So, which TDR should I keep and which should I sell?
They are both showing the ~70mm of 50ohm stripline between the sampling diodes and the magic shorting BNC connector (the latter discharges any static in a cable before it it connected to the tunnel diode). The risetime is ~50ps, the falltime slightly longer.
The differences are:
- the top one has the best flat "internal cavity" response I've seen, but is calibrated in feet - which makes the *10 range setting difficult to calculate mentally.
- the bottom one has slight ripple, but is calibrated in metres - which is much more usable
I would like to keep the top one because of the flat response, but I don't do engineering in feet anymore (except 1ns = 1ft!).
Option 1: simply swap cavities or the front panel. But either of those would disturb the cavity and almost certainly bugger the response.
Option 2: change a few resistors/capacitors to rescale it, and stick a new range scale over the anodised scale.
Option 3: keep the bottom one.
Ah, decisions, decisions. That is a pretty good example of a "first world dilemma"!