John,
I admit to not being aware of the Pepper patent until you
mentioned it. What I suggested in my post was exactly an
interrupted ramp generator.
As you mnetion, the issues are related to : the integrator switching
states ( integrate/hold/reset ), integrator errors such as droop or
offsets in the hold mode, discriminator switching jitter, prop delays
through logic and drivers, etc.
To the extent that these errors are constant with time, temp,
operating
conditions, they imply a bias (offset) in the produced time
interval.
Leakage into the "hold" cap is the big nasty. It's not a lot of pF,
but you may want to freeze it for a long time. The SRS box has the
same problem, essentially sample-and-hold leakage, which gets nasty
for longer delays. In our design the clock-to-trigger information is
stored digitally, so doesn't leak.
The potentially nasty situation is when the trigger signal comes
synchronously with the local oscillator active edge, and the system
may add ( or swallow) a whole clock period. That's why the
synchronizers are needed.
The Pepper scheme adds flats within the ramp, never at the start, so
clock-to-trigger alignment doesn't matter. Of course, you have to
avoid metastability with a double-rank synchronizer or something, to
decide when to insert the flats, but that's no big deal. If you make
the ramp many (say, 5 or so) clocks long, there's no rush to let
things settle and do the logic. Short delays are pure analog ramps, no
clocked pauses at all. Very clever.
<-------- net delay ----------->
+ < compare == done
/
/
/
/
____________________/
/ kill N clocks
/
/
/
/
/
________________________/ < trigger
Pepper was a better inventor than circuit designer. I have a
reverse-engineered schamatic of his original box, and it's a
nightmare. He floated most of the ECL logic 22 volts above ground, so
it could directly switch current sources. Imagine debugging that!
And the way he did output pulse widths was ghastly. A pot jams Vbe
directly into a transistor, which becomes the current source for
another ramp. Wide range log control on the pot!
EG&G reportedly cleaned it up a few years back, but I don't know how
extensive that was. EG&G was absorbed by Perkin-Elmer, who then spun
off Signal Recovery, who still makes the box.
Thanks, Jure Z.
PS : where is the OP , while we talk about "his" problem ?
OPs often disappear. But we can carry on!
John