[time-nuts] Fast frequency counting question

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun May 4 19:21:42 EDT 2008

Murray Greenman wrote:
> 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
> struggle.
> 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
> available.
Amplified mixer noise could easily account for this if the post mixer 
amplifier/filter system isnt appropriately designed.
Optimising the post mixer amplifier and filter will drop this noise to 
well below the counter resolution.
> Any suggestions (a) how to make the measurement; (b) what is causing the
> counter result to degrade in this way?
> Thanks,
> Murray Greenman ZL1BPU


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