Question: Frequency Synthesizer for Mixer Issue

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koukei

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Question:
1. to design one 2.4G frequency synthesizer for Mixer, one proposal is to design 2.4G frequency synthesizer with 2.4GHz LC VCO, directly ouput for Mixer. The other proposal is to design one 4.8G frequency synthesizer with 4.8GHz LC VCO, through one CML divider, generate 2.4GHz frequency, then output for Mixer. Which proposal is better.
2. 2.4GHz LC VCO can output good sine waveform, but if through CML divider, there have distortion in the CML divider, output waveform of CML divider is worse than the waveform directly output from LC VCO. Does it mean that the proposal the first proposal without CML divider is better?

Thanks,
 

If you use this as a LO drive for your mixer, the first option will be more simple and have less overall noise. Anything you add in the chain will add an amount of noise. You want any LO to be as clean as possible for a number of reasons.
 

oscillator Q gets worse at 2 times the desired frequency, but when you divide by 2 you clean up the phase noise by 6 dB, so it is approximately a wash.

The design will be a little harder at 2X the frequency, more parasitic effects.

Some people do the oscillator at 2X the frequency so that they can divide by 2 and generate two LO signals 90 degrees out of phase. If you are using an I/Q mixer front end, this might be the reason to do so.
 

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