[time-nuts] Designing and building an OCXO and GPSDO
Bruce Griffiths
bruce.griffiths at xtra.co.nz
Thu Aug 14 11:40:12 UTC 2008
Luis Cupido wrote:
> Related to this, I have a question in my mind for sometime.
>
> How close to a decent phase-noise setup are we if
> we down convert and use a low frequency spectrum analyzer ?
> (downconverting with the best XTAL LO we can imagine).
>
> My target is microwaves and millimeter waves so I have to
> down convert anyway, Or then I have to live with what my
> spectrum analyzer show me (a tek 492, in my case).
>
> ---
>
> Rephrasing the question, how much better are the low frequency
> spectrum analyzers comparing to the microwave spectrum
> analysers (in phase noise I mean)?
>
>
> Any comments ?
>
> Luis Cupido.
> ct1dmk.
>
> P.S.(I know that I'm limites to 100Hz RBW with my 492... and
> I would like also to measure closer than that )
>
>
Luis
The latest spectrum analyser offerings from Agilent have similar phase
noise floors for both the millimeter wave and low frequency spectrum
analysers.
However the low frequency analysers have a lower "flicker" noise corner.
The noise floor of a good double balanced mixer is still 30-40dB lower
than that a spectrum analyser.
Thus you are stuck with using a low bandwidth phase lock loop to get
down to the mixer noise floor.
Alternatively a dual (first conversion uses analog mixers, 2nd
conversion uses DSP techniques) conversion Costas receiver using 4
mixers and 4 ADCs should go down to -170dBc @ offsets of 100Hz or so
when correlation techniques are employed. Its a pity the TSC5120A doesnt
allow independent access to all 4 of its ADC inputs so a quad of
external mixers can be employed to extend the technique to the
millimeter wave region.
All you need is 4 high resolution ADCs and an offset generator or 2.
A couple of high end sound cards may be suitable at least for testing
the concepts.
Bruce
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