The foregoing assumes an ideal “lossless" cable. In practice, measurements obtained include
the effect of pulse attenuation due to cable loss and requires correction in order to obtain true
return loss.
i.e. True Return Loss = Measured Return Loss – Pulse Attenuation
Since the amplitude of the reflection varies with the pulse width it is essential when determining
Return Loss to use the SAME PULSE WIDTH for all measurements.
This is facilitated by the HELP Menu. Press HELP
and then the appropriate key for
PULSE WIDTH. From the list of options select the pulse appropriate to the measurement. On
returning to the trace the selected pulse width will be confirmed on the display, and the PULSE
key will be inoperative.
The output impedance of the T631 must be set to match the impedance of the cable under test.
7.9.1
MEASUREMENT OF RETURN LOSS (dBRL)
Through the HELP Menu select the IMPEDANCE to match the cable under test.
Similarly select the PULSE WIDTH appropriate to the measurement.
To minimise any possible mismatches the cable under test should be connected directly to the
unit using matched connectors.
T631 OPERATING MANUAL
35
Radiodetection Ltd. 00901209-6
