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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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