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: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] > 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 <[EMAIL PROTECTED]> 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 > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.