[sdiy] frequency doubling

[Tony Clark]

> On a different topic, I'm interested in building a frequency doubling
> circuit.  There was some talk here a while back which I haven't been
> able to find in the archives.  The gist, as I recall, was to create a
> 50% duty cycle square with the same frequency as the original circuit,
> then use leading and falling edges to create trigger pulses.

   Perhaps you should clarify what you intend to double.  There are 
different ways to achieve frequency doubling, but it gets a bit hairy if 
you are trying to also preserve harmonic content!
   For instance, frequency doubling a triangle waveform just involves 
using a full-wave rectifier.  But you can't use that on a sawtooth 
because you'll end up with a triangle!  To double a sawtooth, you need to 
mix it with a square wave that has the opposite polarity so that you are 
really doing nothing more than level shifting the first half of the 
sawtooth waveform UP and the last half DOWN (I'm talking rising sawtooths 
here, reverse that for falling saws...and wear a hardhat for God's sake!).
   For complex waveforms, I have no idea how you could do it and maintain 
the same harmonic content, except by maybe using a frequency shifter, but 
that may be too complex a solution.  :)

>  I understand most of this, but there are two parts of the process I'm
> unclear on.  First, how would I create a 50% square wave from a
> non-symetric input (if its symmetric about a known voltage, I know to
> simply use a comparator).

   If you have an non-symetric input then just send it through a DC 
blocking capacitor to remove the DC component.  Then the waveform will at 
least be near symmetrical around zero (or whatever value you want to bias 
it at).  Or if you can, trim out the DC component in the original 
waveform, but I'm guessing that you can't do that?

  In particular, I have a square wave that's
> not 50% - what do I need to do to it.  The other part is how to convert
> narrow trigger pulses (after doubling) back into something like a square
> wave or triangle.

   Can't help with that one, but if you do have a square wave, getting 
triangle is easy BUT getting the waveform to maintain a constant 
amplitude over the frequency range is much more difficult!  That's why if 
you have the opportunity to use the methods I described earlier, they 
offer the best way to double while preserving the harmonic content.
   You can do a lot of fun things with full-wave rectifiers and level 
shifting circuits.  :)

   Cheers,

   Tony

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