[time-nuts] New topics (was Re: He is a Time-Nut Troublemaker....)
Bruce Griffiths
bruce.griffiths at xtra.co.nz
Wed Dec 24 02:34:42 UTC 2008
Hej Magnus
Magnus Danielson wrote:
> Hej Bruce,
>
>
>> Hej Magnus
>>
>> Thats true, but I had the more general case in mind wherein which one
>> adds coarse fixed delay increments as required and merely uses the
>> variable gain mixer to make a fine adjustment to the phase.
>>
>
> How do you intend that to work? Slightly introduce the signal with a
> different but nearby phase?
>
>
Simple, use a splitter (0 degree, 90 degree, 180 degree as appropriate)
to separate the signal into 2 parts one for the LO, one for the DUT.
Add some fixed delay to the LO branch then use a 90 degree splitter,
attenuate the 90 degree output and add it (a hybrid combiner/splitter)
to the 0 degree output, thus phase shifting the final output by a
small adjustable amount.
>> No real need for sine and cosine weighting for small phase adjustments
>> as the effect on the amplitude will be small and phase noise measurement
>> systems are usually relatively insensitive to AM.
>>
>
> Certainly, but various delay-adjustments will provide different output
> levels, which can be handled in a sub-sequent scaling or by weighing the
> signal of different levels.
>
Probably a simple calibration will suffice when using a saturated mixer
used as a phase detector.
> To achieve an arbitrary (0-360 degree) phase-shift with such a system
> may be a viable question. Running a set of cables and a programable
> multiplexer of some sort would be possible.
>
> Well, that was not what was asked for originally, and also not apparent
> from your original comment. Surely you can achieve better performance
> than this quad-phase circuit provides, but since it was not described
> what type of circuit you intended it never became clear what you meant.
>
> The original question infact was for a system where you do not use fixed
> delays such as cables.
>
I reinterpreted it to mean that some fixed delays would be OK as long as
they don't have to be accurate to within better than a few nanosec.
In most cases ( eg when measuring isolation amplifier phase noise at
10MHz ) such lumped delays probably wont be necessary as the amplifier
delay will be relatively small.
> Cheers,
> Magnus
>
>
Bruce
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