Agilent Technologies 4395A, Manual Change

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Calibration for Network Measurement
Figure A -28. Eective Directivity E
DF
Source match error . Because the measurement system test port is never exactly the
characteristic impedance (50 or 75 ), some of the reected signal is re-reected o the test
port, or other impedance transitions further down the line , and back to the unknown, adding
to the original incident signal (I). This eect causes the magnitude and phase of the incident
signal to vary as a function of S
11A
and frequency . Leveling the source to produce constant
(I) reduces this error , but because the source cannot be exactly leveled at the test device
input, leveling cannot eliminate all power variations . This re-reection eect and the resultant
incident power variation are caused by the source match error , E
SF
(Figure A -29 ).
Figure A -29. Source Match E
SF
Frequency response error . Frequency response (tracking) error is caused by variations in
magnitude and phase atness versus frequency between the test and reference signal paths .
These are due mainly to imperfectly matched receiver circuits and dierences in length and
loss between incident and test signal paths . The vector sum of these variations is the reection
signal path tracking error , E
RF
(Figure A -30).
A-50 Basic Measurement Theory