[sdiy] Cheap frequency counter for oscillator calibration

[Magnus Danielson]

Harry,

On 04/22/2010 03:24 PM, Harry Bissell wrote:
> I don't think a frequency counter is the way to go. they are too slow if they are accurate enough.
>
> I use a good guitar tuner to set the frequency of an external sine wave oscillator, then use
> lissajous patterns on an oscilloscope to beat the unknown VCO against that reference. Its easiest if the VCO
> has sine or triangle outputs, but possible with a ramp. Square waves are nearly useless for this tuning.
>
> You can easily discern about a 4:1 ratio in each direction, if you set the fixed oscillator midrange you can
> get an eight octave range.
>
> Its much easier to tweak trimpots until the pattern stops rotating, and your ear can tell you if you are
> in the right octave range...
>
> Strobe tuners are another option.
>
> I made myself a precision 1V/step calibration box so I can change input voltages easily and be sure
> they are repeatable. this makes the job a lot easier
>
> my vote: frequenct counter is useless...

Unless you always tune to even numbers like 440 Hz and octave variants, 
just making a useful readout is the main head-ache with counters.

Modern counters (such as HP-53131A and Pendulum CNT-90) is reciprocal 
and uses overlapping predictors giving far better values. The key issue 
is that the slew-rate of a typical synth signal is so low that slew-rate 
induced jitter is produced, limiting the resolution. Using a slew-rate 
gain-stage can handle that iusse.

The reason that frequency counters is useless is in my opinion the lack 
of adaptation to our field, they lack the tone and cent scale and they 
lack the input adaptation needed for our type of signals.

A guitar tuner is adapted since it from the beginning needs to handle 
soft signals, quick tuning and the effective resolution scale.

If you where to build your own tuner, using the frequency counter 
approach would certainly be possible.

Cheers,
Magnus