[time-nuts] Designing and building an OCXO and GPSDO

John Miles jmiles at pop.net
Fri Aug 15 13:03:39 UTC 2008


LO phase noise is almost always what limits the noise floor at close-in
offsets, because of the narrow RBW (either analog or digital) typically used
at those offsets to keep the carrier out of the measurement.

Occasionally a high degree of RF attenuation might raise the equivalent
front-end noise floor high enough to dominate the LO's inband noise, but
that's not usually what happens.  Certainly not if you're using a strong
carrier to characterize the analyzer's PN floor.

Phase noise at 100 Hz would likely be dominated by the reference, either the
original 10 MHz reference or a higher-frequency crystal oscillator locked
with a very low bandwidth.  As in Bruce's example, if the first IF is 2 GHz
and the LO is tuned to 3 GHz to receive a 1-GHz signal, there's a 50 dB
penalty on a 10 MHz reference (20*log(3000/10)).  A decent 10 MHz OCXO is
good for about -150 dBc/Hz at 100 Hz from the carrier, so you would expect
to see about -100 dBc/Hz at 100 Hz from the carrier on an analyzer built
with this sort of architecture.

That's a good match for what you see in the PSA data sheet, as well as the
FSU67's.  The 8560E portables are about 5 dBc/Hz worse.

-- john, KE5FX


> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com]On
> Behalf Of Luis Cupido
> Sent: Friday, August 15, 2008 4:34 AM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] Designing and building an OCXO and GPSDO
>
>
> Bruce,
> John,
> ....
>
> And at smaller offsets like 100Hz and less ?
> Shouldn't the improvement be even bigger ?
> Closer to the carrier we are dealing with bigger signals
> so the ADC issues like resolution should be less important,
> and the limiting factor should really be the the
> phase noise of the LO's and etc.
> Am I right ?
>
>
> Luis Cupido.
> ct1dmk.
>
>
>
> Bruce Griffiths wrote:
>
> > Luis
> >
> > The R+S FMU36 has a phase noise floor of around -143dBc/Hz (offset >
> > 10kHz) with a 10MHz input.
> > Whereas the R+S FSU67 has a phase noise floor of around -133dBcdBc/Hz
> > (offset = 10kHz) with a 640Mhz input.
> >
> > There is a definite improvement at lower frequencies but not quite as
> > much as one might have expected.
> >
> > Bruce
> >
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