Keysight N9927-90001 User’s Guide 77
TDR (Time Domain Reflectometry) - Option 215
TDR Measurement Settings
Cable (Correction) Specifications
By default, the FieldFox does NOT correct TDR measurements to account for
the inherent loss of a cable. However, to make more accurate TDR
measurements, the Cable Loss and Velocity Factor values should be
considered.
About Velocity Factor and Cable Loss
— 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.
— 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 0.1
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 TDR measurement present:
—Press Meas Setup 4
.
The following describes Coax Media type. Waveguide Media can NOT be
measured with TDR as waveguide is inherently a high pass device.
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