Hi
Thanks for the replies, and sorry for not explaining better.
The GPSDO main purpose is to
* provide a (factory) calibration for the 0.14ppm on-board oscillator and
* ensure that the AE20401 frequency drift measurements are measured to a much better accuracy than the 0.14ppm oscillator that I want to measure the drift of
Keeping it all on 20MHz seems less of a hack to me than doubling a 10MHz, but then I'm not used to frequency doublers.
1ppm is not difficult, but it still seems like overkill for a sports timer.
For my counter I really like the AE20401. Unfortunately its self calibration is only to within 1ppm, using an external 10MHz source like a GPSDO.
That seems like a poor choice, since you hardly need a GHz counter to develop a sports timer. Instead, get a high resolution reciprocal counter like an HP 5335A or Philips PM6654.
I have an old HP 5335A but there is no way to log frequency drift over time etc with that unless I get a GPIB PC interface, LabView etc. The AE20401 is around €100 and has logging software and USB interface.
But it has an internal 20MHz oscillator. So I was thinking of buying a 20MHz GPSDO and route the signal into the AE20401 instead of its regular oscillator. (Possibly with a buffer/level shifter in-between.) This should make it perfectly calibrated, always....right?
10MHz standard + simple doubler would be easier.
2) the idea of self-calibration with a D-type
Digital mixers have quite a bit of jitter. It would be much better to use a GPS 1pps signal as a gate for calibration.
That is certainly an option as well. Although I understood from another thread on here that the 1ppm duty cycle is extremely low (or maybe it was high?) - anyway the point being that the pulse to be captured needs to be coupled onto the device very well with a high bandwidth connection (i.e. no stray cap/inductance), as otherwise there will surely be a problem with repeatability between pulses? I'm thinking that the timer's 20MHz oscillator will be probed by an external board containing a D-type, and a cable from the 20MHz GPSDO. It will feed the mixed signal back to the timer's CPU for it to self-calibrate by adjusting SPI pot. Even if the 20MHz signal has poor slew rate it shouldn't matter as long as the D-type triggers on every pulse. If the 1pps pulse was used a poor slew rate would introduce larger errors, I believe. But I can try both methods
GPS isn't perfect. TvB has measurements of different GPSDOs:
http://www.leapsecond.com/pages/gpsdo/
Thanks, I'll check that out!
but if your frequency counter is only 8 digits or so, and the TCVCXO instability similar or worse, maybe it doesn't make a difference.
If the 10MHz REF input on your counter is any good you should use it and not bother with the 20MHz hack.
The AE20401 doesn't have a REF input, only a self-calibration. The best oscillator option it has has 1ppm, which is worse than what I'm trying to measure and calibrate. So because the counter has a 20MHz internal oscillator I figured I could kill a few birds with one stone.
If you feed the same 10MHz from a passive splitter to both REF and CH1 input of your counter and collect gap-free data (hopefully it does Pi-counting) you can evaluate the noise-floor of the counter. If it would significantly improve by a 20MHz hack then the REF-input is really poorly designed...
looking for a zero beat-frequency between REF and TCVCXO should work. How do you know if you are below or above? Maybe always approach from the same direction?
Yes, I was thinking to go from one direction.
think real hard about your spec "<< 1ppm" and the TCVCXO you've chosen. Is it realistic to hold calibration within spec for any length of time and over temperature/voltage etc. variations? Good OCXO's age ~few ppb/day.
That is why I want a GPSDO for absolute reference in the lab/factory, as there should be no ageing. The product (timer) will have a VCTCXO, which will of course age, and is allowed to drift/age a few ppm/year, and short term drift 1ppm.