[sdiy] quadature oscillator

[Magnus Danielson]

From: harrybissell <harrybissell at prodigy.net>
Subject: Re: [sdiy] quadature oscillator
Date: Thu, 27 Apr 2006 23:15:57 -0400
Message-ID: <445188ED.9000CAE9 at prodigy.net>

Hi,

> If you run the OTA output to a virtual ground point (like in a state
> variable filter... or dast I say it... its likely that the quadrature
> oscillator has a similar archetecture :^)... then all you need to do is
> use the OTA directly and forget the variable resistor circuit.

Indeed. This was exactly my thought. One has to recall that in a state-variable
filter (propped up for a quadrature oscillator) you have the capacitors sitting
in the negative feedback path of a pair of op-amps. This makes their negative
input being the virtual ground. OK, OTAs have voltage inputs and current
outputs and the resistors in a virtual ground system is there to convert the
voltage into current, so just dropping in a OTA as replacement should is very
simple. As always one has to care about the input voltage of the OTA, so a
classical divide-down resistor network is needed on the inputs in use.

A trick one cna allow oneself is to alternate input between the two OTAs, since
that will acheive the necessary inversion for a two OTA/integrator state-
variable filter setup. The oscillator core should end up having 4 resistors,
2 OTAs, 2 op-amps and 2 capacitors. One could relly on the tanh distorsion on
the OTA inputs for gain limitation, but that has the drawback of distorsion.

Good luck!

Cheers,
Magnus