[sdiy] Frequency meter (was Re: Anyone in the UK used RS Calibration Services?)

Andre Majorel aym-htnys at teaser.fr
Fri Mar 21 12:05:19 CET 2014


On 2014-03-20 21:41 +0000, Tom Wiltshire wrote:

> In a similar vein, I often find myself in need of a high
> accuracy frequency meter for audio and sample clock
> frequencies. 0-25KHz for sure, and 0-200KHz would be nice.
> Presumably there are expensive pieces of test equipment
> available that do this job, but does anyone know of a
> cheap/accurate way to do it? My definition of "accurate" would
> be something like "so I can see errors of a cent or less" I
> suppose - e.g. an order of magnitude better than I can manage
> with my ears. Otherwise there's really no point.

How about software ? 1 cent is about 578 PPM and I think that
the error on sample rate of a sound card is on the order of
+/-100 PPM.

It's probably not going to go up to 25 kHz, though. Some go up
to 192 kHz these days but I suspect that the anti-aliasing
filter is still at 20 kHz.

Sub-audio frequencies, on the other hand, can be measured, even
with an AC-coupled sound card. As long as it's a pulse or
sawtooth or anything with regularly spaced sharp edges. The
edges are turned into spikes by the AC coupling and you just
count the time between peaks. I have some C++ code for Linux if
you want.

-- 
André Majorel http://www.teaser.fr/~amajorel/



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