[time-nuts] Divider circuit for Rubidium Standard

Bob Camp kb8tq at n1k.org
Thu May 21 06:49:15 EDT 2015


Hi

> On May 20, 2015, at 11:27 PM, Alex Pummer <alex at pcscons.com> wrote:
> 
> once upon the time at Gigatronics we compared logic devices noise and found that  TTL were the quietest
> 73
> KJ6UHN Alex

Before the 74AC stuff came along, some flavor of TTL was the best bet. With TTL you needed to be a bit 
carefull about just what family (and in some cases manufacturer) you used. All that picky stuff has pretty much
gone away with fast silicon CMOS, at least among the major outfits. 

Bob

> 
> 
> 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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