NETWORK ANALYZER

B I A S T E EB I A S T E E

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A T T E N U A T O RF I X T U R E

A T T E N U A T O R

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Figure 6-54. Typical Measurement Set up

If the device measurement requires bias, it will be necessary to add external bias tees between the iixed attenuators and the fixture. The internal bias tees of the analyzer will not pass the bias properly through the external fixed attenuators, Be sure to calibrate with the external bias tees in place (no bias applied during calibration) to remove their effect from the measurement.

Because the bias tees must be placed after the attenuators, they essentially become part of the fixture. Their mismatch effects are the same for source match and load match, so the TRL CAL routine will correct for their effects Although the fixed attenuators improve the raw mismatch of the network analyzer system, they also degrade the overah measurement dynamic range.

This effective mismatch of the system after calibration has the biggest effect on reflection measurements of highly reflective devices, Likewise, for well matched devices, the effects of mismatch are negligible. This can be shown by the following approximation:

Reflection magnitude uncertainty = ED + ER&I + WW2 + E~s21S12 Transmission magnitude uncertainty = EX + E&l + EsSllSzl + E&&l

where:

En = effective directivity

En = effective reflection tracking

Es = effective source match

EL = effective load match

Ex = effective crosstalk

E,-r = effective transmission tracking

&,, = S-parameters of the device under test

6-86 Application and Operetion Concepts

Page 377
Image 377
HP 8753E manual Typical Measurement Set up