Date: Sun, 31 Jul 2011 10:20:18 -0400
To: rsgb_lf_group@blacksheep.org
From: Bill de Carle <ve2iq@magma.ca>
In-Reply-To: <4E355332.6010107@iup.uni-heidelberg.de>
References: <4E355332.6010107@iup.uni-heidelberg.de>
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Subject: Re: LF: frequency doubler and divider
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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
>