s f rom d e t e c t o r h e a d s to ampl i f ie r w i t h o u t a n y possible d i s to r t ion .

A P R I L 1 9 5 4 Slonczewski—Precise Measurement of Repeater Transmission 347