[time-nuts] Frequency Stability Analyzer - ZCDs

[Glen English VK1XX]

well. thank you everyone for your contributions !

I had a good night in reading the references.

I agree the cascaded band-limited limiter strategy is eminently suitable.

That LT part looks like an excellent option, of course, 
horses-for-courses caveat applies for freqs and risetimes...

On comparators. Much of the 'noisyness' of comparators comes from the 
the use of a super wideband comparator  say 5GHz, the noise in even a 50 
ohm termination at room temperature is a few tens of microvolts and adds 
a fair bit of noise. I've dealt with this up to 500 MHz  by filtering 
before comparison,  but tricky for GHz ops...

-glen



On 28/07/2019 1:03 AM, Magnus Danielson wrote:
> Hi,
>
> Yes, indeed, so for many purposes the 6957 is probably good enough, and
> actually better than many classical approaches (i.e. direct
> comparators). It is when you design for a fixed or very narrow range of
> frequencies that you should consider rolling your own, assuming the
> performance of the 6957 becomes a limit to what you can achieve.
>
> Cheers,
> Magnus
>
> On 2019-07-27 15:49, Bob kb8tq wrote:
>> Hi
>>
>> Assuming we are still talking about a test instrument that needs to handle a variety of levels
>> and a range of frequencies, the 6957 is probably as good as anything.
>>
>> With a “full up” Collins style circuit, you very much need to optimize for a specific input.
>> Change that and you change the circuit. 1 MHz, 10 MHz, and 100 MHz will each “want”
>> a very different set of parts. Change levels 10:1 and that has an impact ….
>>
>> Even if you *do* get a circuit up and running, take a look at the TC of the caps in all those
>> filter stages. Matching all that up for a valid test is going to be a bit hard. You have a wide
>> range of values and (likely) a range of capacitor types. Not an easy problem to solve without
>> ovenizing the whole beast. Do that and you no longer have a “simple” box … (and no guarantee
>> a basic oven will solve the problem …)
>>
>> Bob
>>
>>> On Jul 27, 2019, at 6:32 AM, Magnus Danielson <magnus at rubidium.se> wrote:
>>>
>>> Hi,
>>>
>>> On 2019-07-27 12:07, Attila Kinali wrote:
>>>> On Sat, 27 Jul 2019 18:21:50 +1200 (NZST)
>>>> Bruce Griffiths <bruce.griffiths at xtra.co.nz> wrote:
>>>>
>>>>> The LTC6957 is a better choice for squaring up sinewaves:
>>>>> http://www.ko4bb.com/getsimple/index.php?id=phase-noise-and-other-measurements-with-a-timepod
>>>> If you want to have a single component ZCD, then I agree.
>>>> Otherwise, a multi-stage Collins like ZCD can perform better.
>>>> Especially, if the input waveform has known properties, then
>>>> the multi-stage approach can properly optimize for those.
>>> The LTC6957 is a multi-stage device with only 4 different bandwidths to
>>> optimize for, so you can do better. It may however be good enough for
>>> many purposes.
>>>>> Comparators are almost always noisier.
>>>>> Oliver Collins wrote a paper on optimising such sine to square converters.
>>>>> I extended the analysis to allow optimisation when the input noise of the
>>>>> cascaded stages arent equal.
>>>> There is one important point with Collins' analysis that hardly gets
>>>> mentioned: His analysis assumes that the output signal of a stage is
>>>> trapezoid. While this is true for high gain settings, it is not for
>>>> low gain settings. Ie in his example with 6 stages, the first three stages
>>>> have a total gain of 23, ie the signal has still significant curvature.
>>>> Thus Collins' analysis the noise contribution of these three stages contains
>>>> significant erros. See the attached paper for details.
>>> The trapetzoid model is a simplification which is better than sine or
>>> square, but not perfect.
>>>
>>> Another thing with Bruce noticed was that it assumed the same noise from
>>> all op-amps, but you can choose different op-amps with different noise
>>> and slope-rates and then you need different formulas, which Bruce produced.
>>>
>>>> Additionally, in a multi-stage ZCD, it is very important to keep the
>>>> duty cycle at 50%, as otherwise the even harmonics give rise to an increase
>>>> of flicker noise due to noise up- and down-conversion. See [1] for details.
>>> This effect has been seen by NIST for dividers, which made them conclude
>>> one needs to end with a divide by 2.
>>>
>>> Cheers,
>>> Magnus
>>>
>>>> 			Attila Kinali
>>>>
>>>> [1] "A Physical Sine-to-Square Converter Noise Model", by Attila Kinali. 2018.
>>>> http://people.mpi-inf.mpg.de/~adogan/pubs/IFCS2018_comparator_noise.pdf
>>>>
>>>>
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