[time-nuts] another Ebay mixup, 5370

[John Miles]

> When using +-TI then the START and STOP events occur on the same edge of
> the input waveform so that the measured time interval is nominally zero.
> When the START and STOP events are separated by 1 period of the input
> signal (100ns for a 10MHz input) the difference in the differential non
> linearities of the START and STOP interpolators are not as well matched
> than when the the START and STOP events are nominally coincident. Thus
> the calculated jitter will be larger for the +TI only setting.

That does make sense.

> The jitter measurement with an asynchronous input signal is more
> representative of the result when measuring signals that are not
> coherent with the internal timebase.
> When using the internal timebase as the input signal the START and STOP
> events will tend to exercise the same narrow region of the interpolators
> so that the calculated jitter will be dominated by noise rather than the
> interpolators differential non linearity characteristics.

That was my thinking as well; with a fixed relationship between clock and
data at the interpolators for each measurement cycle, you'd see the
best-case results, like measuring the residual phase noise of two identical
VCOs with a common clock.  What I'm still hazy on is the effect of the
so-called 'anticoincidence' circuitry.  I thought I had internalized it at
one point, but apparently not.  It seems that we all have to collectively
remind ourselves about the quirks and limitations of the 5370 on this list,
about twice a year. :)

-- john, KE5FX