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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