[time-nuts] Basic question regarding comparing two frequencies

[J. Forster]

There is a cute way to use a scope.

It requires a power splittere, a quadrature hybrid, and two mixers (all
appropriate for the frequencies you are comparing), and an X-Y scope.
Mini-Circuits sells appropriate parts. The stuff is hooked up like this:


              X Axis
       S       |       H
       P ---- MIX ---- Y
REF 1--L               B -- REF2
       I ---- MIX ---- R
       T       |       I
              Y Axis   D

The 'scope display will be roughly a circle if the frequencies are a bit
different and the spot will go around CW or CCW depending on which Ref is
higher.

-John

=============


> 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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