Agilent Technologies E4418B, Руководство по обслуживанию

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Performance Tests
Output Standing Wave Ratio (SWR) Test
2-24 Agilent E4418B/E4419B Service Guide
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
Γ
L
close to zero.
When the thermistor bridge is set to 100
Ω
the RF impedance is 25
Ω
giving
a nominal
Γ
L
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
Γ
1
and
Γ
2
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:
This cannot be directly solved for
Γ
S
because it is a complex quantity. In
order to solve for
Γ
S
additional values of load reflection coefficient are
required with a phase difference of approximately 120
o
. However an
approximate solution using two values of
Γ
L
can give adequately accurate
results provided the phase angle of the source is within ±40
o
of 0
o
or
±180
o
.
Where:
P
1
1
Γ
s
Γ
1
–
2
1
Γ
1
2
–
---------------------------- P
2
1
Γ
s
Γ
2
–
2
1
Γ
2
2
–
----------------------------
=
Where:
P
1
P
2
------
1
Γ
2
2
–
( )
1
Γ
1
2
–
( )
--------------------------
1
Γ
s
Γ
2
–
2
1
Γ
s
Γ
1
–
2
---------------------------- =
Where:
M
1
Γ
s
Γ
2
–
2
1
Γ
s
Γ
1
–
2
---------------------------- =
4402serv.book Page 24 Tuesday, October 14, 2003 3:18 PM