On 11/29/20 3:53 AM, Hal Murray wrote:
[email protected] said:
I don't know about Voyager, but on the SDST and later, the auxosc is a TCXO
with fair to middling performance. A good part of the "art" of communicating
with an "old" spacecraft is knowing/predicting/guessing where the "best lock
frequency" is, because DSN sweeps very slowly through that frequency hoping
the spacexraft receiver acquires it in a <10Hz BW filter. If you don't see
the transmitter frequency (in turnaround mode) jump when acquisition occurs,
you widen up the sweep, or slow it down, or turn up the exciter power. When
round trip time is measured in hours, this is a slow process<grin>
Neat. Thanks.
How often do you contact old spacecraft?
That depends on a lot of factors - one of which is the budget <grin> -
not necessarily for the DSN time, but for the people to build the
commands and handle the telemetry.
Once a week or once a month.
I think I remember a news story about Ed Stone (who's the Project
Scientist on Voyager, as well as former director of JPL, now a Caltech
professor) checking on the data periodically.
How much does the TCXO drift due to
aging since the last time you talked to it?
Not much, by now <grin> That's a well aged crystal.
I happen to have the data for a 49.244 MHz TCXO clock from Vectron that
we used on a radio for the SCaN Testbed on ISS (which has now been
discarded to burn up in the atmosphere).
It's spec was <2ppm first year, <10 ppm for 10 years (obviously, they
didn't test it for 10 years!) at 70C
The actual aging rate was 0.024 ppm first year, epected to be no more
than 0.030ppm for 10 years.
How much does the temperature
change?
Depends on the spacecraft - for Voyager, I'll bet it stays within a
couple degrees. Sunlight is tiny at that distance, and they're not
changing operating modes (power dissipation) all that much. On my LEO
spacecraft, it was about 10-15 degrees every orbit as we went in and out
of eclipse.
What else changes that influences the TCXO frequency?
Radiation has a small effect. Power supply voltage (called "pushing" in
the spec sheet, about 0.01-0.03 ppm for a 0.25V difference on my Vectron
parts). That could be due to a change with temperature or radiation
dose (changes in the reference voltage in the regulator due to TID).
Load impedance changes the frequency (pulling), too.
If you are transmitting on the right frequency, how long does it take the
receiver to lock up?
Seconds - if you're right on frequency it's just how long it takes the
phase to change to match the incoming signal, and that's mostly about
the tracking loop time constant.
Newer transponders have rapid acquisition circuitry which change the
loop filter to be narrower after carrier lock is acquired. This, of
course, depends on the SNR.
In most cases, one starts out transmitting pure carrier, waits long
enough to be sure that the receiver has locked, then go into a data
mode, which moves power from the carrier to the sidebands (if not
entirely suppressing it), so changing the loop bandwidth after
acquisition is a useful thing.
In precision two way ranging, the locked oscillator in the receiver is
used to generate the downlink signal as well, so having very narrow loop
bandwidth helps with having the downlink signal have narrow bandwidth,
too. The uplink signal from the ground is derived from a maser.
In two way mode, the auxosc is ignored (although in modern designs, the
auxosc is also the processor clock, so it keeps running)
_______________________________________________
time-nuts mailing list -- [email protected]
To unsubscribe, go to
http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
