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FieldFox User’s Guide
About Velocity Factor and Cable Loss
o
Velocity Factor is a property of the physical material of a cable. A VF of 1.0
corresponds to the speed of light in a vacuum, or the fastest VF possible. A
polyethylene dielectric cable has VF = 0.66 and a cable with PTFE dielectric
has VF = 0.7.
o
Cable Loss is specified in dB/meter. In addition to the length of the cable,
loss is also directly proportional to the frequency of the signal that passes
through the cable.
The following is an example showing how DTF cable correction works:
The DUT is a 100 meter transmission cable. The Cable Loss value is .1
dB/meter. This means that a signal traveling ONE WAY through the cable will
lose 10 dB of power (100 m * .1dB/m). Because the FieldFox performs this
measurement with 1 port, the test signal travels down the cable and then back,
for a total loss of 20 dB.
After a calibration has been performed, for the purpose of illustrating this point,
connect an OPEN to the end of the cable – a maximum-sized fault - for 100%
reflection of the 300 MHz test signal.
Without compensation
for the loss of the cable, a –20 dB response would be
visible at 100 meters, which is the OPEN at the end of the DUT. This is from 10
dB of loss through the cable in each direction.
With compensation
for the loss using the manufacturer’s specification, the
FieldFox compensates the trace as though the signal traveling through 100
meters was increased by +20 dB. Therefore the response will show 0 dB for 100%
reflection.
How to enter Cable Loss and Velocity Factor
Cable Loss and Velocity factor can be entered using one of the following
methods:
Manually enter cable loss and velocity factor for the measurement.
Select or create a cable file which contains the cable loss and velocity factor.
With a DTF measurement present:
Press
Meas Setup 4
.
Then
DTF Cable Specifications
Select
Cable Corr
o
Auto
Use Cable Loss and Velocity Factor values from a Cable file. See “How
to Edit, Save, and Recall a Cable File” below. This will overwrite a manually-
entered value.
o
Man
(Default setting) Manually enter a value for Cable Loss and Velocity
Factor.
Then:
o
Velocity Factor
Using the numeric keypad, enter a value between 0.01 and
1. Then press
Enter
.
o
Cable Loss
Using the numeric keypad, enter a positive Cable Loss value in
dB/m, then press
Enter
.
