Theory of Operation
3-- 4
TDS1000B and TDS2000B Series Oscilloscope Service Manual
Main Board
The Main board is also called the acquisition board. The Main board of the
four-channel oscilloscopes is essentially 2, two-channel oscilloscopes tied
together through a common microprocessor, and some special interconnects to
support combining the display and trigger systems. For this reason, the focus of
the main board discussion will be the two-channel system, with differences noted
as necessary.
At a minimum, the main board contains attenuators, an amplifier ASIC, a
digitizer/trigger system ASIC, a signal processing/display/system services ASIC,
RAM, flash PROM, a system microprocessor, USB controller, USB RAM,
system communication RAM, and special power supplies. For a four-channel
oscilloscope, the attenuators and ASICs are duplicated. Most of the other aspects
of the circuitry remain unchanged.
Signals from the CH 1, CH 2 and other input connectors pass through attenuators
and an AC-coupling switch to the amplifier ASIC. The EXT TRIG input has an
abbreviated version of this path, lacking some of the attenuator settings and the
AC coupling switch.
The amplifier ASIC contains buffers and variable gain amplifiers, as well as
filters that provide 20 MHz bandwidth limiting. The task of the amplifier ASIC is
to convert from a 1 MΩ single-ended environment in the front end to a much
lower impedance differential (and thus less noise-sensitive) environment for the
acquisition process. The amplifier ASIC assures that the input signal is amplified
to approximately the correct level to allow the fullest possible use of the digitizer.
The acquisition ASIC contains samplers and peak detectors for each input
channel, a common amplifier, an A/D converter, and the trigger logic. The
digitized waveform samples are transferred to the processing and display ASIC.
In four-channel systems, the two acquisition ASICs are interconnected so that a
trigger on one ASIC can cause a trigger on the other.
The processor system adds the microprocessor and flash PROM to the processing
and display system. The processor system interprets the front-panel control
changes detected by the display ASIC, provides control parameters based upon
user setting requests, computes waveform measurements, and manages the USB
interfaces via the dedicated USB controller. Saved setups, waveforms, and
calibration constants are stored in nonvolatile memory sections within the flash
PROM. The processor system shares DRAM with the display system.
Acquisition System
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