[hpsdr] Trade-offs in loop filter design

[John Miles]

> In all cases, a lot of attention needs to be given to the phase
> detector and
> to the way in which the frequency translation is achieved. Sampling phase
> detectors are a better solution than phase/frequency detectors
> and frequency
> dividers.

Can you elaborate on that, Chris?  By sampling phase detectors, you're
talking specifically about harmonic samplers, correct?

I'd never use a harmonic sampler as a phase detector if I had a CMOS
PFD/counter chip available for the frequency of interest.  In my experience,
samplers have widely-varying degrees of conversion loss depending on the
harmonic in use, requiring a lot of IF gain that raises both the circuit's
complexity *and* its phase-noise floor.  You also have to worry about
minimizing the additive noise from the LO comb generator.

Finally, there are deleterious effects arising from the fact that the
sampler is simultaneously downconverting ("folding") at all of the harmonics
it sees.

Would be good to hear any pointers to the contrary...?

-- john, KE5FX


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