just look what is happening in one amplifier which happened to be a bit
to slow for for the passing trough signal. The part of the spectrum --
which can't make it -- although will not show up at the output, will
not disappear, but --if the amplitude is large enough -- overdrive
parts of the amplifier, causing recovery effects and delays. the delay
will influence the propagation of the desired signals too..... That is
particularly important if the desired signals position on the time
range is important.
Also, by comparing the structure of a comparator and an amplifier it is
very good visible, that a comparator, has much less stages which have
all relative high bandwidth and lower gain, and the dominant pole --
which at the mentioned amplifier [LM358 ] is not much higher than 5
[five] Hz, yes you could see it here:
http://www.onsemi.com/pub_link/Collateral/LM358-D.PDF
It could happen that the signal coming out from a miss-used amplifier
look very nice, but it is very likely, that it is not exactly correct,
and there is a good reason, why the solid-state industry produce not
just amplifiers but also compactors, which would need pre-filtering, but
provides the correct output signal
73
KJ6UHN
Alex
who used to be corporate applications engineer at ON Semiconductor,
Intersil Corporation and Elantec
On 4/12/2016 2:00 PM, Charles Steinmetz wrote:
Nick wrote:
At one point, I did try an LM393 instead of a 358. The result was
that noise caused excessive false triggering. The 358, so far as I
can tell, when acting as a comparator lacked sufficient bandwidth
and/or speed to keep up with the noise.
My results also seemed to be on a par with the published results of
other similar investigations (particularly those of tvb).
I'm on record as being in favor of pre-filtering to separate true grid
phase artifacts from local noise, as much as practicable. However, it
would be serendipitous indeed (more like miraculous, actually) if the
slew-rate limit of an LM358 just happened to be exactly the right
degree of filtering. Close enough for a science-fair project,
perhaps, but not a time-nuts-level solution.
Tom uses a Schmitt trigger input (which, as I pointed out yesterday,
guarantees that the "non-zero-cross detector" [or,
"zero-cross-by-proxy detector"] will have AM to PM conversion), and
(last I knew) he does not filter the input (other than the incidental
interaction of the input resistance of his pickup with the input
capacitance of the gate). A proper comparator with a 0v threshold and
a few mV of hysteresis, preceded by carefully designed filtering, can
generate a ZCD output with substantially lower cycle-to-cycle and
second-to-second phase errors due to local noise that is a
significantly more faithful representation of the actual grid phase
and frequency. If what we're interested in is measuring the grid
phase and frequency, rather than the incidental local noise that has
nothing to do with the grid phase and frequency, this is the clearly
better approach, IMO.
Best regards,
Charles
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