Cable Nulling; Cable Definition; Nulling Procedure - AEA VIA Bravo MRI Operating Manual

3.12 Cable Nulling using the included terminators
Any load connected to a coax that is not perfectly matched to the coaxial cable's
characteristic impedance will have its impedance modified by the coax. Cable nulling allows
you to remove the effect of the coaxial cable so that the impedance you read shows the
impedance of the load at the end of the cable, without the cable modifying effects. Cable
nulling is available in the VIA mode, the SWR mode has no cable null feature.
Cable nulling is selected by using one of the <with cable> options when selecting the VIA
instrument mode (see 3.8.4.1). The VIA Bravo MRI will prompt you for the required action
during the cable nulling procedure. Basically, the VIA Bravo MRI takes three readings: open
circuit, short circuit and nominal Z
The data resulting from a procedure that nulls a cable will be specific for that cable. If the
cable is changed, a new nulling must be performed. If the power is turned off, the cable
nulling information will be lost, so a new cable nulling will be required after power is restored.
If you recall data from the memory, you will need a new cable nulling. Note, if you power
back up cable nulling will be disabled, so if you wish to use cable nulling you must set the VIA
to cable nulling after power up (see 3.8.4.1).
3.12.1

Cable Definition

The coaxial cable characteristic impedance, Z
operate properly. This can be done in the Cable Characteristics sub menu (see 3.11.4).
3.12.2

Nulling Procedure

The nulling procedure begins by selecting a <with cable> option when setting up the VIA
instrument mode (see 3.8.4.1). Install the cable to be nulled to the RF connector of the VIA
Bravo MRI unit. Once the instrument mode is selected and the cable is connected, the
procedure can begin.
3.12.2.1 Open Circuit Reading
With the far end of the coax un-terminated (open circuit) press the ENTER key once. A
few seconds later the VIA Bravo MRI will prompt you for the next reading..... (a short).
3.12.2.2 Short Circuit Reading
With the far end of the coax shorted using the included coaxial short, press the ENTER
key once. When this step is complete the display shows that it wants the 50 (or 75) ohm
reading.
3.12.2.3 Nominal Z Reading
With the far end of the coax terminated with the included PRECISION 50 Ohm load
(usually 50 or 75 ohms), press ENTER once. This completes the cable nulling procedure.
To verify that you completed the procedure correctly, notice that the display shows flat line(s)
at Z (and near zero degrees). If that procedure went well, remove the matched load. Now
0
you are ready to take measurements at the end of the coax. (This also needs to be a quality
component, not a "home made" device
.
0
, must be defined for cable nulling to
0
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