Input Protection And Power Meter - Motorola R-2001A Service Manual

Communications system analyzer
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17-1. General. The RF Input Module is subdivided into three isolate circuits; input protection and power
meter, wideband amplifier and frequency converter and duplex generator. A block diagram of the RF Input
Module is shown in figure 17-1 with its schematic shown in figure 17-2.
17-2. Input Protection and Power Meter. RF power to and from the system pass through this section to a
common input/output RF connector (RF In/Out) attached to the module. In the generate or monitor operating
modes the input protection relay is switched so that a low-loss 50-ohm path exists through the module. When
the power monitor mode is selected, the WATT MTR EN line switches the relay so that the input is connected
to a 50 ohm power termination. A detector across a portion of the load provides a DC level proportional to the
input RF level. This level isamplified and made availabletothe system processorforthedetermination of input
power. A thermal sensor monitors the load temperature and signals the processor when safe operating limits
are exceeded. The processor in turn warns the operator that the RF input to the unit must be removed to
prevent permanent damage.
17-3. If power in excess of 200 mW is applied to the system while operating in the 50 ohm load, protecting the
system. A signal line (INPUT PROTECT ACT) to the processor results in an audiable and visual warning to the
operator that the unit is in a protected mode. The warning ceases and normal operation resumes if the RF input
is removed or if the power monitor mode is selected.
17-4. Wideband Amplifier and Frequency Converter. The wideband amplifier provides a leveled RF output
from-3dBm to+13dBm in the generate mode and a +7dBm LO drive in the monitor modes over the 10 KHz to I
GHz frequency range. Primary components of the leveling loop are; the input VCA (Voltage Controlled
Attenuator), the output level detector, and the level comparator. A level control voltage, proportional to the
desired output level, is compared to the actual output level as determined by the level detector. The result of
the comparison steers the VCA maintaining the detected output level equal to the requested output level. In the
generate mode the control voltage is obtained from the front panel RF level control (AM Mod + DC REF). For
generate AM, the modulation signal is summed with the DC control level, causing the RF output level to follow
the modulation signal. Also, in the generate mode the signal from the output level detector(CARRIER + MOD
LVL) is made available for the determination of RF output power and percent of AM. A fixed reference voltage is
switched to the level control input in the monitor modes giving a leveled +7dBm local oscillator drive.
17-5. The VCA on the wideband amplifier board covers the frequency range from 1 MHz to 1 GHz. For
frequencies below 1 MHz, the VCA select circuit clamps the VCA in the minimum attenuation position and
enables a low frequency VCA in the RF Synthesizer. Coincident with the enabling of the low frequency VCA,
the time constant of the output RF level detector is increased assuring proper operation down to 10 kHz.
17-6. The wideband amplifier output is relay switched between the local oscillator port of the input mixer for
the monitor and generate DSBSC modes, and the RF attenuator for the generate mode. An RF sample from the
mixer local oscillator output terminal, at a nominal level of -20dBm, is provided to the duplex generator.
17-7. The frequency converter section consists of the input mixer, the first IF amplifier, and IF filters. In the
monitor mode the desired signal is converted to 10.7 MHz by the input mixer. A two-pole input filter, IF
amplifier, and a four-pole output filter select the 10.7 MHz component at the mixer output. The 10.7 MHz IF
output of the converter is applied to the receiver module.
SECTION 17
RF INPUT MODULE (AH)
17-1

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