[time-nuts] Spec An for phase noise measurements

John Miles jmiles at pop.net
Wed Jan 23 19:51:05 EST 2008


Usually the built-in noise marker will average some nearby trace points
(behind the scenes), convert the result from dBm to dBm/Hz by subtracting
10*log(RBW), and add the filter/detector noise-equivalent-bandwidth
correction.

The latter factor makes the DUT look 2-4 dB worse than the 10*log(RBW)
calculation would indicate by itself.  If it's not being applied by the
marker function you're using, that could explain why the results appear a
bit better than specified.

Similarly, if there are spurs present, or if the marker is positioned on or
near a steep slope in the trace, the averaging process can yield misleading
or inconsistent numbers.

-- john, KE5FX

> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com]On
> Behalf Of Matt Ettus
> Sent: Wednesday, January 23, 2008 4:44 PM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] Spec An for phase noise measurements
>
>
> On Jan 23, 2008 4:26 PM, John Miles <jmiles at pop.net> wrote:
> > Are you using a noise marker that yields dBc/Hz values?  The FFT window
> > function has its own required noise-response correction value,
> so if you're
> > just looking at a marker and doing the log10(RBW) subtraction
> yourself, that
> > could account for the difference.
> >
> > Also, if there is a noise marker, check to see if it reads
> dBc/Hz or dBm/Hz.
> > Most of them read dBm/Hz values, which are obviously only equal to
> > conventional dBc/Hz values if you're measuring a 0-dBm carrier.
>
> I used the analyzer's phase noise function.  It won't make a plot, but
> will do measurements at a few spot offsets.  It's not entirely clear
> how it is calculating these, though.
>
> Matt
>
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