Hi I would not depend on the 10 MHz reference for my phase noise floor at microwaves. I'd do as Bert suggested and lock up a quiet 100 MHz oscillator to the 10 MHz. The "best of the best" 10 MHz OCXO isn't going to be able to beat a 100 MHz over a 100 Hz to >=10 KHz bandwidth. The 100 MHz likely will beat the 10 MHz by 10 to 20db.
Bob On Mar 11, 2013, at 4:25 AM, Rex <[email protected]> wrote: > 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 horrible. > > 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. > > -Rex > > > > On 3/10/2013 5:15 PM, Bob Camp wrote: >> 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 <[email protected]> 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 >>> >>> >>> > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
