[time-nuts] Basic question regarding comparing two frequencies

J. L. Trantham jltran at att.net
Sun Jul 25 08:04:47 EDT 2010


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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