[time-nuts] Fast frequency counting question

Peter Vince pvince at theiet.org
Mon May 5 08:36:26 EDT 2008

Forgive me if this is nonsense, but what about mixing with a pair of 
quadrature signals from another continuously running 40 MHz signal, 
and using the low-pass filtered outputs to directly show the relative 
phase, and then grab that on a sampling 'scope?


		Peter Vince (G8ZZR, London, England)

>Hi all,
>We have a problem at work where one of our engineers needs to measure
>the start-up chirp in an oscillator destined for a GPS application.
>We are looking for ppb level frequency resolution with sample periods
>around 10 - 20us (yes, that's us, not ms!). The chirp is all over well
>inside 1ms.
>The measurement is on a 40MHz oscillator, which is mixed down to about
>198kHz to improve resolution. We can count the down-converted frequency
>in a conventional manner with no problem. The problem is of course that
>at 198kHz the period is ~5us so even an interpolating counter will
>Preliminary tests with a Pendulum CNT90 counter give results that
>deviate +/- 16Hz when the gate time is reduced to 10us. We are using
>GPIB to capture multiple repeated samples, and trigger the counter when
>the oscillator is powered up. We also have an Agilent 53181A counter
>Any suggestions (a) how to make the measurement; (b) what is causing the
>counter result to degrade in this way?
>Murray Greenman ZL1BPU
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