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Theory of Operation
Input Attenuators <1>
This section describes the electrical operation and relationship of circuits in the input attenuator assemblies.
See Schematic <1> in Section 6.
Attenuators
The 11A52 attenuators consist of thick-film resistor networks and hermetically sealed DPDT relays. Each
attenuator also contains a 2.2 µF ac coupling capacitor and a high-frequency-compensation skin-loss network.
Defective attenuation modules are not repairable and should be replaced.
CAUTION
When removing or inserting attenuator modules, take care to unbolt the attenuator-mounting bracket
from the main circuit board first. The attenuator-mounting bracket is designed as an integral part of
the attenuator and supports the large ac coupling capacitor. To promote heat dissipation from the
relay coils, the attenuators are not covered. Handle the attenuator modules carefully.
Each attenuator has four relays. In this discussion, we assume that the #1 relay (Cal/Norm) passes a signal to
the #2 relay (AC/DC). From the #2 relay, the signal goes to the #3 relay (lOX/lX), then to the #4 relay
(lOX/lX). The #3 and #4 relays and associated resistor networks are identical in function, though the #3 relay
resistor network can dissipate slightly more power. Each relay function is explained below:
Calibrate/Normal (#1 relay)
With the Cal/Norm relay in the Cal position (relay not energized), the front-panel input signal is
disconnected and the attenuator input comes from pin 2 of the attenuator socket on the main circuit board. With
the Cal/Norm relay in the Norm position (relay energized), the front panel input signal is connected to the
input of the #2 relay on the attenuator. The signal voltage at pin 2 of the attenuator socket on the main ECB is
disconnected from the input of the #2 relay on the attenuator. The Cal/Norm relay applies the accurate
calibration voltage signal (CALSIG) to the very front end of the 11A52 signal path during Enhanced Accuracy
calibration. To optimize calibration accuracy, the calibration signal follows a signal path nearly identical to
that of the signal applied at the input connector. The Cal/Norm relay is also used in the Off coupling mode.
AC/DC Coupling (#2 relay)
The AC/DC relay inserts a coupling capacitor into the input signal path between the input BNC and the
remainder of the attenuator. When the AC/DC relay is set to AC position (relay energized), the #1 relay is AC
coupled to the #3 relay. Alternately, with the AC/DC relay set to DC (relay not energized), the #1 relay is DC
coupled to the #3 relay. The network of resistors surrounding the AC/DC relay serves to charge the coupling
capacitor while Coupling is set to Off, and to discharge the coupling capacitor when the input signal is
removed from the input connector.
2-4
11A52 Extended Service Manual
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Troubleshooting
