Re(2): V/Oct >> Hz/V converter circuit?

[Magnus Danielson]

From: "jhaible" <jhaible at debitel.net>
Subject: Re(2): V/Oct >> Hz/V converter circuit?
Date: Sat, 15 Jan 2000 19:10:27 +0100

> > Take the expo guts from some v/oct VCO and feed the current output into an
> > op-amp with a resistor from output to (-) input. The current output should
> hit
> > this (-) input. Now, take the (+) and hook it to ground. The output would
> now
> > become the Hz/V. Change the feedback resistor to scale the Hz/V.
> 
> I've built one recently, have not drawn schematics, but it's exactly as
> Magnus describes.
> What I want to add: Use an opamp with low offset voltage for this,
> as you're always fighting errors of low frequency tracking in linear VCOs.
> This is important for *all*stages that follow the expo converter, like glide
> circuits.

Good point.

> Good opamp choices are LF411 or OP-07.

These are not particularly exotic, so it would be not trouble. You could
otherwise make a simple trimming curcuit if so necessary by just taking an
trimmer between +15V and -15V and put a resistor in series and hook this into
the (-) input. The resistor should be fairly high ohmic. The trouble with this
would be to do accurate triming for most people. Some of us are pretty well of
with good DMMs, so we would have no trouble doing it.

Cheers,
Magnus