Performance Tests 2
E4416A/E4417A Power Meters Service Guide 51
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 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 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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