[time-nuts] time-nuts Digest, Vol 196, Issue 31
lstoskopf at cox.net
lstoskopf at cox.net
Sun Nov 29 18:38:13 UTC 2020
Great fun to have the guys who were there share War Stories on how things were done. But I'm the kind of guy who buys some machine tool on eBay and then looks up the name on the WEB to see something about the guy that used it (angle block at GM, etc.) Thanks, N0UU
> On November 29, 2020 at 12:10 PM time-nuts-request at lists.febo.com wrote:
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> Today's Topics:
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> 1. Re: Voyager space probe question (jimlux)
> 2. Re: Voyager space probe question (jimlux)
>
>
> ----------------------------------------------------------------------
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> Message: 1
> Date: Sun, 29 Nov 2020 09:12:13 -0800
> From: jimlux <jimlux at earthlink.net>
> To: time-nuts at lists.febo.com
> Subject: Re: [time-nuts] Voyager space probe question
> Message-ID: <76b6e6e0-787c-60dd-429b-49f62bea8a24 at earthlink.net>
> Content-Type: text/plain; charset=utf-8; format=flowed
>
> On 11/29/20 3:53 AM, Hal Murray wrote:
> >
> > jimlux at earthlink.net 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)
>
>
>
> ------------------------------
>
> Message: 2
> Date: Sun, 29 Nov 2020 09:27:24 -0800
> From: jimlux <jimlux at earthlink.net>
> To: time-nuts at lists.febo.com
> Subject: Re: [time-nuts] Voyager space probe question
> Message-ID: <738ee526-8033-3b10-967b-cbc0012af03c at earthlink.net>
> Content-Type: text/plain; charset="utf-8"; Format="flowed"
>
> For those interested in a "typical" (hah!) TCXO spec for a space radio,
> generically similar to the Electra UHF radios on and orbiting Mars, it's
> attached.
>
> The excessively precise frequency (49.244..... MHz) is because it
> matched a particular channel assignment for S-band, and the idea was to
> have the PN code (which is about 3 MHz, proportional to the carrier) be
> exactly 16 samples long.
>
> This is in pre "we can trust an NCO/DDS" days. When it takes years to
> build your spacecraft, ordering a crystal with a 24 month lead time to
> get the frequency "just right" isn't considered a problem. Historically,
> the SDST used a VCXO with a crystal at the frequency at 8*f0, where f0
> is about 9.xx MHz, and multiplies up by 880 to the transmit frequency
> between 8.4-8.45 GHz.
>
> Today, we use 50 or 100 MHz oscillators (Electra uses 24 MHz, but it's
> an older design) and synthesize the carrier with a DDS feeding a PLL.
> For instance, the Iris cube-sat transponder uses a 50 MHz oscillator,
> and that drives a DDS running at 20 MHz, which is multiplied up in an
> integer N PLL to the carrier frequency.
>
> This is because the missions are shorter development time, and we don't
> want to have to know the frequency until after the radios are built (or
> at least, the oscillators are ordered). For Iris, there were 7 of them
> built for the Artemis-1 mission, and the frequencies are all over the
> space science X-band allocation.
>
> The SDST and older used a DRO as the microwave oscillator, and they just
> don't have the tuning range needed to cover 50-100 MHz tuning range (and
> lordy, we tried a bunch of techniques) - not do mention that DROs have
> noticeable microphonics because the physical cavity is part of the
> resonator.
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