[time-nuts] Fast frequency counting question

[Murray Greenman]

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.

Any suggestions (a) how to make the measurement; (b) what is causing the
counter result to degrade in this way?

Thanks,
Murray Greenman ZL1BPU