[time-nuts] Divider circuit for Rubidium Standard

Richard (Rick) Karlquist richard at karlquist.com
Wed May 20 22:15:12 UTC 2015



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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