[time-nuts] OT: DMTD Question

Bob Camp lists at cq.nu
Mon Jan 25 02:47:49 UTC 2010


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. 

The isolation amps are common base buffers. Not much gain, but quite a bit of isolation. Phase shift / C - need to look into that.

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. 

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. 

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. 

Crazy Stuff ....

So what did I miss that time?


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
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