Theory of Operation
Main Board
The Main board is also called the acquisition board. The Main board of the
4-channel oscilloscopes is essentially two 2-channel oscilloscopes tied together
through a common m icroprocessor, and some special interconnects to support
combining the display and trigger systems. The focus of the Main board
discussion
is the 2-channel system, with differences for the 4-channel models
noted as necessar y.
At a minim u
m, the Main board contains attenuators, an amplifier ASIC, a
digitizer/trigger system ASIC, a signal processing/display/system services
FPGA,
RAM, flash PROM, a system m icroprocessor, USB controller, USB RAM, system
communication R AM, and special power supplies. For a 4-channel oscilloscope,
the attenuators are duplicated. Most o f the other a spects of the circuitry remain
unchanged.
Acquisi
tion System
Signals from the channel 1 and channel 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 th
e AC coupling s witch.
The amplifier ASIC contains buffers and variable gain amplifiers , as well as
filter
s 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 im pedance differential (and thus less noise-sensitive) environment for the
acquisition p rocess. The amplifierASICassuresthattheinputsignalisamplified
to a level that will 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 w aveform samples are transferred to the processing and display
FPGA.
I
n 4-channel systems, the two acquisition ASICs are interconnected so that a
trigger on one ASIC can cause a trigger on the other.
T
he processor system adds the microprocessor and flash PROM to the proces sing
and display system. The processor system interprets the front-panel control
changes detected by the display ASIC, pro vides control parameters based on
user setting requests, com putes waveform measurem ents, a nd 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.
24 TDS2000C and TDS1000C-EDU Series Oscilloscope Service Manual
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