once upon the time at Gigatronics we compared logic devices noise and
found that TTL were the quietest
73
KJ6UHN Alex
On 5/20/2015 3:15 PM, Richard (Rick) Karlquist wrote:
On 5/20/2015 11:22 AM, Magnus Danielson wrote:
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.
The counter front ends seem to be modeled after scope front ends
and scope triggering circuits, where you can adjust the triggering
level. Any jitter in the triggering would normally only affect
the interpolator. The interpolators in general were no great shakes,
so the triggering wasn't the limiting factor.
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.
Magnus
_______________________________________________
ECL is bad because the voltage swing is low; because as you say,
a lot of the circuitry is in the active region all the time, and
because the current source in the emitters generates a lot of
noise.
In the early 1990's, I thought I had proved that the high ECL
noise was mostly common mode and that you could reduce it
20 dB by using a transformer to couple the output. Alternately,
a good differential amplifier with high CMRR would do the trick.
I had actual measurements to back up this theory.
Subsequently, other people tried to reproduce this and could not.
By that time, I had moved on and didn't have the bandwidth to
continue to own the problem.
It would make a nice project for some time-nut to prove or disprove
my hypothesis regarding ECL.
ECL line receivers as squarers are not as bad as comparators, but
are much noisier than 74AC.
Rick Karlquist N6RK
Rick Karlquist N6RK
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