[sdiy] DCO ramp core / capacitor discharge transistors

[Razvan Tataroiu]

> A VCO depends on a rapid discharge time so that it responds properly to
> the control voltage.  If it took 0.1 ms to discharge the integrator's
> capacitor, the high frequencies would be off as the discharge time
> would significantly contribute to the total period of the output
> waveform.

I did some quick calculations and came up with some ideas.
Say T is the VCO period, T=t1+t2, where t1 is the ramp-up time and t2
is the discharge time.
t1 is inversely proportional to the control current i, more precisely
t1=C*U/i, where U is the output amplitude and C is the integrator
capacitor.
Assuming t2 is constant (the discharge current must be much larger
than the charging current), the frequency is f=1/(CU/i + t2). You can
decrease i by a certain constant offset to compensate for t2, namely
i0 = -CU/t2.
You can do that with a resistor from the summing point of the
integrator to a stable positive voltage.
Now f will be strictly proportional to i, f=i/CU.
Assuming t2 is not constant enough (it's affected by i), force i to
zero during the discharge by one of the various means available, for
instance a jfet in parallel with the expo converter input.