[time-nuts] Basic question regarding comparing two frequencies

Steve Rooke sar10538 at gmail.com
Sun Jul 25 08:12:45 EDT 2010


You can always use the ready, fire, aim, approach. Make a tiny
adjustment and see if it makes the Lissajous figure move faster, so
you know your going the wrong way, or slower, and then you'll know
your on the correct path.

Steve

On 26/07/2010, J. L. Trantham <jltran at att.net> wrote:
> The only problem with the lissajous approach is you can't tell if your OCXO
> is high or low relative to the reference figure.  This is resolved by
> triggering the scope with the reference.  If the trace is moving left to
> right, the OCXO is high, and vice versa, IIRC  (I just woke up).  Otherwise,
> it works fine for fine adjustments aligning an unknown oscillator to match a
> known reference.
>
> Joe
>
> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
> Behalf Of Geoffrey Smith
> Sent: Sunday, July 25, 2010 2:58 AM
> To: 'Discussion of precise time and frequency measurement'
> Subject: Re: [time-nuts] Basic question regarding comparing two frequencies
>
> Mark,
>
> I have been measuring the difference between a GPSDO and a HP 10811A TCXO.
> To avoid any triggering issues I put the CRO into XY mode.  The resulting
> Lissajous curve figure flips at the rate of the frequency difference good
> old Wikipedia has the maths.
>
> Just sit and watch the Lissajous and you can adjust the TCXO to have the not
> flip and set accuracies in small fractions of a Hertz.
>
> Geoff
> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
> Behalf Of Mark Spencer
> Sent: Sunday, 25 July 2010 3:29 AM
> To: time-nuts at febo.com
> Subject: [time-nuts] Basic question regarding comparing two frequencies
>
> Hello:
>
> Just for grins I decided to compare the frquency from my GPSDO to the time
> base
> in my 5328A counter.
>
>
> I connected the 10 mhz time base from the counter to channel A of my 100 Mhz
>
> scope, fed the 10 mhz signal from my GPSDO into Channel B and with a T
> adaptor
> also fed this signal into the input of the counter.    I scope to trigger
> from
> Channel B.
>
>
> The drift betwen the two signals on the scope seems to match the error in
> the
> displayed frquency on the counter.  (ie. if the counter shows 9999.9998 it
> takes
> approx 5 seconds for the the wave form on channel A to slip a full cycle
> realitve to channel B.)
>
>
> Is this a reasonable approach or is there a better way to compare two
> frequencies using a scope ?
>
> Best regards
> Mark Spencer
>
>
>
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-- 
Steve Rooke - ZL3TUV & G8KVD
The only reason for time is so that everything doesn't happen at once.
- Einstein




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