another BFO idea (was:RE: Soft sync and medicine to cure it.)

[Don Tillman]

   From: Haible Juergen <Juergen.Haible at nbgm.siemens.de>
   Date: Mon, 18 Sep 2000 13:46:38 +0200

   Now that discarding the BFO concept in frequency shifters looks
   more reasonable than ever (IMO), there is another application that
   beckons just for the opposite: Ordinary audio VCOs with linear thru
   zero capability (LTZVCOs)

   Background: There are various LTZVCO circuits published (including
   my own at http://www.synthfool.com/diy/hj2vco.gif - *not*
   recommended for building) which to my knowledge all suffer from
   "commutating inaccuracies" when the modulation just approaches the
   zero point. The direction switch might be missed or delayed, and
   you get a unpleasant artefact.

I think this is a fundamental shortcoming of a VCO whose state is
determined by a single capacitor.  A quadrature design, with two
capacitors, can enable smooth, well-behaved transitions near zero Hz.
I have a couple of napkin-scribble designs that I need to try out.

  -- Don

-- 
Don Tillman
Palo Alto, California, USA
don at till.com
http://www.till.com