Korg MS 20 f->Volt Converter

[Martin Czech]

Yesterday I took a look at the Korg MS20 frequency to voltage converter.

I think this is how it works:

First the input signal is conditioned, ie. amplified and  bandpass
filtered. A second order highpass A and a 4th order lowpass B do the job.
B cuts away strong harmonics that could fool the detector, and the A
limits the lower end of frequency range, it reduces unnecessary bandwidth
below the expected fundamental of the input signal thus reducing noise.
Unfortunately 4x pots are required for both filters.  The signal then goes
into two 339 comparators, the first working as zero crossing detector,
the second simply as inverter. The rectangular waves of these comparators
are about 180 deg out of phase, they feed a capacitor & diode network,
which forms a positive spike for every negative and positive zero
crossing. These spikes discharge a one-shot via TR2, the one-shot made
out of one 339 and C2. The last comparator does almost nothing, it is
allways pulling a low level and seems to provide a reference potential
for the one-shot. I think this is some kind of temperature compensation.
Now we have a train of constant lenght pulses and the frequency of the
pulses is double input frequency. The 4096 inverter chain does perfect
wave forming of the pulse train (maybe some glitch suppresion also). The
second lowpass filter C (2nd order) follows A (4x pot) but with some
lower frequency, it is there to integrate the pulse train into an average
level. The following enshures low ripple for every cutoff value of A. The
one-shot lenght is critical, because it goes directly into the average
level, temp compensation is therefore logical.

Even if the system reaches a steady state at the output, it suffers from
ripple. So the detection should be quite good provided that A and B are
proprely adjusted, but the output voltage still has ripple, which should
lead to a certain roughness off following vco processing.

I think Tom G. build the f->Volt converter, and maybe some others too.

Any experiences ?
Comments?



m.c.