Performance Tests 2
Keysight E4416A/E4417A Power Meters Service Guide 61
This method for measuring the source match exploits this relationship by
presenting the source with two different values of and measuring the 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 Keysight 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 close to zero. When the thermistor bridge is
set to 100 Ω the RF impedance is 25 Ω giving a nominal of 0.33. The effective
efficiency of the thermistor mount remains constant 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 and and these give the power
measurements of P
1
and P
2
respectively (but the available power remains the
same) then the two relationships can be equated as follows:
Equation 2-6:
Therefore:
Equation 2-7:
The left side of the equation can be replaced by the factor M as follows:
Equation 2-8:
P
1
1 Γ
s
Γ
1
–
2
1 Γ
1
2
–
---------------------- P
2
1 Γ
s
Γ
2
–
2
1 Γ
2
2
–
----------------------=
P
1
P
2
-----
1 Γ
2
2
–
⎝⎠
⎛⎞
1 Γ
1
2
–
⎝⎠
⎛⎞
---------------------
1 Γ
s
Γ
2
–
2
1 Γ
s
Γ
1
–
2
----------------------=
M
1 Γ
s
Γ
2
–
2
1 Γ
s
Γ
1
–
2
----------------------=
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