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low duty-cycle REF CLK for DDS?

Question asked by DanCA on Jul 26, 2011
Latest reply on Jul 26, 2011 by DanCA

I  was told by AD's technical support that REF CLK could not be a  short-duty cycle pulse...though I've now read in these forums that a higher slew rate on REF CLK is better for phase noise performance, so I am now a  little confused. A short, sharp pulse would have a high slew rate, so  why could this not be used as a REF CLK signal? That is, why is there a  requirement of a 45-55% duty cycle (as stated on the AD9912 datasheet,  for example).

 

In  my application, I am using a pulsed laser as the REF CLK source. I have  an ultra-short-pulse pulsed laser on a fast photodiode. The laser  pulses at about 80MHz; the pulses are so short that the electrical pulse  from the photodiode is limited by the risetime of the photodiode (<1 ns). So in the time domain, I have short 80MHz pulses; in the  frequency domain, I have an RF 'comb' with approx. equal components at  80MHz, 160 MHz, 240 MHz, etc. I wanted to use the 80 MHz pulse as the  REF CLK. If I cannot do this, then I will now plan on bandpassing just  one of the harmonics of the RF comb to obtain a sinusoidal signal that I  can then use as a REF CLK, albeit with a lower slew rate. So for  example, I'll filter out a comb line at ~1GHz and use it as REF CLK for  an AD9912 eval board. Are there any problems with this?

 

Thanks  in advance for any help from anyone! I realize this is possibly an  unusal REF CLK source, plus I am a physical chemist, not an EE expert,  so any help would be greatly appreciated!

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