[time-nuts] ? phase comparison or other device

Pete peterawson at earthlink.net
Tue Jun 26 22:38:13 EDT 2007


A few final thoughts.

1. Thanks for the critical view; it does help.

2. Like many time-nuts I have a reasonably good 10MHz source &
sometimes need to check out a newly acquired OXCO to ensure
it can muster 1E9 or 1E10 performance (with 10x headroom).
An SR620 would be ideal, but it's just too many $$ ;even used.
I expect "casual" participants of time-nuts already have a basic,
decent counter e.g. HP5335A & a basic decent synthesizer
e.g. PTS040, Fluke6060(?), HP3335x or 6x. Also, I assumed a
coaxial level 7 mixer & suitable lowpass filter would be available.

3. I read the JPL paper (more than once) & developed the first three
stages (modified for 1KHz bandpass) per their process. At that point
the measured jitter was well under 1ns rms; which was enough to enable
1E12 resolution for 10MHz sources. I deliberately choose the ADA4899-1
opamp since it's characterized for 5V operation, low noise, fast & cheap
enough ($4.30/ea). It was apparent that even with 2 stages the ZCD
was still under 1ns jitter; the risetime wasn't blazing, but it was 
good enough.

4. Without PCB capability (at home & now retired) even this simple
circuit is tough to build; each part adds significantly to the effort when
doing 1-up. So I examined the need for every part in an effort to
minimize parts count, but retain jitter performance. I found that the
opamp overload recovery was more than fast enough to discard the
limiting without measurable deterioration in jitter. Lots of parts went
away; construction became easy.

5. I went TOO FAR. The opamps I had exhibited such low offset that I
DC coupled without thinking about it. WRONG answer (as you noted),
Rookie mistake. I have shown the AC coupling & 2nd stage feedback
resistor in the revised circuit.

6. The ZCD costs <$20 for parts & about 2 hours to build/check out.
It performs well enough to look at stable sources to 2 parts in 1E12
in 50 to 100 seconds and be confident in the data. The noise floor is
easy to measure & verifies functionality.

Is it "well designed" ? NO. Could it be (much) better? Certainly.
Does it work well for it's intended purpose? Yes.

My assumptions about equipment may be out of line. In my case, eBay
supplied everything, except the mixer, filter & ADA4899-1s, so this
effort didn't require much in the way of extra $$. It does what I wanted.

As previously observed, the mixer should have a diplexer between it and
the filter for the mixer higher order products to be terminated properly.
I examined the filter input Z, as terminated, and found it to be from 150
ohms inductive to 1200 ohms inductive from 10 to 30 MHz. This suggests
the use of a feedthrough termination of around 100 ohms as a first order
fix. Using a 93 ohm feedthrough, no improvement, or degradation in results
was noted. This could use more study.

More information about the time-nuts mailing list