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

Needham, Alan Alan.Needham at centrica.com
Fri Mar 21 21:13:10 CET 2014

Some years back I wanted to check my 'el-cheapo' frequency meter, a Thandar TF200 before using it to check an organ tuning aid I built.
I found a circuit online, I think it was called 'the Poor Man's Frequency Standard', from some UK amateur radio website. The circuit was based around stripping the line sync pulses from the BBC transmissions (before digital messed everything up), the author stated that the studio transmissions were disciplined with an atomic clock and by running a PLL at 1MHz locked to this a high degree of accuracy could be achieved.
I guess it would be easy now to do a similar thing with a GPS signal.
I checked a couple of guitar tuners borrowed from a friend against this lashup, one really cheap one and one 'much better quality' one.
Both came out as scarily accurate, better than 100ppm as I remember - and at a tiny fraction of the cost of my organ tuning aid :-(

Ps. Yes, I waited for 100seconds per reading at low frequencies (not reciprocal).

-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Andre Majorel
Sent: 21 March 2014 11:05
To: synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] Frequency meter (was Re: Anyone in the UK used RS Calibration Services?)

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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