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

[John Miles]

> > 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.

> 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.

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