[time-nuts] Divider circuit for Rubidium Standard

[Bob Camp]

Hi

> On May 22, 2015, at 4:48 AM, Gerhard Hoffmann <dk4xp at arcor.de> wrote:
> 
> Am 21.05.2015 um 23:32 schrieb Magnus Danielson:
>> 
>> 
>> On 05/21/2015 12:15 AM, Richard (Rick) Karlquist wrote:
>>> 
>>> 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.
>> 
> Could we agree on a test procedure?
> 
> A friend of mine did some tests for synthesizers in mil. avionics and he told me
> that Motorola's MOSAIC3 process was the worst thing that has hit the planet wrt
> phase noise. That was used for a lot of fast ECL. (Some years have passed since
> he made the test.)
> 
> Comparators have their advantages, too. At least, someone has been thinking
> about dispersion, constant flowthrough time against frequency and overdrive;
> there are even specs that include overdrive. Just that comparators can switch
> cleanly at mV levels does not mean that they are to be used that way.
> 
> More gain may mean more noise voltage, but it also means less time spent
> in the transition region. Once the decision has been made the noise is squelched
> anyway.  And I prefer setting the bandwidth with thin film Rs and np0 capacitors,
> not with oversized junctions.
> 
> The fairest shootout between the logic families that we have is the LTC6957.
> 
> < http://cds.linear.com/docs/en/datasheet/6957f.pdf >
> 
> Probably just bondout options of the same chip. The PECL version wins
> hands-down, LVDS is worst and CMOS is in-between.
> 
> Especially at low offsets PECL is best, that clearly contradicts the
> above-assumed 1/f problem and the lower swing standard of today
> comes out worst.
> 
> regards, Gerhard
> 
> 

The way I’ve tested most of this is with a TimePod or something similar. You start with 
a power splitter of some sort (active or passive) and feed one input to the tester off of one port. The DUT
connects to the other port. Once the signal has flowed through what ever circuits you 
have, it’s output goes to the other input of the tester.

In the “good old days” the test had to be done with two pieces of gear. You ran one set
of tests with a short term stability box. You ran another set of tests with a phase noise
analyzer. The nice thing about the more modern gear is that you can run both tests
at once. That’s nice when you consider that the tests run for > 10 hours  in many cases.

If you want to go *really* old school, you can run a single mixer setup for the phase noise
and a DMTD for the short term. That approach works (been there done that). You do 
need to start by testing some amplifiers. 

I’ve found that the results make more sense if you use a good signal source for the testing. 
An OCXO is generally a good idea. If you have a source with a lot of AM noise, you will spend
time learning about AM to PM conversion …

What to expect: 

Phase noise @ 100KHz can / might get down below -190 dbc Hz. Short term can / might 
hit 1x10^-14 at 1 second. You will need some pretty good setups to get that data. The bad
news is that those numbers are not the bottom of the range :)

You probably will spend some time checking things like square to sine converters. In some 
cases they will be pretty easy in others they will turn out to tell you a lot about how your HVAC
system is running overnight. 

Bob

> 
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