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Service
Model 8901B
Calibration of the Power Sensor is verified by connecting the Sensor Module to the CALIBRATION
RF POWER OUTPUT connector, which outputs a calibrated
1
mW signal from the 50 MHz Power
Reference Oscillator. If desired, a calibration factor can be stored and used to correct subsequent power
readings as explained in the
Detailed Operating Instructions
in Section 3.
RF Input
The Modulation Analyzer measures RF signals in the frequency range from 150 kHz to 1300 MHz and
power levels of -25 to +30 dBm into its 50 ohm input. The voltage, sensed at the input by the RF
Level Detector, is used to help set the proper input attenuation and, if the input exceeds lW, to trip
the Overpower Protection relay. When RF Level (Special Function 35) is selected, the Voltmeter reads
the output from the RF Level Detector. The controller converts the output from' the Voltmeter into
power in watts.
The 5.25 MHz High-Pass Filter is manually selectable. Since the
IF
will generally respond to signals
2.5 MHz and below, the filter eliminates any low frequencies which may be present on the input. For
signals in the range of 150 kHz to 10 MHz, the filter should be switched out.
The Input Attenuator is set to provide the Input Mixer with an optimum input level. The attenuator
pads are set by the Controller which receives signal level information from the RF and IF level detectors
(via the Voltmeter).
Input Mixer and IF
The Input Mixer down-converts the
RF
input to the intermediate frequency
(IF).
The
IF
frequency is
normally the frequency of the LO minus the frequency of the
RF
signal.
The IF is centered at 1.5 MHz for input signals 10 to 1300 MHz. (However, an IF of 455 kHz can be
manually selected.) For signals between 2.5 MHz and 10 MHz the
IF
is 455 kHz. Below 2.5 MHz, the
signal is passed directly into the
IF
without being down converted (unless the 455 kHz
IF
has been
manually selected).
The
IF
Amplifier is a low-noise, 33 dl3 amplifier. When the 1.5 MHz
IF
is selected, the frequency
response is determined by the 2.5 MHz Low-Pass Filter. When the 455 kHz
IF
is selected, the 455
kHz Bandpass
IF
Filter preceeding the First
IF
Amplifier determines the IF response.
The
IF
signal is sent to the the 2.5 MHz Low-Pass Filter where it is buffered and sent to both the
rear-panel
IF
OUTPUT connector and to the FM Demodulator circuitry. The
IF
Peak Detectors sense
the presence of the
IF
signal (and thus the input
RF
signal) during an automatic signal search (via the
IF
Present and Stop Sweep lines). The Voltmeter measures detected
IF
level (via the
IF
Peak Level
line) to help determine the setting of input attenuation.
Selective Power Options (Option Series 030)
The Selective Power Options (Option Series 030) add a set of selectable bandpass filters and a
programmable IF amplifier to enable the instrument to make accurate adjacent-channel power and
single-sideband noise measurements on transmitters and signal generators. For these measurements,
the instrument is first tuned to the incoming signal using the 455 kHz
IF.
The
IF
level is then measured
with the IF RMS Detector after the
IF
is bandpass filtered and amplified. Next, the R F frequency is
offset (as required by the specific test), and the IF level is again measured. The ratio of the two IF
level measurements is the relative adjacent-channel power or single-sideband noise. (Refer to
selective
Power Measurements
in the
Detailed Operating Instructions
in Section 3.)
Two bandpass filters in series define the measurement bandwidth. The First Channel Filters are one
of two pairs of IF filters. The Option Series 030 specifies the pairs of channel filters installed in the
instrument. The pair of Second Channel Filters have bandwidths of 30 kHz (for adjacent channel
power measurements) or 2.5 kHz (for single-sideband noise measurements). Filter selection is listed in
Table 8D-1. The measurement bandwidth indicated in the table is the total bandwidth of the selected
filters. In addition to determining the total measurement bandwidth, the second filter pair remove
broadband noise generated by the preceding IF amplifiers. The Programmable IF Amplifier, governed
8D-2
Service Sheet BD1
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