[time-nuts] DMTD phase shifter

Sat Jul 25 13:36:54 UTC 2009

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 minimise 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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