[time-nuts] Phase shifter circuit for DTMD

Stephan Sandenbergh stephan at rrsg.ee.uct.ac.za
Fri Jul 24 16:10:57 UTC 2009


Hi,
Thanks for the replies. I agree. One is interested in the timing of beat
notes.

But, I'm slightly confused now. It might be a language problem on my side.

Quoting Howe, Allan and Barnes, 1981, "...adjust the phase so that the two
beat frequencies are nominally in phase; this adjustment sets up a nice
condition that the noise of the common oscillator tends to cancel..."

I can see a few issues here:

1) if the beat frequencies are in phase, there will be a very small time
interval between their zero crossings. This might be difficult to measure
with accuracy.

2) I agree that reference oscillator noise will cancel to some extend
because the measurements are done closer to the same time, which makes the
reference oscillator noise better correlated between the start and stop
edges.

3) Also, one would like to compare both clkA and clkB at the same time. Not
one at t=0 and the other at t=1day to exaggerate a little. I'm not planning
to measure atomic standards, or the best OCXO available, so I doubt this
will bother me.

Other than the above, I agree: it is better to have a greater phase offset
between the beat notes.

Does my thinking my sense at all?

Regards,

Stephan.





2009/7/24 Magnus Danielson <magnus at rubidium.dyndns.org>

> Lux, James P (337C) wrote:
>
>>
>>
>> On 7/24/09 6:32 AM, "Stephan Sandenbergh" <stephan at rrsg.ee.uct.ac.za>
>> wrote:
>>
>>  Hi,
>>> Many sources and papers refer to the actual DMTD mixer system (and
>>> zero-crossing detector etc.). But, I've seen very little on the phase
>>> shifter circuits, that are used in conjunction with this system.
>>>
>>> I was hoping some of you might have experience, and perhaps a circuit
>>> example. If it is similar to what is used in audio applications, the
>>> single
>>> opamp, 0 - 180deg, RC-all pass filter, it is simple. However, I assume
>>> there
>>> are more to it than that. It seems such is the case with all precision
>>> time
>>> related things :)
>>>
>>>
>> For RF in coax, it's probably more like a mechanical line stretcher or
>> trombone line. For 10 MHz there are some variable transformer sorts of
>> schemes.
>>
>> One can also do a 90 degree hybrid with a variable power combiner on the
>> output, or, essentially the equivalent, a I/Q or vector modulator. There
>> are
>> also "linear phase modulators" based on diodes. With anything using an
>> active device, one is going to worry about the usual things: noise, aging,
>> etc.
>>
>>
>> All of the "electronic"  phase shifters will require some form of
>> calibration to get better than a few degrees accuracy. In some measurement
>> schemes you don't really care what the phase shift is, just as long as you
>> can get it to quadrature.
>>
>> For the DMTD, aren't you really looking at timing of the beat notes, not
>> the
>> absolute phase?
>>
>
> For DMTD you use a local oscillator which is detuned to produce a beat
> frequency of say 10 Hz. That means that you will scan through all phase
> relationships 10 times every second. For 10 MHz that means about 1 milion
> different phase-relationships. Now, the phase relationships producing a beat
> voltage going through zero will choose themselfs. Thus, no need for a
> phase-shifter when doing DTMD as such. However, it can be wise to shift the
> second input such that it does not occur neat the same time, since that way
> time-correlation could be reduced. However, such phase-steps does not need
> to be made in linear steps, but rather crude steps approximating 0, 90, 180
> and 270 degrees will suffice.. however other means to achieve the same thing
> can be used to create the separation.
>
> Cheers,
> Magnus
>
>
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