Agilent Technologies 8590 Series Service Manual page 375

A14 Log Amplifier
The A14 log amplifier assembly provides the following functions:
• Log Mode. The input signal is logarithmically displayed due to the
sequential response of seven log amplifier stages.
The log amplifier stages have an overall range of 70 dB. This
allows a greater range of signal amplitudes to be simultaneously
displayed.
All seven amplifier stages are at maximum gain for low input
signal levels.
As the signal level increases, the gain of the each 10 dB amplifier
is reduced in sequence, with the last stage dropping to unity gain
first.
The vertical display axis is calibrated in dBm (relative to a
milliwatt) rather than volts.
• Linear Mode. The seven log amplifier stages are biased to operate as
linear amplifiers. Linear gains from 0 dB to 40 dB can be selected.
• The Video Detector. The detector is a half-wave rectifier and filter.
The video signal (VIDEO_IF) has a 0 to 2 volt output that is
proportional to the signal level.
• T/C Supply. A temperature-compensated −8 V reference supply
(−8 VT).
Maintains amplitude stability of the log stages over temperature.
Maintains linear step-gain accuracy by providing a stable voltage
source for the linear step-gain control lines that originate on the
A7 assembly.
A16 Processor/Video and A16A1 Memory Boards
The A16 assembly coordinates the operation of all analyzer assemblies
to perform all analyzer functions. This section briefly describes the
major analyzer functions provided directly by the A16 processor/video
assembly.
• Selection of the input signal for the ADC. The input MUX selects one
of the following:
An analog signal from assemblies installed in the card cage.
The detected 21.4 MHz IF signal (VIDEO_IF).
A +2 V reference used for ADC calibration of the graticule at top
screen.
An analog ground (ACOM) reference used for ADC calibration of
graticule at bottom screen.
Chapter 9
Assembly Descriptions and Block Diagrams
IF Section
375