Microwave Data Systems MDS 4310, Установка, эксплуатация и техническое обслуживание на месте

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5-2 THEORY OF OPERATION MDS 05-2415A01, Rev. A
The output of the squelch gate goes to the modem receive audio amplifier, U5A. U5A is an
amplifier with the gain set by potentiometer R25. R25 is used to adjust the receive audio
level supplied to the modem and remote maintenance boards.
U5D and U5C are an audio lowpass filter that removes noise from the receive audio. The
output of this filter goes to external modems through J1.
SQUELCH
The squelch circuit consists of a high-pass filter, noise amplifier, noise rectifier and a
comparator.
The high pass filter U6A removes low frequencies of the detected IF output. The output of
U6A goes to a gain stage, U6B, which amplifies the high frequency noise. The gain of U6B
is set by R41, which is the squelch threshold adjustment.
The amplified high frequency noise output from U6B goes to a full-wave rectifier, which
compares this DC voltage with a fixed reference voltage. U6C, which rectifies the noise
signal. The output of this stage goes to a squelch comparator, U6D whose output is the
receiver unsquelch sensor (RUS) line, and is used to control squelch gate U26A in the receive
audio path. This gate is also controlled by the RX MUTE signal from U19D.
The RUS logic signal is also fed to the modem and is used to gate the DCD output from the
modem. In addition, this signal appears at J1 pin 10 through a 1 k
Ω
resistor.
POWER SUPPLY
The + 13 volt DC input appears when an external power source is connected to J2. From J2,
the + 13V is conducted to the internal transceiver circuits through F1, a 4 ampere fuse. The
fuse protects the transceiver in the event the external, in-line fuse is either defeated or
replaced with one of a higher rating.
Z4 is a EMI power line filter. C77 is a electrolytic capacitor. These two devices help provide
a noise free power source for the transceiver circuits. A 17 volt zener diode across C77
provides over-voltage and reverse polarity protection.
U38 provides a reg 8 volts for most transceiver circuits. U13 provides a reg 5
volts, which supplies power to the microcontroller and the logic circuitry.
U45 is a low voltage protection device. It will disable the radio when the input voltage
becomes less than 10 volts. The output from U45 drives the radio disable circuit Q15 which
disables the 8 volt and 5 volt regulators, U38 and U13.
A precision reference DC voltage of + 2.5V is supplied by U4; this is used by the FSK
modem circuitry.
The RX/TX signal from the microcontroller a 8 volts to the RF amplifier, high IF
amplifier and tripler circuit in the receive mode, and is shut off in the transmit mode.
Q19 supplies a biasing signal to the transmitter amplifiers and antenna switch. Q19 is turned
on during transmit by the microcontroller.
The main transmit power control signal from the microcontroller, TXE2, is first conditioned
by U46 whose output is sent to the power control potentiometer R69. The wiper of R69 is
connected to U44B which is a power-leveling circuit. U44B drives U44A, Q41 and Q43
which form an adjustable voltage regulator. The output of this regulator feeds the power
control pin on U33