[time-nuts] frequency reference for portable operation

[Bob Camp]

Hi

If you go with one of the better DOCXO's on eBay (spend the full $30 not $15) you should get something that will hold < 0.3  ppb for 48 hours. You would have to do a few things:

1) Keep it on power for a couple weeks ahead of time.
2) Keep it on power the whole weekend. 
3) Make sure it's always in the same orientation (base down or whatever)
4) Put it in something like a cooler to keep the drafts off of it
5) Regulate the supply and efc tightly.

You might have to buy three and sort them, but I suspect not. 

One ppb at 10 GHz would be 10 Hz. The carefully minded DOCXO should keep you within 3 Hz. 

Bob
 
On Mar 10, 2013, at 6:24 PM, Rex <rexa at sonic.net> wrote:

> 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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