I accidentally only replied to an off-list reply with the following information. It was also meant to go to the entire list:
Let me clarify here a bit: The application is: 1) In an unconditioned/semi-conditioned space. The ambient temperature can slew 20*C easily over the holdover period. (Think sun hitting an outdoor enclosure first thing in the morning - the ambient temperature tends to rise quickly). The overall temperature range is -40 to +60C, although those edges are rarely hit, and usually only when fans/heaters fail (more typical is 0-50C). 2) I need to stay within a 'few' microseconds of UTC alignment. No it isn't specced better than that. 4-5 uS is probably a good target. However, 10uS is probably ok and 30uS has been shown to not cause a lot of problems. Beyond that things get bad fairly quickly. Nominal 1PPS UTC is coming from a GPS receiver. The application is ok with a uS or so of cycle-to-cycle jitter. 3) 10 minutes would be a good target. Longer is better, like in any holdover. My basic math has been: 5 uS over 10 Minutes is ~8ppb. (5uS/600). If I can tolerate worst-case of 30uS, then I can get away with 50ppb. The implementation idea today is to clock a FPGA with an appropriately stable clock, then do either a 1Hz PLL or some other similar arrangement in the FPGA designed to track the GPS 1PPS when it's operational, or switch to holdover when it's not. I don't particularly see a good reason to discipline the oscillator when I can effectively measure it's frequency in the FPGA - If I know the measured frequency of the oscillator is 10.0000033425 Mhz, I can output the 1PPS every 10000003.3425 cycles, with each 1PPS aligned to the next clock edge. The jitter introduced by this method isn't going to be a problem in this application. Or put differently: I'm not steering the oscillator, I'm steering the divider in the FPGA. So my spec when I get right down to it is that I could tolerate a clock source which is fine with a worst-case drift of 50ppb over 20*C, but I would be more comfortable with something close to 10ppb. I note that Abracom and probably others make TCXO's which are rated at 50ppb over their entire temperature range, and the charts show that I probably can expect better than that over almost any given fairly narrow temperature range - and the lower power and ease of use are very much plusses. But, they cost more than an low-end OCXO. The main disadvantage of the OCXO's is the higher power needs. And my tendency to say 'well, I've already got a OCXO in the circuit, maybe we should go up to the expensive parts... I apologize for the softness of the specs here and (even worse) in my original message. Right now I'm sort of just playing with the 'is this doable for a reasonable cost, and what are the cost/benefit tradeoffs' game. And trying to get something I can play with and throw into the "environmental chamber" (aka converted old freezer) for testing. On Wed, Sep 23, 2020 at 6:20 PM Forrest Christian (List Account) < li...@packetflux.com> wrote: > I'm hoping some of the list are familiar with the newer > currently-available oscillators available from the distributors. > > Some background: > > I'm starting to play with a short-term 1PPS holdover application, mainly > to cover up brief GPS signal loss events (a few minutes at most). > > Since I may at some point want to make numerous copies of this, I'd rather > start with a new oscillator as opposed to digging through the junkbox or > buying surplus. Lower cost (<$100 ideal, lower is even better) and power > consumption is good. My short-term stability requirements indicate to me > that I'm probably looking for an OCXO instead of a TCXO, although it looks > like some of the best TCXO's out there are in the range which will work in > most cases (50ppb over temperature (or better)). > > I suspect I'm going to handle the holdover in a FPGA. As such, I don't > think I'm going to bother trying to voltage control the oscillator. Seems > like it will make the overall circuit simpler and I don't have to worry > about a whole bunch of control circuit temperature compensation. > > I've dug through enough datasheets at this point, my eyes are glazing > over. I also know that the spec sheets are only part of the story when it > comes to oscillators. I also have found over the years that often the part > I'm looking for (that 20ppb Temperature-stable TCXO for a reasonable price) > exists if you know the vendor to look at... > > So any specific recommendations or pointers toward a brand/type would be > appreciated. > > -- > - Forrest > -- - Forrest _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.