Hi Again more or less in order:
I'm trying to keep things as simple as I can at least to start. That rules out the clean up loop oscillator at least in the beginning. It is a good idea, and eventually I'll probably put one in. I guess I'm going to need to do some looking on transformer feedback high isolation amps. Everything I've seen so far on hight isolation has been straight / no feedback stuff. The loading at RF on the mixer does reduce the audio output, but it improves the isolation / match on the mixer. You trade one for the other. Looks like some kind of local temperature stabilization might be a good idea for the audio band limiting stuff. It's after the down convert, but some of the time constants are indeed very long. I suspect that silica dielectric cable is outside the budget constraints on this project. Cheap foam coax in a spool on the floor or tiny stuff in the box, possibly with better temperature control are about the only two choices. --------- Another very real choice is to simply move the goal post a bit. Pushing the 1x10*-12 point to 10 seconds from 1 second could turn out to be the only economical basement alternative. Off to bed .... Bob On Jan 24, 2010, at 10:26 PM, Bruce Griffiths wrote: > Bob Camp wrote: >> Hi >> >> More or less in order: >> >> The beat frequency is coming out of a rubidium. Hopefully it's fairly >> stable. It won't be super quiet for 1 or .1 second tau. It looks like the >> counter will be a FPGA time tagger, so the beat note frequency will drop out >> for free. >> >> > A cleanup loop may be useful to improve the offset source short term > stability. > The cancellation of offset oscillator noise in a DMTD is imperfect. >> The isolation amps are common base buffers. Not much gain, but quite a bit >> of isolation. Phase shift / C - need to look into that. >> > You can achieve similar isolation together with lower noise and distortion > with a transformer feedback CE stage. > Transformer feedback CB stages have even lower noise coupled with low > isolation, however they can be useful for amplifying low level signals ahead > of a high isolation amplifier. >> Mixer loading likely would be as I've done it before. Resistive termination >> at RF and fairly high impedance at audio. Resistor here and there to improve >> the match at RF. LC filtering adequate to suppress the RF stuff on the >> output of the mixer. Single pole R-C for audio bandwidth control. Big >> capacitors and small resistors for low noise. >> >> > That's one of the worst terminations possible from the noise perspective. > To lower the noise its essential to reflect the sum frequency back into the > mixer. > Resistors in series with the mixer LO and RF inputs will then be required to > improve the mixer input VSWR. >> Until I've measured them I'm not sure of the floor of the limiters. Before I >> get into that I want to be fairly sure I'm not over spec'ing them. If 100 ns >> is as good as 3 ns it's not as hard a problem. >> >> > You can take the published phase noise for unspecified mixers as a lower > limit. > The noise in the flicker region for the mixers (eg those from minicircuits) > that use integrated diode quads may be somewhat higher. > Initial measurements on a HP10534B (uses discrete diodes) appear consistent > with the typical noise specs for a low level mixer. >> The issue of the group delay is an interesting one. I believe that people >> have been getting good results with coax line for the phase shift. I'm a bit >> conflicted on the coax. 15 meters of small diameter stuff will fit in the >> box (maybe), but it's not super stable.. If I go foam coax then the phase >> shifter gets pretty big. If I go with some kind of LC setup, temperature >> stability would likely be an issue. >> >> > NIST's measurements indicate that lowest delay tempco is achieved with a > powdered silica dielectric. > Specialised fibres can have very low delay tempcos. >> Crazy Stuff .... >> >> So what did I miss that time? >> >> Bob >> >> > Bruce >> >> >> On Jan 24, 2010, at 9:01 PM, Magnus Danielson 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? >>>> >>> Remember that you *must* measure the actual beat frequency, since you will >>> need that to calculate the beat-gain. If it is between 5 and 10 Hz >>> the for a 10 MHz source your gain is 2E6 and 1E6 respectively, which is a >>> factor of 2 difference or 6 dB. So, your measurements will be inprecise >>> from that factor alone by +/- 3 dB. The remedy is fairly easy to come up >>> with, measure the input frequency and beat frequency for each arm. The best >>> thing is naturally to ensure that the beat frequencies of both arms is >>> fairly close. EFC steering of either source may work well enought in >>> open-loop mode during measurement (with the added benefit of not do >>> spectral interference with the phase noise which locked loop does). >>> >>> How do you control the input levels to the mixers? >>> >>> Do you have any isolational amplifiers? >>> >>> How do you load and pre-filter the mixer outputs? >>> >>> You haven't convinced me of the expected performance of the limiters. >>> >>> I'm not sure it will be your biggest problem, but the way you phase-shift >>> can be of importance for the decorrelation loss. >>> Phase-shifting such that group-delay moves noise in time will be >>> problematic, since then the decorrelation gain of having phases coincide >>> will be partly lost since it is the group-delayed variant of the transfer >>> oscillator against the current-time transfer-oscillator (both delayed by >>> each detector arm, but only differnces is important). Vector-adding phase >>> delays could work around that. The optimum delay setting for cancelation >>> may not be to fully phase-adjust the leading edge. >>> >>> That is what just popped up in my head at least. >>> >>> Cheers, >>> Magnus >>> >>> _______________________________________________ >>> 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. >> >> > > > > _______________________________________________ > 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.
