H Heuer Instruments UMDT51A, User Manual

Page: 74 / 104

Making Measurements
LT51 User Manual 62
Pulse width
The pulse width can be quite short for short cables, but needs to be longer for
greater length cables so that enough energy is introduced in the cable. If the
energy is too low, there won’t be enough energy in the return wave to be
detected reliably. The LT51 allows a choice between 50 and 2500 ns. The
2500ns pulse width will produce enough energy for a 5 to 10 km cable:

Press the [f3] soft-key to select a PulseWidth between 50ns, 200ns, 500ns,
1000ns, 2500ns.
NOTE: The LT51 automatically chooses the pulse width appropriate for each
TDR range selected (5000ns for max. Range). However the pulse width
can still be changed independent of TDR range.
Adjusting the TDR display
There are several options for optimising the view of the TDR waveform on the
LT51 LCD. The transmit pulse will be seen at the left of the display when the
horizontal zoom is set to 1x.
Cursor
Use the knob to move the cursor across the screen (ensure that any dialog
boxes are cleared by pressing the key). The distance from the LT51 RTXab
terminals to the event marked by t
he cursor is always shown next to the “Mkr:”
text. Always place the cursor to the left side of an event to determine the
distance to the event.
NOTE: Always subtract the length of the test leads from the distance
measurement for highest accuracy.
Vertical zoom
The size of the features in the TDR display (ie. how tall or small they are) can be
set by varying the vertical zoom in multiples of 8x:

Toggle the [f4] soft-key to set Zoom to Vertical.

A Dialog box appears with the text “Vertical Zoom”.

Turn the knob (anti) clockwise to (decrease) increase the vertical zoom .

The bar on the vertical axis adjusts to indicate the active zoom size.

Press the key (or any key) to clear the dialog box.
Note:
Vertical zoom only changes the size of the displayed waveform. It does
not increase / decrease the amplitude of the TDR pulse which remains
fixed.
Tip:
If Zoom is set to Vertical, pressing the knob or key, is a shortcut to
enable the zoom to be set by turning the knob.