Rick,
On 05/20/2015 01:10 AM, Richard (Rick) Karlquist wrote:
On 5/8/2015 2:19 PM, Bob Camp wrote:
On May 7, 2015, at 11:09 AM, Attila Kinali <att...@kinali.ch> wrote:
On Wed, 06 May 2015 18:09:03 -0700
"Richard (Rick) Karlquist" <rich...@karlquist.com> wrote:
A standard input on a frequency counter is not a very demanding
thing in the hierarchy of
TimeNut signals. You can drive any of them with some pretty simple
logic gate based
circuits. No need to spend a lot of money.
Logic gate, yes. Comparator, no.
This reminds me a lot of a similar discussion a couple of weeks ago.
(where the issue boiled down to noise bandwidth)
What is the problem with a comparator vs a logic gate?
What makes the logic gate supperior?
Attila Kinali
The comparator as a squarer circuit is folklore that unsophisticated
users want to believe in, because it is seemingly the easiest way
to get the job done. Wouldn't it be wonderful to be able to put in
any signal from -30 dBm to +15 dBm and get a perfect square wave
out with no effort? Unfortunately, what a comparator looks like
is a very high gain differential amplifier that is slew rate limited.
The threshold voltage input must be extremely low noise or it
will introduce jitter. Even if the input pin is clean, there is
internal noise. Driving it will a low level signal will produce
a jittery output for obvious reasons. The trouble is that if you
drive it with a high level signal, the jitter doesn't go away because
the input stage is already in saturation. Also, the effective noise
figure of the comparator is usually high. Making the comparator
faster exacerbates the problem. Read papers on "zero crossing
detectors" such as John Dick's 1990 paper in PTTI and you will
see that a comparator is the exact opposite architecture from
the optimum one. I hope that clears up the question.
The older HP counter manuals explained it very nicely too, as they
illustrated the slew-rate & amplitude noise to time-noise conversion.
What do amazes me is the fact that I've yet to see a counter input
channel which takes care to square up the signal properly, they rather
provide the comparator after the obvious damping and AC-blocking
conditioning. I can't even recall that there where much such shaping as
a side-product.
Regarding logic gates: it is not so much that there is something
magic about gates; actually ECL gates are lousy. It is just that
comparators are so bad that almost anything else is better.
Now, remind me why ECL is lousy, I can't recall there being very high
gain in them, but fairly high bandwidth and they stay in the linear
operation region.
PS. Sniffed the heat from a 1979 ECL based PM6674 counter as I was doing
some checkout before put it in the hands of a friend.
Cheers,
Magnus
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