Only for the Nuts,
ZCD have been discussed at great length this time and before, and bandwidth
can be a major issue, but still much is being left out.
With a little care, one can easily get to 1ns type accuracy, with the
various suggestions,
but that only gives 1e-9 / sec of accuracy, not even close to what is needed
so as not to degrade the short term accuracy of a basic OCXO like a HP10811.
For that it takes sub-picosecond type phase / time stability and
repeatability, and to get to that level, it takes a whole lot more
considerations.
If you want to see how bad things really are, try and check out the
sub-picosecond drift that even a very small temperature change or signal
amplitude change can have on any of the Schmitt triggers or the suggested
multi-stage band-limited ZCD designs.
ws
***************
Good, I've learned also that bandwidth can matter and that a ZCD test can
done by comparison: feed the counter or TSC or TimePod or 'scope with your
source signal and 2 cables, then insert the ZCD and see the difference.
Actually I'm interested in a ZCD to feed the FPGA from the OCXO, I'm using
a 74HC04 with feedback followed by a regular 74HC04.
On Sat, Jul 21, 2012 at 4:15 AM, John Miles <jmiles at pop.net> wrote:
> I see that from one way or the other, we always end up in a TimePod. OK,
> then the TimePod has no comparator, no trigger but has A to D
conversions.
> Is the A/D conversion process supposed to be threshold-free?
Hey, everybody needs at least one or two TimePods. :) You can use a
TimePod
or TSC 512xA to measure additive jitter, or for that matter a mixer and a
delay line. But these instruments will all do the job by making a phase
noise measurement, then integrating the plot to find the equivalent RMS
time
jitter. This means that you'll have to decide what limits of integration
you want to use. A counter, on the other hand, will give you the total
jitter seen across its entire front-end bandwidth, so there is less
thinking
involved.
The trouble is, any good shaper or ZCD will have very low jitter, perhaps
too low for even a Wavecrest-class TIC to measure. This is what Wenzel's
quick-and-dirty differential amp with a pair of 2N3906s looks like, when
the
splitter test mentioned by Bob is performed with a TimePod, TSC or other
phase noise analyzer:
http://www.wenzel.com/documents/waveform.html
http://www.ke5fx.com/wenzel_shaper_resid_jitter.png
That's about 100 fs of additive jitter, measured between 0.1 Hz and 100
kHz.
Because the broadband floor is relatively high, a great deal of the total
jitter comes from the higher decades. (The circuit's jitter contribution
between 0.1 Hz and 100 Hz is only about 10 fs.)
A counter will not be limited by the 100 kHz or 1 MHz integration range of
a
TimePod or TSC 5120, so you might see enough jitter to be noticeable on a
Wavecrest in the 1 to 10-ps neighborhood. But maybe you only care about
jitter at lower offsets... in which case the counter will make your shaper
look a lot worse than it really is.
For instance, if the reason you're investigating ZCDs is because you want
to
build a DMTD, then you may be more interested in a residual ADEV plot
instead. The pair of bipolars contributes white and flicker PM noise, so
its residual ADEV at t=1s isn't too different from the residual jitter in
the ADEV measurement bandwidth, which was 500 Hz in this case:
http://www.ke5fx.com/wenzel_shaper_resid_ADEV.png
It's worth noting that I made these measurements on a TSC 5120A. The
phase
noise measurement could have been made on a TimePod, but the residual ADEV
plot could not, as it's below the TimePod's ADEV floor.
To me, this says that there are better ways to spend one's time than
designing a fancy multistage ZCD. The important thing is to consider how
much bandwidth is really required in your application, and whether/how it
should be limited.
-- john, KE5FX
Miles Design LLC
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