[sdiy] ASM-1 VCO: Sensitivity of linear FM Inputs?

[Grant Richter]

> I too ran into this - the circuit as designed only produces a narrow sweepo,
> and I am interested in classic Buchla style complex FM.  Does anyone know
> how many octaves the Lin FM input on a Buchla will sweep with the audio
> signals as they are configured in THAT system?
> 

Think of it as a linear current source. The FM range cannot exceed the bias
current or the current source will switch off, shutting off the oscillator.
That sounds bad. So for a typical 10 uA bias current, the linear FM range
has to be less than 10 uA, or the oscillator will stall at the low end with
max linear FM.

Look for the resister from the reference transistor op-amp summing node to a
supply or reference. Then calculate bias current.

Don Buchla was an Electronotes subscriber, 258 linear FM is no different
than any Electronotes VCO. Though Don may have used a "hotter" bias current
since the 258 timing cap is 4700 pF instead of 1000 pF (roughly 2 octave
lower than most Bernie Hutchins designs with a 10 uA bias). That may allow a
deeper modulation index.