# [time-nuts] Fast frequency counting question

Magnus Danielson magnus at rubidium.dyndns.org
Sun May 4 16:14:51 EDT 2008

```From: "Murray Greenman" <murray at rakon.co.nz>
Subject: [time-nuts] Fast frequency counting question
Date: Mon, 5 May 2008 07:16:32 +1200
Message-ID: <BE50C3A72EBA61449C804B2C8B4AE381D05516 at NEPTUNE.rakon.net>

> 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?

Have you amplified the mixed signal in order to create a good slew-rate?
If you are measuring the 198 kHz sine directly you will have a limited slew-
rate, and noise will cause trigger-jitter.

Amplitude also plays in. The slew-rate of a sine is A*sqrt(2)*2*PI*f where
A is the RMS amplitude of the sine and f is the frequency. Trigger-jitter is
t_jitt = N/S where N is the RMS noise voltage and S is the slew-rate.

A simple test may be to dampen the signal, like making it half the level.
If the jitter doubles (you may benefit from testing in the period measurement
mode for this) then you are certainly slew-rate limited in your measurement.

Start there. Sorry if I am a bit lazy and is not prediciting weither you are
limited of that property or not. You should be aware that the time accuracy
of the counter may be limiting for your application. You might need a much
better counter. The CNT-90 says 100 ps in the specs. This is a combined total
of trigger jitter, other internal errors and of resolution. The internal
resolution is however better than 100 ps. For a gate time of 10 us will 1 ppm
resolution require a 10 ps trigger jitter. It is quite extreme.

I still wonder what purpose this measurement has. Does the initial chirp really
affect your design? It should not.

Cheers,
Magnus

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