[time-nuts] Re: Is the practical quality of a 10 Mhz reference determined by the quality of the fundamental or by the quality of the zero crossings?

[Richard (Rick) Karlquist]

A lot of stuff to unpack here.  I can make a few general comments:

IMHO, you should make use of the whole sine wave, assuming you have a 
sine wave to start with.  Precision frequencies should always be
distributed as sine waves.  The problem of deriving a square wave
from a sine wave without degrading phase noise has been discussed
many times on this reflector.  Spoiler alert:  it is non trivial :-)
Only attempt it if absolutely necessary.  Like if you have to
generate a low jitter clock signal for an ADC.

In a former life, I worked for the HP Santa Clara Division in the
frequency counter section.  Frequency counters always started with
a high speed comparator that generated a square wave from the
incoming sine wave.  This royally messed up the phase noise, but
frequency counters don't claim to measure phase noise.  I remember
someone had a wet dream where they would use the counter's comparator
like a zero cross detector, and then they could do DSP on the zero
crossings and recover all the information that was in the original
signal, based on the Nyquist theorem.  Fortunately, this ridiculous
idea never went anywhere.

Regarding PLLs:  You can't go wrong using a double balanced mixer
as a phase detector, if you want the best performance.  That's what
I did in the HP 5071 Cs standard.  Of course, that kind of phase
detector is not self acquiring, so I added a circuit using a couple
of flip flops that detected (1) if the loop was out of lock and (2)
if so, which direction it was out of lock. and (3) added an offset
of the correct polarity to the phase detector to make the VCO slew
into lock and then (4) removed the offset once the loop was locked.
There are whole books written about PLL's that you can read.
Clock recovery from data is another huge topic.  I even hold a
patent on a clock recovery circuit for 40 Gb/s data.  Way too much
complexity to discuss here.

Rick N6RK

On 3/24/2023 6:24 AM, Erik Kaashoek via time-nuts wrote:
> For one of my projects I was requested to make a presentation about 
> measuring phase and frequency
> Part of the presentation is about measuring phase and frequency for 
> which I could use a lot of excellent material from various sources.
> I did run into one small problem when trying to explain why the 
> PhaseStation phase measurement method (decimated I/Q down mix to zero 
> Hz) is ok compared to previously zero crossing methods such as used in 
> interpolating reciprocal counter.
> When using 10 MHz reference in a modern measurement device, is the lock 
> on the reference done by direct conversion to a square wave (some simple 
> digital circuit like a limiting amplifier) or are more advanced clock 
> recovery approaches used that look only at the fundamental and use all 
> information in the 10 MHz fundamental, just like the Phase Station is 
> doing?
> In what category would a PLL for clock recovery fall? Is the PLL looking 
> to the fundamental and ignoring noise on the zero crossings by using all 
> available information or is it plagued by the same problems as a zero 
> crossing clock recovery?
> I hope someone with knowledge on clock recovery could help out here. 
> Many thanks in advance.
> Erik.
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