[time-nuts] another Ebay mixup, 5370

[John Miles]

> > My 5370A currently has the following characteristics:
> > With the EXT 10MHz connected to the input jitter is 15ps.
> > With an FTS1200 5MHz input signal jitter is 46ps.
> > With an external GPS locked 10MHz input signal jitter is 45ps.
> >
> > This illustrates the danger in believing the jitter measured with a
> > coherent input signal.
> >
> > Bruce
>
> Thanks much for the measurements. I agree with the
> conclusions.
>
> There have been several threads recently where people
> use a counters' reference output as one of the inputs.
> I'm not sure where the idea comes from, but it seems
> ill advised to me.

In the case of the 5370, page 3-12 of the manual is where it came from ("15.
Press STD DEV and +/- TI switch.  Display should read less than 100 ps (this
reading is the instrument's jitter).")

At some point between my original 2316 edition and the 2904 edition scanned
by David Kirkby, they apparently changed this step to call for pressing the
+TI switch, unless the +/- symbol failed to scan properly.  I get about
10-15 ps more jitter in +TI ONLY mode for whatever reason.

The last time I studied the manual, I remember being convinced that the 5370
would be relatively immune to clock-correlation effects if the interpolators
were set up properly.  The trouble is, I tend to remember how this thing
works for all of about 3 days after I put the manual back on the shelf...
and the last time I looked at it was several months ago.  All I can say
right now is that I believe that slew rate at the zero crossings is a bigger
concern than whether the signal being measured is correlated with the
internal clock.

I would almost put money on the problem with Said's unit being dirty
switches or another basic signal-conditioning issue.

-- john, KE5FX