[hpsdr] Sampling Phase Detectors

[Chris Bartram]

Hello John

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

Not specifically! I'm talking about a phase detector based on a sampler. One 
which uses an impulse to operate a switch, as in a sampling oscilloscope. 

> 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.

Most of the time, I'd agree with you. One of the problems with the phase 
frequency detector is that when the loop is locked, it behaves more like a 
frequency locked loop close to carrier than a phase locked loop. 

One very revealing way of looking at that in practice is to use a PFD in a 
loop which tries to lock a crystal oscillator to an external standard such as 
GPS. Use a linear phase detector such as an ex-or gate or a DBM and you'll 
see the loop phase-lock solidly. Use a PFD and the uncertainty manifests as 
greater frequency/phase inaccuracy. 

Look if you will at some of the simpler GPS locked standards, such as the one 
published by James Miller, G3RUH. I once, misguidedly, tried to use a PFD in 
such a device, and it was a couple of orders of magnitude worse than the 
ex-or gate which replaced it... 

For simple synthesisers the PFD represented a huge step forward when it was 
first introduced, however, it came at a cost. The very close-in phase noise 
is degraded by uncertainties introduced by the detector, and shows as greater 
than expected spurious FM. That's why many IC implementations of the PFD add 
some form of dead-band reduction circuitry. Most make some improvement, but 
few, if any completely solve the problem. In the old days, we simply hung a 
resistor from the PFD output to ground in order to improve the spurious FM, 
and fought the reference frequency sidebands! The levels of spurious FM 
generated by the PFD wouldn't matter in a 'normal' synthesiser, but if we're 
trying to go for the best...

In practice, the folding effects you posit don't seem to be that significant, 
presumably because in a harmonic locking system everything is synchronous. 
Also, with a decent VCO, the sampler should only be seeing a single 
sinusoidal signal... For the 1GHz case which we've been discussing over the 
last few days, I've looked at the use of packaged commercial DBMs as a 
sampler. Within their frequency range they work well, as might be expected, 
on odd-order harmonics, and seem to essentially be operating as samplers 
rather than harmonic mixers. I need to understand that better. To do 8x for a 
125MHz oscillator/1GHz source will require a different sampler topology, 
though.

Don't forget that a sampler will have a sensitivity which (at least in 
theory!) is independent of harmonic number. Harmonic mixers are a different 
beast... A major problem is generating a suitable pulse. 

The use of sampling phase detectors for locking microwave oscillators has been 
standard practice for decades. FWIW my 24GHz system relies on a commercial 
'brick' which uses a sampler to lock a 12GHz VCO to a VHF oscillator with 
remarkably good results. I agree that careful design is needed, but if that 
is accepted, the sampling detector removes the noise contributions of the PFD 
and counter, and replaces them with potentially significantly smaller 
contributions of its own. The free lunch principle still applies! In this 
case we have to trade some circuit complexity for performance.

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

I hope I've provided some indications in this very late night email. This 
isn't a straightforward problem, but I'm convinced that the harmonic sampler 
is the right way to go. Either that, or back to multiplier chains...

Vy 73

Chris
GW4DGU