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 <mag...@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.griffi...@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
_______________________________________________
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to
http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
_______________________________________________
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to
http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
_______________________________________________
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to
http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
_______________________________________________
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to
http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
_______________________________________________
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to
http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
