Bruce Griffiths wrote: > John Miles wrote: > >>> Probably impractical as the sampling noise is so high that it will >>> require tens of thousands of measurements to get the noise down to the >>> required level and meanwhile the temperature etc will have changed >>> significantly. >>> >>> >> Heh, that's a good point, you can't just sit there and trigger the thing >> 10,000 times per second for 24 hours... >> >> >> >>> Also the relatively low ADC resolution and nonlinearities >>> will tend to limit the effective resolution in determining the zero >>> crossing locations. >>> >>> >> I stand by my assertion, though, that all you're going to get with high >> sampling precision is a lot of well-characterized random noise determined >> almost solely by outside factors. It's an interesting question, but one >> that can't be answered without knowing more about the symptom that started >> the whole thread. >> >> -- john, KE5FX >> >> >> > If the oscillator ADEV were to follow a 1E-12/Tau law into the microsecond > region then at 10us the ADEV will be around 1E-7. > > A measurement system noise sampling jitter of a little less than a > picosecond should suffice to characterise this. > > It is unlikely that a GPS TCXO has that low a noise it may be perhaps > 10x this. > Thus measurements of the startup transient characteristics to the 1ppb > level in 10us windows are perhaps somewhat futile. > > The 16Hz variation in a ~ 200kHz signal is equivalent to noise of about > 4E-7 at 40MHz with a 10us gate. > > A substantial part of this may well be due to the TCXO and local > oscillator phase noise with some contribution due to amplified mixer noise. > > More detail on the end users measurement techniques are in order to see > exactly how and what they are measuring is required. > If in fact they want to characterise the zero crossing jitter more > accurately during startup subpicosecond (but not 10 femtosecond) system > noise can probably be justified. > This is easy to achieve with a high resolution (14-16 bit) ADC directly > sampling the 40MHz signal. > > 100x smaller measurement system noise is perhaps only justified if one > were observing the startup transients of a much higher Q oscillator > however these would last much longer so in practice perhaps it isnt > justified. > > Bruce > >
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