On Thu, 28 Dec 2006 21:49:59 +1300, Dr Bruce Griffiths <[EMAIL PROTECTED]> wrote:
>Rex wrote: >> The 15 MHz generation looks very similar to the other boad I described >> in another message on an FRS-C unit. The active source again seems to >> come from an Altera PLD. Maybe it divides 10 MHz by two, and (guess) >> narrows the pulse for better 3rd harmonic? > >Narrowing the pulse width does little for the amplitude of the third >harmonic component. >There's already plenty with a 50% duty cycle square wave. >A waveform with duty duty cycle of 1/6 has the same 3rd hamonic >component amplitude as a 50% duty cycle waveform. >Narrowing the pulse to a very small duty cycle tends to flatten the the >frequency comb whilst reducing the amplitude of all components. >see: >http://www.wenzel.com/pdffiles/choose.pdf > >Bruce Yeah, the harmonic amplitudes vs pulse width chart in that Wenzel doc is exactly what I was thinking about when I wrote. I didn't actually look at it though. I see now 3rd is good at 50% and the other harmonics are mostly low. Guess I was trying to find a reason why the source for the 15 MHz filter seemed to be coming out of a PLD. Maybe it's doing other more complicated functions that I haven't yet figured out, and divide by 2 for the 10 MHz was just easy to throw in. Like I said, seems about like the implementation I saw in that other board. Never did work out all the PLD was doing there either. PLDs are hard to guess out. I guess, when I get around to powering it up, I can put a scope on the signal at that point and see if it is just 5 MHz square. Good chance I'll just use the LPRO anyway and dump the rest of the box. Can't think of a reason why I need 15 MHz and I can't see much else useful in the rest of the supporting circuitry. The metal box is kind of interesting. _______________________________________________ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts