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Performance Tests
Output Standing Wave Ratio (SWR) Test
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 200Ω giving an impedance at RF of 50Ω and Γ
When the thermistor bridge is set to 100Ω the RF impedance is 25Ω giving
a nominal Γ
of 0.33. The effective efficiency of the thermistor mount
L
remains constant under these two conditions so that the power ratio
between operation at 200Ω and 100Ω can be accurately measured.
If the two reflection coefficients are Γ
measurements of P
1
the same) then the two relationships can be equated as follows:
Equation 2-6:
2
1 Γ
Γ
–
s
1
--------------------------- -
P
=
P
1
2
Γ
1
–
1
Where:
Therefore:
Equation 2-7:
2
(
Γ
)
1 Γ
P
1
–
1
2
------
------------------------- -
=
--------------------------- -
P
2
(
Γ
)
1 Γ
1
2
–
1
Where:
The left side of the equation can be replaced by the factor M as follows:
Equation 2-8:
2
1 Γ
Γ
–
s
2
M
=
--------------------------- -
2
1 Γ
Γ
–
s
1
Where:
This cannot be directly solved for Γ
order to solve for Γ
S
required with a phase difference of approximately 120
approximate solution using two values of Γ
results provided the phase angle of the source is within ±40
o
±180
.
2-24
and Γ
1
and P
respectively (but the available power remains
2
2
1 Γ
Γ
–
s
2
--------------------------- -
2
2
Γ
1
–
2
2
Γ
–
s
2
2
Γ
–
s
1
because it is a complex quantity. In
S
additional values of load reflection coefficient are
Agilent E4418B/E4419B Service Guide
close to zero.
L
and these give the power
2
o
. However an
can give adequately accurate
L
o
o
of 0
or
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