Frequency Synthesizer for Modulating the Frequency and Time Scales of Crystal Clocks on Orbiting Satellites

Abstract

Silver Spring, Maryland summary C ' r y s t a l o s c i l l a t o r s u s e d i n o r b i t i n g satellites to provide a working time scale are s u b j e c t t o drifts i n frequency that cannot be predicted more thsrl a few days a t a time. A frequency synthesizer has been developed that w i l l translate t h e i n p u t f r e q u e n c y t o the output frequency according t o precise mathematical formulation. This device provides the means t o conv e r t t h e s a t e l l i t e c r y s t a l o s c i l l a t o r t o a prescribed frequency that w i l l make t h e s a t e l l i t e c l o c k agree i n r a t e t o c l o c k s on t h e ground. A frequency tuning span of 65 parts per m i l l i o n i s covered i n 38 m i l l i o n d i s c r e t e s t e p s . The movemen't a c r o s s t h i s t u n i n g range i n the s a t e l l i t e is c o n t r o l l a b l e from t h e ground e i t h e r t o s p e c i f i c s t e p s o r t o a rate of progression through the steps. Short term i n s t a b i l i t i e s a t t r i b u t e d t o t h e s y n t h e s i z e r have a one sigma deviat i o n o f 1 p a r t i n IOU averaged over 1 second. Technical Description Block Diwram The b a s i c approach t o t h e s y n t h e s i z e r is shown i n t h e s i m p l i f i e d b l o c k d i a g r a m o f F i g u r e 1. The divide by A is a counter that overflows and resets when it reaches a number s t o r e d i n a r e g i s t e r . The a c t i o n o f overflowing generates a d i g i t a l signal whose fundamental frequency is fo/A. T h i s s i g n a l is combined i n a d i g i t a l v o l t a g e c o n t r o l l e d o s c i l l a t o r ( s i n g l e s i d e band mixer) t o generate a d i g i t a l wave form whose sum frequency is:This waveform is divided by B as above and i n j e c t e d i n t o a v e r y c a r e f u l l y b a l a n c e d a n a l o g s i n g l e s i d e band mixer.The output frequency is then:where B i s the course frequency adjustment and A i s the fine frequency adjustment.The r e s o l u t i o n o f t h e device over i t s operating range is shown i n F i g u r e 2. Having i d e n t i f i e d t h e b a s i c s t r u c t u r e , t h e n e x t l e v e l o f d e t a i l is shown i n t h e n e x t b l o c k diagram, Figure 3. Here w e see t h e A and B counters and the two s i n g l e s i d e band mixers. These counters are cont r o l l e d b y t h e c o n t e n t s o f A and B h o l d i n g r e s i s t o r s . By ground command, Command Data Words are i n j e c t e d i n t o t h e Command Data Word holding register. This word c o n t a i n s t h e d a t a f o r a h o l d i n g r e g i s t e r and t h e i d e n t i f i e r as t o where it belongs; A o r B. There is another means provided for control, through the onboard computer interface. Software programs and d a t a i n j e c t e d from t h e ground, permits t n e computer t o r a i s e o r l o w e r t h e v a l u e o f d i v i d e r number A. Provision has been made f o r t h e n m b e r A to operate between two limits 6300 and 13469. liken e i t h e r limit number i...

Nova satellite time experiment

“…The satellite clocks and ground station clocks contribute to the error. A typical high-performance Cs standard in a laboratory environment has been shown to be stable to 4 parts in 10 •n per day; the satellite oscillator in ground tests proved to be predictable to 3 parts in 10 •3 per day when the drift is compensated, as shown in Figure 5 [Rueger and Bates, 1974;Taylor, 1974]. Therefore the Nova system using Cs ground standards should be able to hold 30 ns on daily comparisons limited by the satellite oscillator performante.…”