Re: LF: frequency doubler and divider

[Andy Talbot <andy.g4jnt@gmail.com>]

No, that doesn't work to give a true square wave.   If the RC delay is not exactly 50% you get a repeated double pulse waveform and not a series of equally spaced 2.F pulses.    On dividing this by 2, you end up with exactly what you had before.

 

Been there  - done that.. :-)

 

The only practical way is to double the frequency, filter it to remove all odd order terms and the half fundamental then square it up and divide.

 

Andy

www.g4jnt.com

 

2011/7/31 Stefan Schäfer <schaefer@iup.uni-heidelberg.de>

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