Pitch -> CV conversion

[Martin Czech]

One application of a pll is frequency multiplication.
Now, I toyed arround with full wave precision rectifiers a while ago.
If you rectify a tri wave, this will give another dc shiftet
tri wave, but with double frequ.
I cascaded that circuit three times. Works good at low frequ.
but at the higher end the waveforms get strange, due to nonideal
op-amps.
I remember that some book suggested a speed improoved precision rectifier
by having the frequency compensation outside the amp in such a way,
that compensation is switched off when the voltage is near 0 Volt,
that is near the critical "no feedback" point of the rectifier.
I think that the compensation cap was not connected directly to the amp
output, but to the diodes in some way.

Any experiences out there ?
What opamp could be used ?

m.c.