Section 2-Theory Of Operation; Block Diagram - Tektronix 11A52 Service Manual

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Theory of Operation
This section describes the circuitry used in the 11A52 Two Channel Amplifier. First we discuss the amplifier at
the block diagram level, using the block diagram shown in Figure 2-L The description then continues with
details of relationships among major blocks and their subparts. Schematics of all major circuits are given in
Section 6, Schematic Diagrams and Circuit Board Illustrations. Stages are outlined on the schematics with
wide shaded lines. Refer to the schematics throughout the following descriptions.
Block Diagram
The following discussion should aid in understanding the overall concept of the 11A52 Two Channel Amplifier
before individual circuits are explained in detail. Figure 2-1 is a block diagram of the 11A52. Each major
circuit in the amplifier is represented; only basic interconnections among individual blocks are shown. The
number in each block is the schematic number on which the block is shown.
The 11A52 is a wide bandwidth two-channel amplifier plug-in. Signals to be displayed are applied to the CH
1 and CH 2 input connectors. Since the CH 1 and CH 2 amplifier circuits are identical, only the CH 1 amplifier
is described here.
The input signal is applied directly to the CH 1 Attenuator. The attenuator is a hybrid containing resistive
dividers, an ac coupling capacitor and relays. The output of the attenuator drives the CH 1 Amplifier input.
The Amplifier IC provides gain switching, bandwidth limit filters, and separate outputs for the display,
trigger and auxiliary signal paths.
The 11A52 plug-in unit functions are accessible only through the oscilloscope. Since there are no mechanical
switch settings, the 11A52 does not "remember" deflection factor, offset, or bandwidth settings when the host
oscilloscope power is shut off, although it does remember internally stored calibration constants. Past settings
are therefore stored in the host oscilloscope and are transmitted to the plug-in at power-up. All
communications between the host oscilloscope and plug-in are conveyed over a set of serial data lines at the
plug-in/ oscilloscope interface connector.
The 11A52 contains a microprocessor (MPU) that interprets the communications described above and initiates
changes to the Amplifier settings. The MPU also constantly monitors the input Overload Sense and the Probe
Data communication lines. The MPU operates by means of an internally stored program in its ROM (Read Only
Memory), which is not alterable by either the plug-in or the host oscilloscope. The MPU stores the plug-in
calibration constants in RAM (Random Access Memory). RAM memory can be rewritten, and is backed up by a
battery so the internal calibration constants are not lost upon power down.
The MPU communicates with the host oscilloscope through a peripheral IC called the Housekeeper IC. The
Housekeeper IC converts the 8-bit data bytes from the MPU into a serial data signal that is sent to the host
oscilloscope. Likewise the serial data signal from the oscilloscope is converted by the Housekeeper IC into 8-
bit data bytes for the processor to read. The Housekeeper IC also updates the analog control voltages, and
controls the operation of the Amplifier.
11A52 Extended Service Manual
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