[time-nuts] OT: DMTD Question

Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Jan 25 03:10:47 UTC 2010


Bob

Whilst one can use the limiter design spreadsheets perfected by Roberto:
http://www.ko4bb.com/~bruce/ZeroCrossingDetectors.html 
<http://www.ko4bb.com/%7Ebruce/ZeroCrossingDetectors.html>
(bottom of page)

One needs to know the mixer phase noise spectrum in the vicinity of the 
beat frequency (not specified by most manufacturers) and its gain in 
order to design an "optimised" limiter.

This can be done using a low noise preamp together with a sound card and 
suitable spectrum analyser software.
The noise and gain need to be measured with the proposed mixer IF port 
termination.
Using a capacitive IF port termination can significantly reduce the 
mixer output phase noise.

Using a mixer that allows the IF port ground to be isolated at low 
frequencies from the RF and LO grounds is useful in minimising the 
effects of ground loops.

Mixers characterised for use as phase detectors tend to have lower drift 
than other mixers.

Your estimate for phase shift tempco of the limiter and associated 
filters is wildly optimistic.
If one uses a single pole RC low pass filter using 100ppm/C tempco 
components then its equivalent phase shift tempco (at the mixer RF 
input) will be around 0.8ps/C if the filter 3dB is equal to the beat 
frequency.
If one increases the filter cutoff frequency its output noise increases 
as its equivalent phase shift tempco decreases. Finding capacitors with 
lower tempcos will be problematic given that the capacitance must be 
relatively high to keep the noise from the series resistor down.

Ensuring that the the temperatures of the limiter input filters track 
will reduce the differential tempco.

Low frequency isolation of the counter drivers grounds may be necessary 
to avoid crosstalk from the large currents used to drive the counter inputs.
This may require using a digital isolator of some kind.
The jitter of the isolator may be significant.
Suitable low jitter optical isolators are available.
The delay tempco of such an isolator may also be an issue.

The phase shift tempco of a well designed RF isolation amp should be 
less than 2ps/C.
SpectraDynamics claim about 1.5ps/C.

Bruce

Bob Camp wrote:
> 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
>>
>>
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>>      
>
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