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Theory of Operation

3-2

TDS 420A, TDS 430A & TDS 460A Service Manual

For example, if 50 Ω input termination is selected and the input is overloaded,

the processor system switches the input to the 1 MΩ position. The preamplifier

in the A05 Attenuator amplifies the input signals.

Probe Coding Interface. Probe coding interface signals pass through the

A06 Front Panel to the processor system.

Jumper. Signals from the attenuators pass through the A08 Jumper board to the

acquisition system.

Acquisition System. The acquisition system converts the analog input signals to

digital signals and controls the acquisition process under direction of the

processor system. The acquisition s ystem includes the trigger, acquisition timing,

and acquisition mode generation and control circuitry.

D1 Bus. The acquisition system passes the digital values representing the

acquired waveform through the D1 bus to the A09 DSP board (TriStar). This

happens after a waveform acquisition is complete if the digital signal processor

(DSP) requests the waveform.

Processor System. The processor system consists of a 68020 microprocessor that

controls the entire instrument. It includes the firmware program for the oscillo-

scope. The firmware can be reprogrammed by using the GPIB and an external

software package. The processor system also includes a GPIB interface. The

processor can display text and symbols (that is, cursors, but not waveforms) by

passing them to the A02 Display board.

DSP. The digital signal processor processes each waveform as directed by

software downloaded from the system processor. Waveforms to be displayed are

passed on to the A02 Display board.

Display. Text and waveforms are processed by different parts of the A02 Display

circuitry. The display sends the text and waveform information to the A26 Moni-

tor assembly as a video signal. The display circuitry also generates and sends

vertical (VSYNC) and horizontal (HSYNC ) sync signals to the A26 Monitor

assembly. A VGA-compatible video output is available at the rear of the

oscilloscope.

Tektronix TDS 420A Service Manual

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