[time-nuts] Fast frequency counting question

Poul-Henning Kamp phk at phk.freebsd.dk
Sun May 4 16:10:37 EDT 2008

In message <BE50C3A72EBA61449C804B2C8B4AE381D05516 at NEPTUNE.rakon.net>, "Murray 
Greenman" writes:
>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

Consider getting a high-rate ADC (check the GNU-radio USRP for instance)
which can sample your 40MHz directly, then do the analysis in software.

Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.

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