[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?

[djl]

Rick: Just the guy to answer a q for me(?).  I've a batch of 80's Hp 
boatanchors that perform well. They have 10 MHz reference inputs and 
outputs. I have wondered for some time if daisy chaining these devices 
from a reference would gradually keep adding phase shift down the chain 
output to input as opposed to tees, that is more than just the cable 
lengths.
Thanks
Don


On 2023-03-24 11:01, Richard (Rick) Karlquist via time-nuts wrote:
> 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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