Picosecond Time Difference Measurement System

Recently a t ime d i f fe rence measurement system was developed i n the Frequency & Time Standards Section o f the Nat ional Bureau o f Standards and independently a t the Nat ional Research Council which shows more than three orders of magnitude improvement over that which is cur ren t ly ava i lab le comnerc ia l l y . Cur ren t s ta te -o f the-ar t t ime d i f fe rence measurement devices have specif ied accuracies of about 1 nanosecond. Measurement p rec i s ion and potent ia l accuracy o f be t te r t han 1 p icosecond has been demonstrated i n t h i s new t ime d i f fe rence measuring device. This has s i g n i f i c a n t i m p l i c a t i o n s i n frequency and t ime metrology using: state-of-the-art frequency standards and c locks. A b r i e f r e p o r t was given on t h i s measurement system a t t h e PTTI Planning Meeting a t t h e Naval Research Laboratory on the 5 th o f December 1974. This paper wil give more d e t a i l e d c i r c u i t diagrams necessary t o b u i l d up such a measurement system. T h i s p a r t i c u l a r measurement system has t h e advantage tha t i t can measure t ime d i f fe rences w i th accurac i e s o f a few picoseconds and w i t h r e p e t i t i o n r a t e s ranging from a few m i l l i seconds to as slow a r e p e t i t i o n r a t e as would be desirable, thus expanding convenient measurement o f time domain s t a b i l i t i e s o f frequency and time standards over several decades w i t h o n l y one measurement system. The system i s a lso very amenable t o s e l f c a l i b r a t i o n and se l f -no i se ana lys i s . Spec i f i ca l l y , a f r a c t i o n a l f r e q u e n c y s t a b i l i t y o f a b o u t 10 l 6 was measured f o r t h e n o i s e o f t h i s measurement system a t a sample t i m e o f io3 S . I n t roduc t i on & Perspective When making measurements between a p a i r o f f r e quency standards or clocks, it i s o f t e n d e s i r a b l e t o have l e s s n o i s e i n t h e measurement system than the composite noise i n t h e p a i r o f standards being measured. This places str ingent requirements on measurement systems as the s ta te -o f the-ar t o f p rec is ion f requency & time standards has advanced t o i t s c u r r e n t l e v e l . [l! As wil be shown, perhaps one of the greatest areas of d i s p a r i t y between measurement system noise and the noise i n c u r r e n t s t a n d a r d s i s i n t h e a r e a o f t i m e d i f f e r e n c e measurements. Comnercial equipment can measure time d i f fe rences to about 10-los, bu t the t ime f l uc tua t i ons second t o second o f s t a t e o f t h e a r t s t a n d a r d s i s as good as 10-13s. The d i s p a r i t y i s u n f o r t u n a t e because i f t i m e d i f ferences between two standards could be measured w i t h adequate precision then one may a l so know the t ime f luctuat ions, the f requency d i f ferences, and the f re quency f l u c t u a t i o n s . I n f a c t , one can se t up an i n t e r es t i ng h ie ra rchy o f k inds o f measurement systems: 1 ) those that can measure time; x ( t ) ; 2) those tha t can measure changes i n t ime o r t ime f l uc tua t i ons 6x ( t ) ; 3) and 4 ) those that can measure changes i n frequency o r those t h a t can measure frequency, V ( y 7 (+vo ) / W O 1; f requency f luctuat ions , 6v (6y f 6v/v, ) . As depicted i n Table 1 i f a measurement system i s of s ta tus 1 i n th is h ie rarchy , i .e . , i t can measure t i m e , then t ime f luctuat ions, f requency and f requency f luctuat ions can be deduced. However, i f a measurement system i s on l y capable o f measur ing t ime f luctuat ions (Status 2 Table l ) , then time cannot be deduced, but frequency and f requency f luc tuat ions can. I f frequency i s being measured .(Status 3 Table l ) , then nei ther t ime nor t ime f l uc tua t i ons may be deduced w i t h f i d e : i t y because e s s e n t i a l l y a l l commercial frequency measuring devices have "dead time" (technology i s a t a p o i n t where t h a t may soon chan e wi th data processing speeds t h a t a r e now avai 1 able?. Dead t i m e i n a frequency measurement des t roys the oppor tun i ty o f in tegra t ing the f rac t iona l frequency t o g e t t o " t r u e " t i m e f l u c t 2 a t i o n s . O f course, i f frequency can be measured, t h e n t r i v i a l l y one may deduce the f requency f luc tuat ions. F ina l ly , i f a system can on ly measure f requency f luctuzt ions (Status 4 Table l), then nei ther t ime, nor t ime f luctuat ions, nor f requency can be deduced from the data. If t h e f r e q u e n c y s t a b i l i t y i s t h e p r i m a r y concern then one may be p e r f e c t l y happy t o employ such a measurement system, and s i m i l a r l y f o r t h e o t h e r s t a t u s e s i n t h i s measurement hierarchy. Obviously, if a measurement method o f S ta tus 1 could be employed w i th s ta te -o f the -a r t p rec i s ion , t h i s wou ld p rov ide the g r e a t e s t f l e x i b i l i t y i n d a t a p r o c e s s i n g . The dual mixer t ime d i f fe rence sys tem se t fo r th in th is paper i s purpor ted to be such a method. Before d iscuss ing th is method i n d e t a i l , we wil br ie f l y rev iew the o the r measurement method examples o f Table 1. The measurement method example o f S ta tus 2 i n Table 1 i s shown i n block diagram i n F igure 1 . This method as described elsewhere [2-61 has proved very use fu l in ana lyz ing the spec t ra l dens i ty , S ( f ) , o f t h e phase ( t i m e ) f l u c t u a t i o n s i n p r e c i s i o n o s c i l l a t o r s ; @ i s t h e phase i t . radians and f i s t h e F o u r i e r frequency. We use "phase ( t ime) " because they are l i n e a r l y and s imp ly re la ted by: cp where v. i s t h e o s c i l l a t o r ' s nominal carr ier f requency. The "keystone" i n al t h e s t a t e o f a r t measurement systems reviewed i n t h i s paper i s the low-noise Schot tky barr ier d iode mixer . The s ignal f rom an o s c i l l a t o r under t e s t i s f e d i n t o one p o r t o f a m i x e r . The s ignal f rom a r e f e r e n c e o s c i l l a t o r i s f e d i n t o t h e o t h e r p o r t o f t h i s m i x e r . The s igna ls a re i n quadra tu re , t h a t i s , t h e y a r e 90 degrees ou t o f phase so t h a t t h e average vol tage out of the mixer is nominal ly zero, and the instantaneous vol tage corresponds to phase f luc tua t ions ra ther than to the ampl i tude f luc tua t ions between the two s igna ls . The o u t p u t o f t h i s m i x e r i s fed through a low pass f i l t e r and them a m p l i f i e d i n a feedback loop, caus ing the vo l tage contro l led osc i l la tor ( re fe rence ) to be phase l o c k e d t o t h e t e s t o s c i l l a t o r . The gain i s ad jus ted to ob ta in a very loose phase lock condi t ion. For ' t imes shor ter than the a t tack t ime o f the loop a v o l t a g e f l u c t u a t i o n wil be p ropor t i ona l t o a phase o r t ime f l uc tua t i on wh ich i s equ iva len t to the cond i t i on where the osc i l la to rs a re " f ree Funn ing" ( the at tack t i m e i s the inverse o f 271 t imes the uni ty gain bandwidth o f the loop). In turn the output o f the low no ise amp l i f i e r may be fed t o a spectrum analyzer, for example, t o measure the Four ier components of the phase f l uc tua t i ons . A l te rna t i ve l y , t he ou tpu t of the lowno i se amp l i f i e r may be fed t o a vol tage to f requency converter which i n t u r n i s f e d t o a counter. A f r e quency f l u c t u a t i o n , 6v , as read on the counter i s p r o p e r t i o n a l t o a phasg ( t i m e ) f l u c t u a t i o n , 6vc &X(t ,T) a l lowing one t o also analyze the f luctuat ions