[time-nuts] Fast frequency counting question

John Miles jmiles at pop.net
Sun May 4 19:55:29 EDT 2008

> The quickest and easiest way to achieve the required resolution is to
> buy a high speed (~100MHz) sampling ADC evaluation kit from Analog
> Devices, Linear Technology etc, use a low phase noise crystal oscillator
> (eg Wenzel ULN or equivalent performance device) or equivalent bandpass
> filter the output and use as the sampling clock source. Then either:
> 1) Sample the 40MHz signal directly (possibly after some bandpass
> filtering) then post process the raw data.
> 2) Use  a mixer to produce a ~1MHz output bandpass filter it and sample
> this signal  (amplifying if necessary) with the ADC then post process
> the raw data. This will achieve lower noise. However the mixer LO has to
> have very low phase noise.
> High speed ADC evaluation kits are readily available (at least from
> Linear) and considerably cheaper than a timer counter with equivalent
> resolution and noise f such an instrument is available at all.
> The LTC evaluation kits include a board with local sample storage and a
> USB interface to a PC.

The last time I looked at those eval boards, though, they were pretty
adamant about not releasing API specs, so you couldn't use them as platforms
for your own DAQ applications.  Irritating, but I can see why, because
they're probably just breaking even on the boards.

Would be nice if that's no longer the case.  Failing that, one of the GNU
Radio USRP boards might be the most economical way to go.

Also: the effect to be studied must be coming from some type of circuit
built with some standard semiconductor process, which in turn is driven by
some sort of clock.  If you go nuts with sampling precision, aren't you just
going to get more information than the underlying process is capable of
generating?  (I'm probably not phrasing that very well, but you get the
idea, hopefully.)

-- john, KE5FX

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