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Model 8902A
General Information
Programmability
The Measuring Receiver is completely programmable via the Hewlett-Packard Interface Bus (HP-
IB).
This, coupled with the diversity of measurements the Measuring Receiver can make, the speed
with which these measurements can be made, and the flexibility of the Special h c t i o n s , make the
instrument ideal for systems applications. In many instances it can reduce the number of instruments
in a system, speed measurements, reduce complexity and improve accuracy.
When the Measuring Receiver is in remote, the front-panel annunicators make it very easy to determine
the state the instrument is in, whether it is in the
talk,
listen, or service request state.
1-10. PRINCIPLES
OF
OPERATION USING A SIMPLIFIED BLOCK DIAGRAM
The Measuring Receiver is a calibrated, superheterodyne receiver, which converts the incoming signal
to a
fixed, intermediate frequency
(IF),
which is then demodulated. As in a radio receiver, the Measuring
Receiver contains an RF amplifier, a local oscillator
(LO),
a mixer, an
IF
amplifier and bandpass filter,
a
demodulator (detector or discriminator), and audio filters (tone controls). The Measuring Receiver,
however, contains additional features which make it much more versatile:
0
automatic tuning,
0
selectable measurement mode: signal frequency, power level, or modulation (AM, FM, or Phase
0
RF power level measurements on tuned signals to -127 dBm (0.1 pV),
0
relative RF level measurements on out-of-channel signals to -129 dBc (Option Series 030),
0
selectable audio detector (peak, average, or
rms
responding),
0
audio counter,
0
audio distortion analyzer,
0
measurement calibrators (AM,
FM, or
power level), and
0
HP-IB programmability.
Modulation
(QM)),
The entire operation of the instrument is governed by a microprocessor-based Controller. The Controller
sets up the instrument at turnon, interprets keyboard entries, executes changes in internal hardware,
and displays measurement results and error messages. The computing capability of the Controller is
also
used to simplify circuit operation. For example, it forms the last stage of the Counter, calculates
the AM or FM generated by the AM and FM Calibrators, and converts measurement results into ratios
(in
%
or
a).
The Controller also contains routines useful for servicing the instrument.
RF
Circuitry
The RF input signal normally enters an external Sensor Module such as an HP 11722A. (See Figure
1-2.) For all measurements except RF Power, the Sensor Module routes the signal to the RF input
conector of the Measuring Receiver. For the
RF
Power measurement, the input signal passes directly
into the Power Sensor, which converts the
RF
power absorbed by the
RF
Power Sensor into a low-
frequency, chopped, ac voltage whose amplitude is proportional to the average RF power. The Power
Meter amplifies the chopped signal and converts it to a dc voltage which is then measured by the
voltmeter. (The voltmeter includes the Audio Peak Detector, Audio Average Detector, Voltage-to-Time
Converter, and Counter.) The calibration of the Power Meter can be verified by connecting the Sensor
Module to the CALIBRATION RF POWER OUTPUT connector on the front panel. (The 50 MHz
Power Reference Oscillator is an accurate 1 mW reference.)
re v.24AUG8
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1-9
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