Stefan:
I think you'll find Andy is right :-)
As I understand your circuit description, the output of the XOR gate
will be a fixed-duration (function of your RC time constant)
positive-going pulse triggered by each rising and falling edge of
your input signal. Dividing that by 2 isn't going to produce a
perfect 50% square wave. You can visualize it more easily by drawing
it out on paper and taking an extreme example, e.g. say your input
signal is 10% high, 90% low. You'd have to set your RC time constant
to a much shorter period. The XOR output will be 2 short pulses
close together, then a long delay, then another 2 pulses close together.
Bill VE2IQ
At 09:05 AM 7/31/2011, Stefan wrote:
LF, VLF,
I just re-invented the wheel ;-) Just by thinking i found one
possible way how to double and divide a rectangular signal, to
achieve an accurate 50 % duty cycle :-)
One could take a simple XOR gatter like a CD4070 and a RC circuit.
The time constant has to be about the half of the period of the
wanted frequency, e.g. 3.6 us for 137 kHz. The output of the RC
circuit (voltage across C) and the original signal is fed to one
single input of the gatter. Then the output frequency will be 2x f input.
All this can be found in the internet as well, of course. But if you
don't know about that possibility you don't know what to seach in
the web too. At least i haven't found that method by searching and
it also wasn't discussed here in the recent time. Furthermore i
never saw it on a PA design by amateurs i found in the web.
The output signal of the 4070 is then fed to the well known
frequency divider consisting out of e.g. a CD4013. Then output
frequency will be the same as on the input but the duty cycle will
be accurate 50%!
This is one method to (re-) obtain a suitable drive signal for a H
bridge PA or class D PA after passing the driver signal through an
optocoupler or so. I'm sure i will use that method now in several
circuits, like in the new LF PA that still causes some problems,
excactly on that matter!
Later i found on the German wiki site:
http://de.wikipedia.org/wiki/XOR-Gatter#Frequenzverdopplung
73, Stefan/DK7FC
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