Monitor Mode; Monitor Mode; Rf Input; 2.5.3 Recei Ver - Motorola R-20010 Maintenance Manual

A block diagram of the wattmeter is shown at the end
of the section in Figure 2-4.
The RF input signal is first attenuated by a 14-dB
attenuator and then is loaded with a 50-ohm, 16-dB
attenuator. A sample of the input RF voltage between
t he two atte nuators is detected by the power detector,
to give a de out put proportional t o the peak RF volt-
age. T he amplifier following the detector buffers and
amplifies the detected voltage to provide the RF
INP UT POWER signal to the processor. The proces-
sor then determines and displays the RF input power.
A te mperature sensor located near the flange of the
RF load alerts the processor when the load tempera-
ture exceeds 80°C. The processor responds to the
OVER T EMPERATURE signal by dis playing a
wa rn ing message on the CRT and by sounding the
audi ble alarm.
2.5
MONITOR MODE
2.5.1 DESCRIPTION
In monitor mode, RF signals from an antenna or
from a transmitter can be checked
directly for fre-
quency error, signal strength, modulation level, and
spectral content. Also provided is off-the-air decoding
of two-tone or 5/6-tone paging, private line (PL) , dig-
ital private line (DPL) or mobile telephone signaling.
Another feature is Scan Lock, which allows the Sys-
tem Analyzer
to
acquire a received signal of greater
than -30 dBm.
A block diagram of the monitor mode is shown at the
end of the section in Figure 2-5.
2.5.2 RF INPUT
For signals less than 100 mW, the RF signal to be
moni tored is applied to t he Antenna port; for signals
greater than 100m W, to the RF In/Out port. The port-
select switch routes one of these signals to the step
attenuator.
In the monitor mode, the RF input from the step
attenuator is connected to the RF port on the receive
mixer. The outpu t from the Wideband Amplifier is
switched to the L.O. port on ·the receive mixer. The
processor programs the RF Synthesizer for an output
frequency t hat is offset 10.7 MHz from the frequency
to be moni tored. The offset may be above or below the
center frequency, as selected by the front panel's Image
switch. The 10.7- MHz difference signal at the IF port
of the receive mixer is fi ltered by a 20-MHz low-pass
filter. The filte r out put provides the 10.7-MHz IF sig-
nal
to
the Receiver board (A8). The overall gai n of the
RF Input mod ule (A17) is -8 dB ± 2 dB.
2.5.3 RECEI VER
The 10.7-MHz IF signal is first filtered and ampli -
fied on the Receiver board, providing a bandwidth of
2-5
280 kHz. This signal is split between the linear IF and
the logarithm ic IF by an active power splitter. The lin-
ear IF provides for demodulat ion of AM, FM, and SSB
signals. The logarithmic IF provides two functions:
spectrum analyzer and signal-strength indicator.
2.5.3.1 Linear IF
The linear IF bandwidth is determined by a wide/
narrowband filter which follows the power splitter. The
modulation-acceptance bandwidths are ± 6 kHz for
narrowband and ± 100kHz for wideband. Next, the IF
signal is down-converted to 700 kHz by an active
AGC'd mixer. Immediately following the second mixer
is t he 700-kHz second IF amplifier. The output level
of the amplifier Is rectified to detect amplitude mod-
ulation and to provide AGC control on t he second IF
amplifier, t he second mixer, and the delayed-AGC
amplifier. T he second IF signal is applied to the FM
demodu lator and to the BFO mixer for SSB demodu-
lation. The L.O. signal fo r the BFO mixer comes from
a 700-kH z (
±
3 kHz) oscillator on this board. T his
oscillator is controlled from the front panel.
Demodulated audio from the AM, FM, or SSB
demodulator is routed to the audio filters by the audio-
select swi tch, which is under processor cont rol. The
audio filters consist of a selectable-bandwidth low-pass
fi lter (300 Hz, 3 kHz, or 20 kHz), and a selectable-
bandwidth high-pass fi lter (300 Hz or 3 kH z). The
selected demodulator output is used to provide three
receiver out puts: 1) The VOL CONTROL AUDIO sig-
nal is the unfi ltered version of the selected demodu-
lator output.
It
goes to the Volume control on the front
panel, to an amplifier on the Scope/DVM Control
board (A 7) and finally to the speaker; 2) The DEMOD
CAL AUDIO out put is the filtered version of the
demodulator output and provides calibrated audio
levels for determining modulation leve l; 3) The
DEMOD OUT signal is also a filtered version of the
demodulator outpu t a nd dri ves th e front panel's
DEMOD OUT port.
2.5.3.2 Logarithmic IF
The 10.7-MHz IF input signal is down-converted to
460 kHz by mixing the input signal with the 10.24-MHz
signal from a second local oscillator. The second local
oscillator is provided by a crystal oscillator on the
Receiver board. Two outputs are provided by the log-
arithmic amplifie r and detector. These out puts pro-
vide a de voltage that is proportional to the logarit hm
of the 10.7-MHz IF input level. One output, SP EC-
T RUM ANALYZER VOLT, is the vertical input to the
scope for t he spectrum-analyzer display. The other
output, SIG STRENGTH VOLTAGE, is measured by
the digi tal voltmeter (DVM) on the P rocessor Inter-
face boa rd (A ll) to provide the signal strength (i n
dBm) of the 10.7-MHz IF input signal. The loga rith -
mic am plifie r has a dynamic range of approximately 80
dB, coveri ng input levels from -110 to -30 dB m.