[sdiy] VCO Linear FM input - DC or AC coupled?

René Schmitz synth at schmitzbits.de
Tue Jun 27 11:00:10 CEST 2023

On 26.06.2023 22:34, Neil Johnson via Synth-diy wrote:
> When you add a DC blocking capacitor to the summing opamp input 
> (assuming you're using an inverting opamp circuit) what you have done 
> is create a crude differentiator.  It's crude because, unlike an ideal 
> differentiator, the gain at daylight is not infinite.  Certainly the 
> gain at DC is 0.  How good a differentiator it is depends on the 
> resistors and the opamp (and for lols, opamps are crude integrators).
Worse. Not crude, but poor.

At Daylight, its phase shift goes to 0, so no differentiation at all. 
The only point where it is a perfect differentiation is at DC (turning 
any constant into 0) as you say, and close to that the gain is zero.

Near the corner frequency we can turn a sine into a 45° shifted cosine 
or sine depending on how you want to look at it. Halfway there. But not 
the derivative.

(And definitely not phase modulation. For this you'd need the derivative 
at ALL frequencies.)

Whats much more important here is a rectification effect:

Because the opamp summing node that the linear FM input goes to, has 
different current sourcing/sinking capabilities in either direction, a 
DC offset develops across the cap.

Happens even if your modulating waveform is perfectly centered and 
symmetrical (i.e. DC free).



synth at schmitzbits.de
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