[sdiy] frequency counter

[Magnus Danielson]

David G. Dixon wrote:
> Hey Team!
> 
> Has anyone here ever built an audio range frequency counter?  I'd like to
> build a reasonably accurate frequency counter module for my synth to use for
> VCO tuning.  Any info would be much appreciated.

OK, it is doable. What you need is some suitable fixed oscillator of 
sufficient stability. For your work I think a normal fixed DIP-14 
crystal oscillator will do just fine. +/- 100 ppm is off the shelf and 
should probably do what you need.

What you should do is to measure the count of the reference oscillator 
for one or a number of cycles. The number of cycles depends on the 
frequency, you would like sufficient amounts of them to get the 
precission you need. It's really simple

f = events/time

where events is number of trigger events from the start to the stop of 
the measurement, i.e. the number of cycles of the signal. Time is the 
number of cycles in the reference oscillator times the period of the 
reference oscillator.

For a 100 Hz tone to have 5 digits of resolution, it would take 10 
cycles to achieve a time of 100 ms being 100000 cycles of 1 us length.

Using a higher clock naturally shortens the time for the same 
resolution, or you can get better resolution for the same time. High end 
counters use linear interpolation techniques to virtually achieve the 
equalent of several GHz of counter clock while using a much lower 
"coarse clock". The highes coarse clock I know of is 500 MHz, but that 
instrument is a bit special. 10-250 MHz is more common.

Very important is the input stage and the trigger jitter. Use the 
sharpest slope (on a sawtooth-wave, use the reset pulse and not the 
ramp) and then apply gain by limiting amps.

A 8253/8254 counter chip can make minor wonders for you along with some 
processing. Found in several synths and at least one high end counter.

Cheers,
Magnus