[sdiy] Frequency multipliers

[Eric Brombaugh]

On 06/05/2010 10:09 AM, David G. Dixon wrote:
>> 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).

Another approach if you're using a PLL to lock onto the input frequency 
for multiplication would be to multiply it by an additional factor of 
say N where N = 2^k. Then drive the resulting fast squarewave into a 
k-bit counter and then into a wavetable ROM and DAC.

That would give you not only frequency multiplication, but also 
wavetable lookup with clean address transitions. There would be some 
challenging issues in the post-DAC reconstruction filter, but some folks 
might also like the sound without.

Granted - this is probably more digital circuitry than most SDIY folks 
want to deal with. Plus, audio-frequency PLL circuits with wide lock 
range tend to suffer from some settling time issues, so pitch tracking 
might be a bit wobbly. Could be an interesting project though...

Eric