The dual 123 plus one gate package - two cheap cans - clearly wins
over the ECL idea.
And a 221 beats a dual 123 in any application where you don't need to
retrigger the monostable. As Jim Thompson pointed out, the 123 is
crummy monstable. He didn't bother to point out that 121 is a whole
lot better if you don't need to retrigger.
The advantage of the ECL part lies in the quality of the pulse edges
it generates, and the stab\ility of the delay, particularly against
power rail noise. The 121 and 123 are essentially analog comparators
looking at a relatively slow ramp. Any noise on the ramp or the
voltage the comparator is using as a reference create a lot more
jitter than the same power rail noise would create in the ECL system,
and the power rails in an ECL system are pretty much guaranteed
quieter than the power rails in a TTL system.
And the ECL system does lend itself to self-calibration schemes, where
you calibrate the delay generating engine from time to time by getting
it to produce a pulse-width modulated waveform, where the repeat time
is controlled by a much more stable clock - which could be derived
from an off-air standard, traceable back to something at the local
National Bureau Standards.
Here you could go for a 3MHz repeat cycle and use the delay engine to
vary the high time from say 35nsec to 285nsec. Digitise the filtered
DC content of the waveform and you've calibrated the delay engine
against the 3MHz clock.
In practice you'd derive the 3MHz clock from a good 10MHz clock and
generate two additional 135nsec and 235nsec "high" period waveforms by
adding in one or two periods of the 10MHz clock, allowing you to
interpolate between exactly known PWM waveforms and the waveforms
being calibrated.
You can find and calculate out more subtle errors by repeating the
procedure with slower clocks - say 2.5MHz and 2MHz.
The Micrel delay line chips (SY89xxx) are better than the Onsemi ones,
but still not a very good way to do this.
It would be a whole lot better way - if more expensive and complicated
- if you designed it right.
This would work, for both the 2 Hz ticker and the pulse generator:
http://www.highlandtechnology.com/DSS/T560DS.shtml
Perhaps. Does it do any self-calibration? TTL is a a bit of a problem
if you are serious about getting accurate low-jitter timing signals.