12-10 Service Guide
Theory of Operation 8719ET/20ET/22ET
Digital Control Theory 8719ES/20ES/22ES
RF Network Analyzers
Main CPU
The main CPU is a 32-bit microprocessor that maintains digital control over the entire
instrument through the instrument bus. The main CPU receives external control
information from the front panel or GPIB, and performs processing and formatting
operations on the raw data in the main RAM. It controls the digital signal processor (DSP),
the front panel processor, the display processor, and the interconnect port interfaces. In
addition, when the analyzer is in the system controller mode, the main CPU controls
peripheral devices through the peripheral port interfaces.
The main CPU has a dedicated flash ROM that contains the operating system for
instrument control. Front panel settings are stored in SRAM, with a battery providing at
least 5 years of backup storage when external power is off.
Main RAM
The main random access memory (RAM) is shared memory for the CPU and the digital
signal processor. It stores the raw data received from the digital signal processor, while
additional calculations are performed on it by the CPU. The CPU reads the resulting
formatted data from the main RAM and converts it to GSP commands. It writes these
commands to the GSP for output to the analyzer display.
EEPROM
Electrically-erasable programmable read only memory (EEPROM) contains factory set
correction constants unique to each instrument. These constants correct for hardware
variations to maintain the highest measurement accuracy. The correction constants can be
updated by executing the routines in Chapter 3 , “Adjustments and Correction Constants.”
Digital Signal Processor (DSP)
The digital signal processor (DSP) receives the digitized data from the A10 digital IF. It
computes discrete Fourier transforms to extract the complex phase and magnitude data
from the 4 kHz IF signal. The resulting raw data is written into the main RAM.
A18 Display
The A18 display is an 8.4 inch LCD with associated drive circuitry. It receives a +3.3 V
power supply from the A19 GSP, along with the voltage generated from the A20 backlight
inverter. It receives the following signals from the A19 GSP:
• digital TTL horizontal sync
• digital TTL vertical sync
• blanking
• data clock
• digital TTL red video
• digital TTL green video
• digital TTL blue video
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