8163A/B, 8164A/B, 8166A/B User’s Guide 187
Return Loss Measurement 7
subtraction becomes inaccurate when the T-value is not sufficiently high.
Therefore, it is imperative for measurements of high return losses that the
T-value is as high and as stable as possible. A "rule of thumb" is that one
can only measure 5-10 dB higher return losses (lower reflections) than the
T-value, meaning if a T-value of 65 dB is achieved after calibration it is
possible to measure accurate return losses up to 70 or 75 dB. The
equation for the T-value is as follows:
Where:
P
ref
is the reflected power from the measurement path at the return loss
diode when making a reference calibration.
P
para
is the backward-flowing power from the measurement path at the
return loss diode when making a terminated/parasitic calibration.
M
ref
is the power at the source monitor diode when making a reference
calibration.
M
para
is power at the source monitor diode when making a
terminated/parasitic calibration.
RL
ref
is the return loss reference value set by the user for the reference
calibration.
These parameters are made available using the "Show Calibration " menu
item, or using the GPIB query:
SENSe [n ]:[CHANnel [m ]]:RETurnloss:CALibration:VALUES?
Figure 105 Reflectance Calibration using RL Reference
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