HEWLETT PACKARD (HP) E4417A
Service Manual

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 200� giving an impedance at RF of 50� and �L close to zero. When the thermistor bridge is set to 100� the RF impedance is 25� giving a nominal � 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� and 100� can be accurately measured. If the two reflection coefficients are �1 and �2 and these give the power measurements of P1 and P2 respectively (but the available power remains the same) then the two relationships can be equated as follows: Equation 2-6:

P1

1 � �s �1 1 � �s �2 --------------------------- = P --------------------------1 � �1
2 2

2

2

1 � �2

2

Where: Therefore: Equation 2-7:
2 P 1 ( 1 � �2 2 ) 1 � �s �2 ------ ------------------------- = -------------------------P2 ( 1 � � 2) 1�� � 2 1 s1

Where: The left side of the equation can be replaced by the factor M as follows: Equation 2-8:

1 � �s �2 M = -------------------------1 � �s �1
2

2

Where: This cannot be directly solved for �S because it is a complex quantity. In order to solve for � S additional values of load reflection coefficient are required with a phase difference of approximately 120o. However an approximate solution using two values of �L can give adequately accurate results provided the phase angle of the source is within ±40o of 0o or ±180o.

Agilent E4416A/E4417A Service Guide

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