[time-nuts] What is a Time-Nut grade Zero Crossing Circuit?

[Richard (Rick) Karlquist]

John Miles wrote:
>>> Modern ECL parts aren't necessarily that bad compared to the old MECL
>>> stuff.
>> My experience goes all the way back to the MECL 1000 series that was
>> discontinued 30 years ago.  I designed many synthesizers around them
>> for Zeta Labs.  Every newer family of ECL line receivers has been faster
>> and had worse phase noise, in my experience.
> 
> Which is odd because the jitter specs have gotten better -- at least, going
> by the promises and hype in the data sheets.  It sounds like the newer ECL
> parts' wider bandwidth is folding more noise into the output signal.

The older parts had no jitter specs.  Jitter specs assume a logic
waveform input, not a sine wave input.  Many jitter specs refer to
pattern jitter of data, which does not apply to clocks.  Also, jitter 
increases at low frequencies in practice, even though in theory it 
should not.  Like I said, this topic is very tricky.

Rick Karlquist N6RK



> 
>> This is a very tricky topic.  When measuring the phase noise of a non
>> sine wave, there are dependencies on how the measurement is done.
>> What is the measurement bandwidth?  Etc.
>>
>> In some cases, the noise is mostly common mode, and therefore will
>> depend on the common mode rejection ratio (if any) of your measurement
>> circuit.
> 
> I'm measuring it with a 3048A, feeding the DBM directly from one of the
> MC100EL16P's output pins via a 0.1 uF cap.  Both output pins are tied to
> ground with 200 ohms, per Q12 at
> http://www.pulseresearchlab.com/faqs/ecl_ques/ecl_Q9-Q12.htm .
> 
> Input-wise, I just tried a T1-1 balun instead of the single-ended
> termination I was using before, and got exactly the same results (floor at
> circa -148 to -150 dBc/Hz at 100 MHz, but only -140 dBc/Hz at 10 MHz.)
> That, I thought was interesting.  -148 dBc/Hz was always the 'rule of thumb'
> for the older ECL families from what I've read, and since it's not sensitive
> to input configuration or power-supply bypassing, it must be the process
> floor.

I measured phase noise on an MC100E131 drop from -145 to -165 by taking 
the output differentially.  This was done in 1992.  Others have
been unable to repeat this result, even using some old 1992 parts I
kept.  So who knows?  Todd Pearson of Motorola said that they have
seen this drop with differential outputs.


> 
> There was no LC or other bandpass filtering at the input, but the sources
> are decent-quality OCXOs in both cases so I don't think I'm feeding it too
> much broadband noise to begin with.
> 
> Maybe another T1-1 at the output would help, but I don't see any reason to
> think so.
> 
> -- john, KE5FX
> 
> 
> 
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