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

Richie Burnett rburnett at richieburnett.co.uk
Fri Mar 21 10:18:37 CET 2014

Another resounding vote for reciprocal counting here. It's definitely the way to go for quickly measuring audio frequencies to a decent precision.

If you're not familiar with the technique it works like this... You accurately measure the time taken for one period of the input signal, instead of the more common frequency measurement technique of counting the number of cycles in a fixed period like 1 second. Provided the timebase for the period measurement is a high enough frequency, you get a high resolution measurement of the period after just one cycle of the input waveform. Then you simply do the calculation f=1/T to get the frequency from the period. It's much quicker than counting the number of cycles over 10 seconds to get just 0.1Hz resolution!

Some frequency counters switch between reciprocal counting and normal pulse counting depending on the frequency range selected by the user. Some still use reciprocal counting at high frequencies but switch to timing multiple periods of the input waveform.

My work has one of the GPS disciplined 10MHz reference clocks that Neil mentioned, and that is what I used for the VCO drift experiments I described here a few years back.  It is very useful for this type of work but I probably wouldn't have shelled out the 800 quid of my own money if it hadn't been already available. Modern oven controlled crystal oscillators are plenty accurate compared to the drifty analogue VCOs I was measuring!

If it was me I'd look to buy a decent reciprocal counter cheaply second hand Tom. Designing the finicky analogue signal conditioning front end and doing the reciprocal calculations on a cheap micro actually isn't that much fun.


Sent from my Xperia SP on O2

Neil Johnson <neil.johnson71 at gmail.com> wrote:

>Hi Tom,
>> 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.
>What is your definition of "cheap"...?
>I picked up an HP 5334A counter off ebay a while back for not much
>which has 8-digit precision.  For audio use and your sanity you really
>want a reciprocal counter so that at low frequencies you aren't
>waiting too long for a reading (the reason why I upgraded from my old
>Racal Dana counter).  If you want high accuracy, then you're entering
>the realm of GPS-disciplined reference clocks to give to a very
>accurate 10MHz timebase.  Or you may be happy enough with the internal
>crystal oscillator.
>What you get in a fancy box are things like a great analogue front
>end, multiple channels, various triggering and counting modes, maybe
>some maths functions, various gate functions, etc.
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>Synth-diy at dropmix.xs4all.nl

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