Model 8901B
Service
Service
Sheet
BD2
BLOCK
RF
PRINCIPLES
OF
OPERATION
General
The RF Block Diagram documents the Power Meter, Power Reference Oscillator, RF Input, IF
Amplifier, and Local Oscillator. The diagram also shows a typical Sensor Module connected to the
Modulation Analyzer’s INPUT and SENSOR connectors.
Sensor
Module
The Modulation Analyzer
is
designed to be used with an external module containing a Power Sensor,
RF switch, and interconnecting cable. The Sensor Module permits direct connection of either the
Power Sensor
or
the RF INPUT
to
the RF source being measured. The direct connection to the
Power Sensor minimizes power measurement errors that
result
from insertion and mismatch losses in
interconnecting cables and connectors. The instrument’s Controller switches the RF signal between the
input to the Power Sensor and the front-panel RF INPUT depending on the measurement being made.
This internal switching eliminates the need for manual reconnection and shortens measurement time.
The RF Power Sensing Element in the Power Sensor converts the absorbed RF power to a dc voltage
proportional to the average power level. The element may be a thermocouple
or
a diode. Because the
voltage from the RF Power Sensing Element is very small, dc amplifiers, which are subject to
drift,,
are
unsuitable for the first stages of amplification. Instead, the voltage is converted to a
220
Hz
ac signal
by the Chopper and then ac coupled into the
AC
Amplifier. The AC Amplifier
is
actually integrated in
the first stage of Amplifier
1
in the A53 Power Meter Assembly.
The Power Meter circuitry in the Modulation Analyzer
is
designed to
work
with
a
wide variety
of
power
sensors with varying frequency and sensitivity ranges. The power sensors contain a Sensor Identifier
Resistor which is unique for each type. The particular sensor
is
identified by driving the resistor with
a known current and measuring the voltage across it.
If
no Power Sensor
is
connected, the instrument
makes RF level measurements using the RF Peak Detector in the A15 RF Input Assembly. (Refer to
RF
Level,
in the
Detailed Operating
Instructions
of
the
Operation and Calibration Manual.)
Power
Meter
Assembly (A53)
The Power Meter Assembly contains circuitry for amplifying and detecting the small ac signal from the
external Power Sensor and for controlling the external input switch and Power Reference Oscillator.
AC Amplifiers
1
and
2
and Attenuators
1
and
2
accurately amplify the chopped dc voltage from the
external
RF
Power Sensing Element. (The
AC
Amplifier in the Power Sensor forms the first stage
of
Amplifier
1.)
The amplifiers have
a
bandpass response tuned to
220
Hz
to minimize noise. Table
8D-3
lists the attenuation for the power meter ranges. (The ranges are not listed in terms of absolute
power because the sensitivity of the power sensors varies with type. Also, the attenuation in
dB
is the
voltage attenuation; a step of
20
dB
of attenuation corresponds to a
10
dB
step in power level.) Special
Function 10 controls selection of the attenuators.
Service Sheet
BD2
8D-15
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