Hi I agree that the limiters may not actually be "20 ns good". It's certainly well worth checking. I'm also thinking that there may be a compromise in the limiter chain to reduce the phase noise issue. What I'm trying to do here is come up with a "noise floor" number that's adequate before I start.
The isolation amp, mixer, and phase shifter all operate at RF, so a picosecond change does indeed get me a 1x10^-12 at the output. The limiter operates at audio, so I've already got the down conversion so a picosecond there is less of an issue. Of course the first limiter is going to be a whole lot more time unstable than the RF stuff. What I'm getting around to is that the counter really does not have to be a SR-620, or even a 5335 to do the job. The problems lie elsewhere. A time tagging FPGA with a 100 MHz clock would do the counting job quite nicely. It also would not be terribly hard to build. I'll grant a 10 or 100 ps/C delay variation with such a gizmo, but it runs at audio, so it's after the 1x10^6 downconversion gain. Running time tags also takes care of issues like measuring the actual beat note frequency. Here's my guess for temperature stability of the setup; 1-10 seconds < 0.1 C >10-1000 seconds < 0.2 C >1000-10,000 seconds < 0.4 C (that may be a stretch) > 10,000 - 100,000 seconds < 1 C That would give me: Mixer : 1ps to 10 sec, 2 ps at 1000 sec. Phase shifter: half the mixer if I use a switch on the transformer for inversion. Isolation amps: something to look at That would give me a limit from mixer and phase shifter of: 1.4x10^-12 at 1 sec 1.4x10^-13 at 10 sec 2.8x10^-14 at 100 sec better than 1x10^-14 at 1000 sec and beyond I suppose that if those numbers were 10X worse than that once the rest of it shows up, I would stabilize the temperature of the setup. So what's still missing? Bob On Jan 24, 2010, at 8:43 PM, Bruce Griffiths wrote: > Bob Camp wrote: >> Hi >> >> I realize that this is a bit off topic from the flow of the last few days. I >> can only claim temporary insanity. Any comments about the temporary modifier >> in that sentence being unneeded will of course be ignored... >> >> Assuming that: >> >> 1) I have a DMTD setup of the "basement engineering" variety. >> 2) The beat note is> 5 Hz and< 10 Hz >> 3) The DUT's are all worse than 1x10^-12 at one second tau (no hydrogen >> masers in the basement) >> 4) The offset oscillator is at least 2x10^-11 at one second tau. >> 5) The DUT's all put out 10 MHz >> 6) My counter will resolve 10 ns (= I could do better) >> 7) The limiters are good enough to not be an issue relative to the counter's >> 10 ns. >> 8) The zero crossings are phase shifted to be close, but not so close I arm >> after I start during a run. >> 9) Regardless of the tau involved, nothing I'm looking at will be better >> than 1x10-14 >> >> My down conversion from 10 MHz to 10 Hz gives me a 10^6 multiplication. >> 10 ns is a part in 10^8 at one second. It's a part in 10^7at 0.1 second (10 >> Hz). >> First order, I should be able to hit (7+6 = 13) a part in 10^13 at less than >> 1 second. That's significantly better than the DUT's. I don't need anything >> better in the counter or limiters to measure what I'm looking at. Even if >> the limiters are 2X worse than the counter, I'm still at the don't need >> better level in terms of counter and limiters. The offset oscillator is >> going to cause some second order issues regardless of the limiters and >> counter, but it still should be "ok". >> >> Next up: >> >> If I phase shift one of the DUT's by 360 degrees, the beat note does the >> same. All I need is 100 ns of phase shift to get everything lined up. I >> could do it with 180 degrees of shift and an phase inversion switch. >> >> I'm assuming (phase shifter and DMTD stuff) can fit it all in a 2x4x8" box >> - I don't need a new bench to hold it all ... >> >> So what did I miss? >> >> >> Bob >> >> >> > Once you have built the DMTD you need to measure its noise floor. > > How do you ensure that the limiters actually achieve a jitter better than > 10ns? > With a < 10Hz beat frequency this is actually quite difficult to do given, > typical mixer and amplifier noise. > Low frequency ground loop noise can be a major problem with low frequency > beat signals. > > Some limiting factors for long tau: > > 1) Mixer phase shift tempco (can be as large as 10ps/C) > > 2) Limiter phase shift tempco (principally determined by phase shift tempco > of first stage filter). > > 3) phase shifter tempco > If you use coax the tempco for 100ns delay may be as large as 10ps/C. > > 4) Delay tempco of RF isolation amplifiers required to prevent injection > locking. > > Another factor not often considered with DMTD systems is the effect of phase > noise aliasing. > The limiter bandwidth of a typical DMTD necessarily exceeds the Nyquist limit. > > RF shielding between the 2 DMTD channels to avoid crosstalk and injection > locking is important. > > Bruce > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
