Frequency changing stuff...

Don Tillman don at
Tue Oct 31 19:33:40 CET 1995

   Date: Tue, 31 Oct 1995 13:14:34 +0000 (GMT)
   From: "D.V. Mitchell" <dvm1 at>

   I've got a question, I've thought up two projects, one of which will
   be too complicated for analogue, so I'll probably use DSP's...  

And you're not going to tell us what it is?
   the other can be done in either analogue or digital (I hope)...  I'ts
   not really a synth circuit, (well for the application I'm going to use
   it for, but the end circuit could be modified to give some effects...
   Basically I need to get a full range audio signal, and increase it's
   frequency by two or three hz...

   Does anyone have any ideas, of how such a feat, could be achieved with
   analogue electronics...

It takes two multipliers (ie., ring modulators), a local sine
oscillator with quadrature outputs (0-degrees and 90-degrees) and a
pair of phase shift networks for your input signal.  It's an
electrical implementation of the geometric identity for cosine of the
sum of two angles.

So it's not a trivial project.

   Also I built one of them 'State Variable Filters' In the TI OpAmp data
   book...  As my maths isn't all that hot, I used the rules of thumb
   (given below it), and built the filter...  But now, it sort of works,
   (sometimes), but then other times all the signal paths seem to 'clog
   up' with the supply, and inbetween D.C. voltages...  Very confusing...

You understand that the phrase "clog up with the supply" doesn't tell
us a heck of a lot. :-)

  -- Don

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