[time-nuts] frequency reference for portable operation
rexa at sonic.net
Mon Mar 11 04:25:03 EDT 2013
I do the stuff on your list that is easy. Controlling the environment --
orientation, ambient temp, etc. -- is not worth the extra effort. Much
of what I discussed is that perfection isn't necessary.
The quality of the oscillator probably matters a good bit, but recently
I am learning, don't trust word of mouth or "specifications". Over the
last few years I have accumulated many "decent" OCXOs. But I don't have
a base standard 10 MHz reference that I consider pristine
phase-noise-wise and until recently had no way that I trusted to make
any kind of phase noise measurements. Recently I got into a project
where I built a board that uses a LMX2541 chip to make 3600 MHz using a
10 MHz reference with a pretty wide loop bandwidth. The board multiplies
the 10 MHz reference up to a point where a good SA can see the phase
noise. Measuring that on my 8566 SA with John Miles software, I learned
a couple things.
First - My measurement setup doesn't give real accurate phase noise
measurements compared to passing my DUT and sources on to someone with
real quality instrumentation.
Second - Comparing my measurements to the very good equipment, it is
clear that my measurements give a close approximation to the good one's,
only not exact across 10 Hz to 10 KHz. But my measurements are good for
a qualitative feel within, say 5 dB, and certainly good for relative
comparison of the contribution from different 10 MHz reference sources.
So, I have bought a lot of 10 MHz OCXOs from eBay over the last few
years. The best phase noise baseline reference I have found so far is my
Z3805. I have lots of OCXOs in the 2x2x1.5 inch size. Many had good
specs pointed at by the listings or word of mouth. When I used them with
my board and SA most were pretty crappy compared to the Z3805. A couple
of the ones I bought were Morion MV89As. - supposedly good, but what I
saw didn't look very great. One of the best ones I have is a small
2x2x.75 inch Wenzel I bought a few years back. It has a custom part
number of 500-11935. But don't buy by name. I recently picked up a 1x1x3
Wenzel 10 MHz with sma output connector and its phase noise looks pretty
Some old Isotemps were decent, but not as good as the Z3805 and I
haven't measured some 10811s and 10554s I have in the back of my box
because they are harder to feed DC-wise.
My point is, I collected a lot of OCXOs that are not nearly as good as I
thought they would be. But all would probably make reasonable references
for frequency stability. Not sure if the level of not-great phase noise
from many of them would be noticeable if they were used to lock a good
10 GHz radio. If I was younger, I'd probably go for that experiment. --
In reality, don't know if I ever will.
On 3/10/2013 5:15 PM, Bob Camp wrote:
> 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.
> 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
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