[time-nuts] DMTD phase shifter

EWKehren at aol.com EWKehren at aol.com
Sat Jul 25 13:17:38 UTC 2009


One more comment the phase shifter is not included in that calculation.  
Bert Kehren
 
 
In a message dated 7/24/2009 11:30:22 P.M. Eastern Daylight Time,  
bruce.griffiths at xtra.co.nz writes:

EWKehren at aol.com wrote:
> On the chance that I may be totally  wrong let me suggest the following. 
In  
> my opinion you are  looking at the wrong information. When you mix a 10 
MHz  
> signal  with a signal offset by 1 Hz you are subtracting from a 10 E 7 
> signal  10  E 7 and as a result get 1 Hz representing 10 E 7. Counting it 
with a  
> counter  with 1 ns resolution, you in effect get at the last  digit 10 E 
16. 
> Obviously  some of the last digits are irrelevant  due to noise and 
trigger 
> error. Using two  mixers with the  common offset oscillator you want the 
two 
> phases to be as close   as possible to reduce the noise contribution of 
the 
> offset  oscillator. That is  why you want the phase shift. Measuring the 
time  
> difference between the zero  crossings in my opinion is not the  way to 
go.
> I am inspired by the simplicity of the NBS unit (thank you  Corby for the 
 
> info) to lay out a PC Board; reading some of the  comments I would 
appreciate 
> any  links to more info on that  subject that may make the design more 
> effective. Any  help would  be greatly appreciate. 
> Bert Kehren  WB5MZJ
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Bert

With a  classical dual mixer system, the only useful measurement is the
time  difference between the zero crossings of the 2 beat frequencies.
The  limiters used distort the signal and amplify the zero crossing slew
rate,  thereby precluding meaningful measures of anything else but the
zero  crossing times.
Time stamping the zero crossing times for each channel may  be more
useful rather than just measuring the time differences.

The  zero crossing circuit employed by NIST is far from optimum (Oliver
Collins  paper on the design of low jitter hard limiters wasn't published
until May  1996), some of the JPL designs are better but are still some
way from  optimum.
The timing jitter and phase drift of the ZCD output limits  performance.
JPL achieved a zero crossing jitter of around 40ns in their  1Hz beat
frequency ZCDs, it should be possible to reduce this by a factor  of 3 or 
so.
There later DMTD systems use a beat frequency of around 100Hz  or so with
a 100MHz mixer input frequency.
The zero crossing detector  filter component tempcos will limit the phase
shift stability for larger  tau as will the mixer phase shift tempco (~
ps/C).
It is also essential  to avoid low frequency ground loops that affect the
2 zero crossing  detectors.
The mixer IF port grounds need to be low frequency isolated from  the RF
grounds of the other 2 ports.
One needs to know the phase  detector output noise in the flicker region
in order to optimise the zero  crossing detector design.
Unfortunately phase detector flicker noise  specifications don't appear
on the datasheet so one has to measure  it.
One can do this using a low noise preamp for a sound card or a  spectrum
analyser.
Using an apprpriate mixer IF port termination (a 50  ohm or even 500 ohm
resistive termination may be far from optimum) will  affect the beat
frequency signal jitter.

If one eliminates the  limiters (however the distortion probably needs to
be kept down) then, in  principle a COSTAS receiver could be used to
measure the phase shift  between the beat frequencies independently of
the individual beat  frequencies and proximity to a beat frequency zero
crossing. However ADC  noise will limit the performance unless one uses 2
or more ADCs per beat  frequency and uses correlation techniques to
eliminate the ADC  noise.

Bruce


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