[sdiy] Jürgen Haible living VCO core

[Neil Johnson]

Hi Liam,

> 1. Resistors R17/R18 in series with the capacitor on the integrator: R18 is marked as being a trimmer for high frequency tracking. Any idea how that works? My guess is it prevents the op-amp output resetting all the way to zero, and more so at higher frequency, and thus a shorter journey back up to the trigger point of the comparitor. So it counteracts the tendency to flat at higher frequency by reducing the amplitude --- is this right?

"Franco Compensation".  At higher charging currents the resistor
introduces a voltage into the feedback loop, which also appears on the
output, causing the comparator to trigger earlier than it would
otherwise, and so causing the oscillator to run slightly faster.  It
is to compensate for the finite capacitor discharge time.

> 2. What is the purpose of D15?

Think about what could happen when power is first applied to the op-amp.

Neil