[time-nuts] Frequency Stability Analyzer - ZCDs

[Magnus Danielson]

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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