[time-nuts] Divider circuit for Rubidium Standard

[Magnus Danielson]

On 05/21/2015 12:15 AM, 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.

Depends on the signal.

>> 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.

Yes, it is bound to have 1/f noise with it's 50 Ohm current load.
I was thinking about the continuous current, as I do know of the gating 
effect. Today there is other interface standards having lower swings 
than ECL.

> 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.

Interesting.

Don't have a lot of ECL lying around, but some toys that might measure 
things.

Cheers,
Magnus