How does a frequency counter work?

[circuitmangler]

I was wondering if someone could describe the theory of operation behind a frequency counter. I understand how a "digital" frequency counter works -- the input signal is assumed to transition between high and low logic levels, and the input pulses the clock signal of a counter. However, it seems that there are frequency counters which can measure the frequency of "any" signal. If this is the case, what are these actually measuring? And what would happen if the signal is a superposition of two pure frequencies? From all the schematics I've seen, it seems that the input signal is first filtered or conditioned before it is passed to what I've described as a "digital" frequency counter. If this is true, what is the nature of this input filter?

Many thanks in advance.

PS. Are there any "frequency counters" which will essentially compute a Fourier transform to determine the largest component frequency in a signal?

 

[Warpspeed]

Usually any frequency counter has some sort of input signal conditioning that turns your input signal into a square-wave. So it just counts the cycles.

If you fed white noise into something like that, you would just see garbage random numbers. There is really no point in trying to measure the frequency of a complex rapidly fluctuating signal, because it probably does not have one.

What you need is a spectrum analyzer that shows the relative amplitude of all the frequency components present in the signal. It could be a fourier system, or be an analog system that either has multiple filters, or a swept heterodyne system.

 

[IRstuff]

That's effectively what a spectrum analyzer does.

TTFN

 

[jbartos]

Suggestion: Visit

http://www.intio.or.jp/jf10zl/counter.htm

etc. for more info

 

[circuitmangler]

Thanks everyone! That clears up a lot.