[time-nuts] frequency reference for portable operation

Rex rexa at sonic.net
Sun Mar 10 18:24:22 EDT 2013

I agree with using an OCXO for amateur radio operation.

The main activity in the US is the 10 GHz and Up contest which takes 
place over two weekends, mid-August and mid-September. I've been active 
in most of them for the last 15 years. Full participation requires 
operating something like 12 hrs each of the two weekend days. 
Occasionally I have operated into the wee hours Saturday night. Some 
people go to a high location like a mountain top and stay there all 
weekend. Many rove, driving hundreds of miles, or some combination of 
the two strategies. Very few have AC power available so operate on 
batteries for the two days, though sometimes with the option of charging 
batteries between stops or by just running your engine or a generator. 
By contest rules, to count as a new contact, at least one of the two 
stations needs to move 10 miles or more from a previous location -- 
hence the rover strategy.

To reduce your levels of uncertainty in making contacts, two things 
matter, antenna pointing accuracy and frequency accuracy and stability. 
Being exactly on frequency is nice, but being off a few hundred Hz at 10 
GHz is usually in the radio passband and good enough. So in my 
experience a good OCXO is fine. It is accurate enough, very stable for 
many minutes of contact operation, has good phase noise and moderate 
power consumption. I think most people in the contest are using OCXOs. I 
never checked exact accuracy, but I think, even with a lot of driving 
and temperature extremes my rig stays within a few tens of Hz at 10 GHz. 
A few operators use rubidiums. To my thinking, the extra accuracy is not 
really needed and the extra power consumption is not worth going that 
way if you are running off batteries. Usually, the ones available tend 
to have a bit worse phase noise than a good OCXO too, though I'm not 
sure if enough worse to matter in real contacts.

I have thought about taking a rubidium along to power on occasionally 
and calibrate the OCXO but never found my OCXO frequency to be an issue 
so never bothered to take the rubidium. There are applications like 
microwave EME where very weak signals are extracted by post-processing 
the data of a long contact in the noise level. In that case rubidium 
accuracy is needed for very narrow bandwidth contacts.

You mentioned operating while driving. A few people have the omni 
antennas to do that and I have worked some of them. For that the 
frequency accuracy becomes moot. At freeway speeds the doppler shift at 
10 GHz is very significant in the audio range. Because of that, the 
mobile-while-moving contacts are usually made in FM mode with wide 
bandwidths and no need for very accurate frequency. That mode can't do 
the long distances of dishes and narrow SSB or CW but it has worked 
better than I would have expected. Also, the 10-mile rule tends to make 
the FM mobile less useful and it usually happens as an experiment while 
someone is driving home at the end.

One side note about doppler. Often several guys roam in small packs. To 
begin a contact, often one station will put up a steady carrier for the 
other end stations to find. Often the rovers are set up near a freeway 
and a station receiving near the guy sending steady carrier will hear 
whoops in the steady tone caused by doppler bouncing of the signal off 
the freeway traffic.

In my view, using GPS locked oscillators has the same disadvantage of 
power consumption as the rubidiums. If you are in one location (a 
mountain top, etc.) for long periods of time, it might work, especially 
if you have AC available, but for roving, with the many locations, I 
would think it would either not give you much accuracy or would cause a 
big operational time penalty for multiple surveys. I'm not aware of any 
roving operators around here using GPS (except for location, which 
almost everyone now uses to determine their operating location).

A few operators, get by with an only moderately stabilized frequency. 
This might be a "brick" oscillator with its so-so internal oven. It's 
better to get out than to stay home thinking about better options. The 
poor frequency control is usually on new operator's rigs and was more 
common 10 years back. If the frequency is fairly stable, then the 
drifting can be reined in by going out with other hams who have good 
frequency to calibrate from, or if you have a beacon in range that you 
can find to establish your offset.

So, yes, OCXO is the way most hams lock their mobile microwave rigs.

-Rex, KK6MK

On 3/10/2013 7:23 AM, Jim Lux 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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