[time-nuts] OT: DMTD Question

[Bruce Griffiths]

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