HP 8901B Operation And Calibration Manual page 23

Model 8901B General Information
0
audio distortion analyzer,
0
measurement calibrators (AM, FM, or power level), and
0
HP-IB programmability.
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
d B ) .
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 H P 11722A. (See
Figure 1-2.) For all measurements except RF Power, the Sensor Module routes the signal to the
F W
input connector of the Modulation Analyzer. 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.)
The Power Sensor
is
unprotected against and
is
easily damaged by
sudden, large overloads. Refer to Table
1-1
under RF Power, Supplemental
Characteristics, RF Power Ranges of HP 8901B Modulation Analyzer with
HP
11
722A Sensor Module, for information on maximum operating lemls.
When the RF Peak Detector senses that the input signal level exceeds lW, it opens the Overpower
Relay. This is done without intervention of the Controller. The output from the
RF
Peak Detector, read
by the voltmeter, is used to set the Input Attenuator to optimize the level applied to the Input Mixer.
The Input Mixer converts the input signal to the intermediate frequency
(IF).
For frequencies greater
than 10 MHz, the
IF
is 1.5 MHz with the Local Oscillator
(LO)
tuned 1.5 MHz above the input
frequency; an IF of 455 kHz can be manually selected for this frequency range. The 455 kHz IF is
selected automatically for input signals between 2.5 MHz and 10 MHz. Below 2.5 MHz, the input
passes directly through the Input Mixer without down-conversion.
NOTE
For the input signal to pass through the Input Mixer without down-
conversion, the LO must still be present to turn the mixer
diodes
on. A n
LO
frequency of 101.5 MHz
is
arbitrarily used. Thus the instrument will
respond to input frequencies of
100 or 103
MHz
as
well as frequencies
between 150 kHz and 2.5 MHz.
The instrument can be manually tuned to a desired signal even in the presence of larger signals, although
filtering may be necessary since low-frequency signals pass directly into the IF. The
RF
High-Pass
Filter can be inserted (via a Special Function) in the RF path for this purpose.
To measure the input frequency, the Counter measures the frequency of the LO and the frequency
of the IF from the output of the IF Amplifier and Filter. The Controller computes and displays the
difference between the two frequencies. For input frequencies below 2.5 MHz, only the IF is counted,
which equals the input frequency.
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