[time-nuts] Low temperature coefficient capacitors for DMTD
Lux, Jim (337C)
james.p.lux at jpl.nasa.gov
Tue Jan 26 01:18:54 UTC 2010
Finding an old 1992 MTT paper by Mysoor, et al., that describes the design of the Deep Space Transponder (DST), which is what's in Voyager..
There's a second order lowpass filter in the tracking PLL. The two time constants are 3556 seconds and 0.0556 seconds (time constant of the pole, timeconstant of the zero, respectively), the loop gain is 2.2E7 1/sec.
The loop bandwidth is about 9 Hz for weak signals and 90 Hz for strong signals (50dB above threshold).
There's a fair amount of information on the tradeoffs made for phase noise and Allan deviation in the papers. The Allan deviation was predicted to be 2.5E-11 for 0.01 second, 2.6E-13 for 1 second, and 2.5E-15 for 1000 seconds. (that's the incremental deviation, assuming a perfect input signal)
This is, of course, an all analog transponder design. These days, we do all the loop tracking in the digital domain with a sampled signal and regenerate the carrier with an NCO. Don't have to worry about those 75 microfarad capacitors in the loop anymore (but, on the other hand, we have other things to worry about now...<grin>) We can probably do about an order of magnitude better on ADEV these days, with a digital radio, and maybe 2 orders of magnitude better with an all analog design.
James Lux, P.E.
Task Manager, SOMD Software Defined Radios
Flight Communications Systems Section
Jet Propulsion Laboratory
4800 Oak Grove Drive, Mail Stop 161-213
Pasadena, CA, 91109
> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On Behalf Of Lux, Jim (337C)
> Sent: Monday, January 25, 2010 4:55 PM
> > Why would JPL study obscure production details of polycarbonate
> > film capacitors in 1990, if they fell of the market six years earlier ?
> Oh my.. a bit of research in Xplore finds that all the gory details were in the very section I work in
> at JPL (we build radios for deep space missions)
> Well, the original incident triggering the study was in 1978 on Voyager 2, when they weren't able to
> get an uplink lock to the spacecraft. The telemetry from the spacecraft indicated that the loop
> filter in the carrier tracking loop was having problems, most likely from leakage across the 75 uF
> capacitor. They figured out a workaround to estimate the best lock frequency using temperature and
> Doppler estimates, and Voyager continues on its happy way out of the solar system. (FWIW, the carrier
> tracking loop has a bandwidth of a few Hz, I think.)
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