Agilent Technologies E4416A Service Manual page 72

This method for measuring the source match exploits this relationship by
presenting the source with two different values of
power delivered to the load under the two conditions. The load used is a
thermistor mount that can be operated at two different resistances and can
accurately measure the power absorbed. The thermistor mount type Agilent
478A with Option H75 or Option H76 normally operates at a DC resistance of
200 Ω giving an impedance at RF of 50 Ω and
thermistor bridge is set to 100 Ω the RF impedance is 25 Ω giving a nominal
Γ
of 0.33. The effective efficiency of the thermistor mount remains constant
L
under these two conditions so that the power ratio between operation at 200 Ω
and 100 Ω can be accurately measured.
If the two reflection coefficients are
measurements of P
same) then the two relationships can be equated as follows:
Equation 2-6:
1 Γ
–
--------------------------- -
P
1
1 –
Therefore:
Equation 2-7:
(
P 1 –
1
----- - ------------------------- -
P
(
1 –
2
The left side of the equation can be replaced by the factor M as follows:
Equation 2-8:
M =
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and P respectively (but the available power remains the
1 2
2 2
Γ 1 Γ Γ
–
s 1 s 2
--------------------------- -
= P
2
2 2
Γ Γ
1 –
1 2
2
2
Γ )
1 Γ Γ
–
2
s 2
= --------------------------- -
2
2
Γ )
1 Γ Γ
–
1
s 1
2
1 Γ Γ
–
s 2
--------------------------- -
2
1 Γ Γ
–
s 1
Performance Tests
Γ
and measuring the
L
Γ
close to zero. When the
L
Γ Γ
and and these give the power
1 2
2
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