Long winded answer to your question: I'm sort of an "Advanced beginner" time-nut or maybe a bit beyond that.
In relation to your question: It really depends on your goals. The following is a high level overview of your options, and is intentionally lacking and/or simplifying some details. There are lots of twists and turns one can go down on each of these options. And there's a few spots which I'm making generalizations which may or may not be exact. In the way of "precision" 10Mhz clock sources, I have 2 thunderbolts, a PRS-10 rubidium oscillator, and a BG7TBL GPSDO. Oh and a couple/few OCXO's which are undisciplined. I want a cesium clock but haven't happened to end up with one yet. You're somewhat familiar with the thunderbolts, let me compare the others first. The OCXO's are undisciplined, but aren't that bad of a 10Mhz reference. Especially if you can trim them to a good quality reference - such as your thunderbolt. Depending on your needs, this might be good enough for you. Generally, once the OCXO is warmed up you can adjust it to match the reference, then as long as it isn't shut off it should stay very close to the same frequency. Of course it will drift over time and perhaps as a result of other external forces (temperature, humidity, voltage, vibration, etc). Of course you can then re-trim it to the right frequency. How much and how often you need to do this will depend on the oscillator. A surplus OCXO can be had for well under $100. A good quality pre-aged surplus one is likely better than a new out of the box one (even of the same time). Other members of the list have much more experience with which are the best, but I personally have found that the oscilloquartz ones are typically acceptable for my needs. A rubidium oscillator is like an OCXO on steroids. But if you think of it as a very very very low drift OCXO then you've probably got the right idea. For comparison a good OCXO will drift in a day about the same as a rubidium will drift in a year. You can often get a Stanford PRS10 in good condition for around $250 or so. The challenge is that any atomic clock has a certain limited life due to the physics package "wearing out" in some form or another, so it's a bit higher risk than just buying a good quality OCXO. Before I get to the GPSDO's let me mention about the relative ease of trimming the above two types of references. If you have a two channel oscilloscope, it is rather easy to trim these. You plug the reference clock into one channel, set the scope to trigger off of that channel (so the waveform is stable). Then you plug the clock you are trimming into the second channel. You then adjust or 'trim' the oscillator until the waveforms do not move (or beat) in relation to each other. If you don't have a two channel oscilloscope, there are other ways to do this as well. Now back to the GPSDO's. The thunderbolt you have is a OCXO 'disciplined' by the GPS's 1PPS output. It's the same as trimming the OCXO on a continual basis such that the OCXO output always has 10 million cycles per 1 second. There are other GPSDO's that use other types of oscillators as well (rubidium, vcxo, etc). The method of trimming the oscillator varies from GPSDO to GPSDO, but the effect is typically the same: that is to adjust the 10Mhz output to something close to 10Mhz. As I mentioned I have a trimble thunderbolt and a BG7TBL GPSDO. The BG7TBL GPSDO is a GPSDO designed by a Chinese ham, and each edition seems to be a little different, typically using a u-blox GPS receiver and seemingly whatever surplus OCXO they can get their hands on. Mine happens to have a russian OCXO in it. The quality of these seem to be rather good, although there is a bug which affects mine and other earlier ones which cause the long-term frequency to be very slightly off. The very slightly off means it runs at ~9.999999999 Mhz instead of 10Mhz. I beleive that There are other designs out there as well from other vendors. A search on a certain popular auction site for "GPSDO" reveals a lot of options. I've had discussions with a couple of people who have compared several of these and apparently the consensus is that they're typically all decent for general use, and some are better than others, but none that they've tried are especially bad. Certainly they should be good enough for a reference clock for most people. If I didn't have a collection of oscillators already I'd probably pick up 1-2 of these of varying types. Maybe a thunderbolt and one of these. One final caveat, which may or may not be applicable to you: In some applications, certain types of noise can be a problem. Depending on how you're using the clock you may actually find that a disciplined oscillator is not the right solution. Generally I'm timestamping events and I'm more interested in stability instead of precision. So, even if the frequency of my clock source is off by 1-2% it isn't a big deal, as long as it stays off by exactly the same amount (within some level of precision of course). But if my clock frequency is always changing, then I'm introducing noise into my measurements that I can't adjust out. Depending on the quality of the disciplining method, the very act of disciplining a clock can introduce more noise than if you just let it freerun. Hope my rambling helps. On Mon, Jul 8, 2019 at 2:12 PM Frank O'Donnell <[email protected]> wrote: > Hi, I'm extremely new to the topics covered on this list, but am finding > reading the message traffic to be highly educational. I have some > questions that I'd like to put out, and apologize in advance for them > being at a greatly simpler level than most of the discussion here. > > I came to time-nuts by way of an interest in frequency measurement as a > radio ham. To supply a 10 MHz signal to my gear I bought a used Trimble > Thunderbolt. I always like to have a backup, so am thinking of obtaining > another device (either a GPSDO or something else that can provide a 10 > MHz signal). For a beginner, is there something else that offers > accuracy/precision at least as good as a Thunderbolt and doesn't cost > more than a few hundred dollars at most that you might recommend? > > I also have some questions about using the Trimble, but will put those > in a separate message. > > Thanks, > > Frank O'Donnell > > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > -- - Forrest _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
