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The step attenuator provides coarse power control for the source signal. It is an
electro-mechanical attenuator, controlled by the A7 CPU, that provides 0 to 55
changing the level of the incident power in the reference path.
The output of the step attenuator is fed into the S4 transfer switch. This is a
solid-state switch. It switches between the port 1 and port 2 measurement
paths, automatically enabling alternate forward and reverse measurements. In
addition, S4 provides an internal termination for the measurement port that is
inactive.
The DC bias tees provide a means of biasing active devices with an external DC
voltage connected to the rear panel DC BIAS CONNECT ports. The DC voltage is
applied directly to the center conductor of the test port connectors. A blocking
capacitor ensures that the bias current goes only to the device under test, and
not back into the source. Likewise, an inductor in the bias path prevents RF
from being imposed on the external DC supply.
The test signal goes into the through-line arm of the couplers, and from there
to the test ports and the device under test. The coupled arm of the couplers
carries the signal reflected from or transmitted through the device under test
to the receiver for measurement. The coupling coefficient of the directional
couplers is nominally 20 dB (40 dB at 50 MHz).
Theory of Operation 12-21
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