[time-nuts] frequency reference for portable operation

[Bob Camp]

Hi

Remember - OCXO's are going to be acceleration sensitive. As you bump about on back roads, the oscillator is likely moving around by a few ppb. If you are after a hertz at 10 GHz, that's a lot. 

Your GPS will be off by a fairly predictable amount based on it's idea of it's location. If you have a 3M position accuracy, then you will likely have 10 ns of time error. With a 10 second loop, you are right back to a ppb. 

A good Rb should be able to hit a sub ppb accuracy at 10 minutes.

No perfect answer. 

Bob



On Mar 10, 2013, at 10:23 AM, Jim Lux <jimlux at earthlink.net> wrote:

> Asking here on behalf of a friend..
> 
> With respect to portable amateur microwave operation.. you want good close in phase noise (so you can use narrow band filters) AND good frequency accuracy (so you can find the signal)>
> 
> the typical operation is "drive somewhere, operate a bit, drive somewhere operate a bit" repeated (contacts from different grid squares/peaks/what haveyou"
> 
> My instinct is that this is an application for a nice quiet OCXO on a battery.  Adjust the frequency before you set out against a good reference and just go from there.
> 
> Surplus Rb references are apparently also popular, but I think they keep those on battery too (that is, you need to be ready to go 10 minutes after arriving, and I don't know that a Rb is "settled in" that quickly).
> 
> So the question from my friend was with reference to GPS disciplined oscillators.  Would that do any better?  I'm used to GPSDOs in a fixed location where you have time to do long term averaging.
> 
> And what about truly mobile operation (there are folks in the SF bay area apparently doing 10GHz mobile ops.. slotted WG radiator on the roof of the car, etc.)
> 
> What sort of 1pps timing accuracy do you get from a GPS "on the move". I assume it would have the usual 10ns sort of uncertainty (in that the mfr specs don't say "only with the antenna fixed in one place for N hours").  10ns is only 1E-8 of a second. Presumably one can average a bit over many pps ticks.
> 
> 
> I've got a bunch of Wenzel Streamline units, and they typically do 1E-10/day aging and 1E-9 over temp.  Assuming the temperature doesn't vary a "lot", seems like the OCXO is "better" than the GPS, at least in a 1-2 day time frame. (and, of course, isn't that just what a GPSDO is, in holdover mode, anyway)
> 
> The Rb is good to 1E-11 over the short run (assuming it's been "calibrated" recently) but I notice that the PRS10 data sheet says 7 minutes to 1E-9, so in the "non continuously powered" mode of operation, it's not all that wonderful.
> 
> 
> The Rb is definitely higher powered.. The PRS10 is 2+ amps at 28V to start, and 0.6 to run.  15-16 Watts is a lot to keep on a battery. (Assume you run off a pair of 7Ah 12V batteries.. that gives you 10-12 hours).
> 
> The Wenzel is a couple watts (after a 5W warmup).  The GPS is a LOT lower power. The Garmin GPS 18x is 0.45W, of course the 1pps on that receiver is only specified to 1 microsecond.. A moto Oncore UT is a bit less than a watt and claims <100ns (with SA.. showing the age of the datasheet I have).
> 
> 
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