[time-nuts] DMTD phase shifter

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Jul 25 23:25:27 UTC 2009


Brian

See:     http://www.ko4bb.com/~bruce/ZeroCrossingDetectors.html
<http://www.ko4bb.com/%7Ebruce/ZeroCrossingDetectors.html>
Just cascade several limiter /filter stages to achieve the required gain.
Inverting stages are perhaps easier to use.
A table specifying the voltage gain and filter time constant for each
stage is given on this page.
The example assumes that the input noise spectral density for each stage
is 10nV/rtHz.
In practice its desirable to measure the phase detector output spectral
noise density.
When the spectral noise density isn't the same for each stage (NB noise
for the input stage should include the phase detector noise(at its low
pass filter output)) the optimum gain and filter time constant
distribution is different to that given in the table.
Without measurements of the phase detector output noise its not possible
to calculate the optimum gain and filter time constant for each stage.

Bruce

wa1zms wrote:
> bruce-
>
> do you have a favorite ZCD schematic that you can share?
>
> many thanks in advance!
>
>
> -Brian, WA1ZMS
>
> On Jul 25, 2009, at 9:36 AM, Bruce Griffiths
> <bruce.griffiths at xtra.co.nz> wrote:
>
>> 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
>
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