[time-nuts] Method for comparing oscillators

Ulrich Bangert df6jb at ulrich-bangert.de
Mon Aug 3 15:31:00 UTC 2009


John,

I see you in the danger to confuse accuracy and stabilility. "Accuracy" of
an oscillator and "stability" of an oscillator are (albeit the fact that our
wishful thinking usually expects both from a good oscillator) two completely
different things that you should not mix against each other. 

Your oscilloscope method (without proper handling of phase ambiguities)
measures a compound of both properties and is not well suited for stability
measurements. You have to realize that one of the oscillators that you are
going to compare may be totally inaccurate so that you will see lots of
phase changes in time occuring. Nevertheless this inaccurate oscillator may
be perfectly stable running on its wrong frequency. Do you see the
difference?

Best regards
Ulrich Bangert

> -----Ursprungliche Nachricht-----
> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von John Green
> Gesendet: Montag, 3. August 2009 16:59
> An: time-nuts at febo.com
> Betreff: Re: [time-nuts] Method for comparing oscillators
> 
> 
> I have studied the dual mixer approach and the consensus is 
> that it is the most accurate method. However, it seems pretty 
> difficult to obtain that accuracy. I do have some DBMs with 
> IF response down to DC. I don't have  a 10811 but do have a 
> pretty good oscillator to use for the offset. The problem 
> comes in with the time interval counter. The only thing we 
> have is an old 5328A. I believe, at this time, DMTD is just 
> not possible for me to do. My oscilloscope method seems to 
> work pretty well. I can't produce graphs showing frequency 
> stability but that isn't a big deal for me. I just want to be 
> able to compare a Rb source to a GPSDO and look at several 
> OCXOs either stand alone or in equipment we have here. If I 
> figure correctly, an error of 1e-12 is 1 Hz every 27.7 hours 
> if comparing 2 10 MHz sources. I don't have to wait for a 
> full cycle to occur, I can see pretty small phase 
> differences. Let's say I can see a 10 degree change. That 
> would cut the observation time down to just over 3 quarters 
> of an hour. Not bad. Most OCXOs will move a lot more than 
> that so shorter times would work for them. I know from past 
> experience that this works pretty well for looking at warm up 
> performance. My first experiment will be the hardest. I am 
> going to check a couple of Rb sources against a Tbolt. I'll 
> let you know how this works out. Thanks for all the input. 
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