[sdiy] Frequency multipliers

[David G. Dixon]

> A phase locked loop (4046 etc) with a counter in the feedback path can
> produce integer multiples of your input frequency, in-phase.  These will
> be square waves but could be shaped.

A square wave cannot be shaped directly into a sine.  If a square wave is
fed to a sine shaper, it produces a square wave.  It can be integrated to
produce a triangle wave, and then this can be shaped into a sine easily.
The trick is to get a triangle of constant amplitude.  For this, some sort
of automatic gain control circuit is required (see US Patent 3,982,189).  

Alternatively, one could use something like Harry Bissell's EFM
pitch-to-voltage converter, but alter it so that the held voltage drives a
saw or triangle core directly, with resets driven on every other clock
cycle.  This would give two periods of a saw or triangle with every sampled
voltage which could be shaped into a sine. I haven't figured out how to do
this yet, but I have lots of future plans for Harry's amazing circuit.