Yes when I was measuring the noise floors of my 5370B's, the noise floor appeared to be noticeably better when the same oscillator was used as both the time base for the 5370B and the signal source for the test. Sorry I'm on the road right now (in Mexico City) and can't post a plot showing this. (For the original poster the list archives also contain some good information about the importance of matching signal levels to the counters in question and other related information which may also affect the results of these tests.) Best Regards Mark Spencer > > Message: 3 > Date: Sun, 4 Nov 2012 21:30:41 -0800 > From: "Tom Van Baak" <tvb@LeapSecond.com> > To: "Discussion of precise time and frequency measurement" >     <time-nuts@febo.com> > Subject: Re: [time-nuts] Measuring gpsdo vs itself > Message-ID: <C5FCD557A7C9416E8AA5E36FBF30AB2A@pc52> > Content-Type: text/plain;    > charset="iso-8859-1" > > Ah, I see what you mean now. Yes, that setup can give you a > rough estimate of the counter's noise floor. > > I can't give you specific numbers but one danger with this > sort of test is that the input and the timebase are > artificially locked together (i.e. fixed phase relationship) > through the common reference. Your measurements may thus > show artificially less noise than a real-life case of > independent input(s) and reference. > > This can happen if your sub-ns counter is based on > interpolators. Because the input and the timebase are locked > in phase, the counter lands near the same point of the > interpolator scale on every single measurement, rather than > experiencing the noise (and non-linearity) of the entire > scale. > > /tvb >