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Model 8901B
Service
Service Sheet BDI
BLOCK
0
Overall Instrument
PRINCIPLES
OF
OPERATION
NOTE
The following discussions cover the principles of operation of the Modu-
lation Analyzer. Each discussion is based on and referenced to a service
sheet. A n introductory-level discussion
of
the principles
of
operation can
be found in Section 1 of the Operation and Calibration Manual under the
title, Principles of Operation for Simplified Block Diagram.
General
The Modulation Analyzer is a general-purpose, tuned, signal-measuring instrument. The Overall Block
Diagram breaks up the instrument's functions so they correspond to those of a receiver: RF Block,
IF Block, and Audio Block. The Digital Block (using the receiver analogy) roughly corresponds to
the human operator. Each of the functional blocks is shown in greater detail in Service Sheets BD2
through BD5.
Sensor Module
During normal operation. the RF input signal is applied to an external Sensor Module such as the H P
11722A. The Input Switch in the Sensor Module routes the RF signal to the Power Sensor when the
RF Power measurement mode is selected. The Power Sensor converts the RF input signal to a chopped
dc voltage which is proportional to the average power level of the signal. For other measurement modes,
the switch routes the input signal to the INPUT connector of the Modulation Analyzer. The Sensor
Module also contains a resistor which provides a means for the Modulation Analyzer to identify the
type of Power Sensor in the Sensor Module.
Power Meter
The Power Sensor and Power Meter amplify the dc voltage from the Power Sensor's transducer so the
voltage can be measured by the Voltmeter in the Audio Block. To minimize the effect of dc drift in the
amplifiers (which may even exceed the dc voltage from the transducer itself), the transducer's output
is converted to an ac voltage by a chopper in the Power Sensor. The chopper drive comes from the
220 Hz Multivibrator. The resultant ac signal is ac coupled to the narrow-band Input Amplifier, and
the gain of this amplifier is autoranged by the Controller. The amplified ac signal is converted back
to dc by the Synchronous Detector, which full-wave rectifies the ac signal by rechopping it in phase.
Noise on the resultant dc signal is filtered by the Noise Filters to stabilize the displayed reading. The
heaviest filtering
is
done on the most sensitive ranges and results in an increase in the measurement
response time. Before an RF power measurement is initiated, the switch at the output of the Power
Meter connects the Sensor Identifier Resistor circuit to the Voltmeter. The voltage across the resistor
indicates the type of Power Sensor present.
The Zeroing DAC is set by the Controller to cancel any dc offset of the Power Sensor when no input
is
applied. In the zeroing sequence, the RF Input to the Power Sensor
is
switched out, a power reading
is taken, and the Zeroing DAC
is
automatically adjusted for a zero reading. (In actual implementation,
the Zeroing DAC is adjusted for a pre-determined offset voltage reading. The offset
is
subtracted out
in subsequent power measurements by the Controller. The Voltmeter then operates in its most linear
region.)
Service Sheet BD1
8D-1
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