[time-nuts] Lucent RFG-RB

[Rex]

On Thu, 28 Dec 2006 21:49:59 +1300, Dr Bruce Griffiths
<bruce.griffiths at xtra.co.nz> 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.