[time-nuts] DMTD phase shifter

EWKehren at aol.com EWKehren at aol.com
Sat Jul 25 14:34:06 UTC 2009


Bruce, what I envision is two identical channels made up of offset  
isolation amplifier, mixer/filter/amplifier and  zero detector/counter module. The 
Isolation and Mixer modules  would be copies of the NIST circuit using later 
components. The two counter  modules talk to a separate counter/u processor 
through opto isolators. The fifth  counter is part of that module and uses 
info from the two counter modules to  also determine the phase difference 
between the two oscillators being monitored.  This seventh module also handles 
the USB interface. If you want to I will  redraw the block diagram and a 
detailed circuit of the counters and send it to  you direct.   Bert
 
 
In a message dated 7/25/2009 9:37:25 A.M. Eastern Daylight Time,  
bruce.griffiths at xtra.co.nz writes:

Bert

If you intend to use a USB interface then its probably  essential to use
an optical isolator or equivalent between the zero  crossing detector
(ZCD) output and the counter input to minimize potential  low frequency
ground loop problems. JPL found that low frequency ground  loops limit
the performance if the zero crossing detector outputs aren't  isolated
from each other.

A resolution of better than 1E-14/tau  should be relatively easy to
achieve with a good ZCD design.
Maintaining  such resolution for long tau will be dependent on the
stability of the  mixer and ZCD temperatures.
Mixer drift can be as large as 10ps/C or more  at 10MHz.
The tempco of the low pass filter components in the first few  stages of
the ZCD will dominate the ZCD phase shift tempco.

Using a  linear amplifier and filter in front of a zero crossing detector
built into  the counter is not the best way to implement such a system.

The  amplifier filter limiter chain should be designed to suit the  offset
frequency.
The goal is to increase the zero crossing slope to a  point where the
noise of the counter input circuitry doesnt contribute  significant jitter.
To do this linear amplification is counterproductive,  every stage of
slope amplification needs to incorporate a low pass filter  and clamping
to reduce the output noise whilst increasing the output slew  rate. As
long as the signal level is sufficient to ensure that each stage  is
driven well into limiting, linear amplification serves little  purpose
other than increasing the noise by more than  necessary.

Bruce

EWKehren at aol.com wrote:
> Bruce, Thanks  for your comments. It is my opinion that a dual mixer 
system  
>  can be built including five counters for a material cost of less than $  
> 200. The  computer interface would be USB. My challenge is the  
programming of 
> two u  processors and the software that need to  be written for the 
computer. 
> How ever  there is so much  expertise in this group to make it happen as 
a 
> joint effort, if   the interest is there. By integrating a dual channel 
counter 
> with  each mixer,  focusing on a zero crossing detector that is part of 
the  
> counter section and  isolated from the mixer/filter/amplifier it  should 
be 
> possible to realize better  than 10 E 14. The counters  would count 100 
MHz. 
> $200 would cover 7 PC  boars, 7 Alu  enclosures and all electronic 
components 
> if 100 units get  build.  The price break will be in the PC board cost. 
> Offset  oscillator obviously would  not be included.
> As you pointed out  there is room for improvement on the zero detector.  I
 
> have done  the counters on an other project and if there is interest I  
will  
> gladly  share my information and thoughts.
> 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
>>   **************A Good Credit Score is 700 or Above. See yours in just 2 
 
>>     
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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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