Microwave Data Systems MDS 4310 Installation, Operation & Field Maintenance page 90

The transmitter can be turned off, independent of microcontroller control, by the synthesizer
out-of-lock signal. An out-of-lock condition at the synthesizer will bring the O/L line high,
overriding the power control signal TXE2.
TRANSMIT POWER AMPLIFIER
The power amplifier chain of the transmitter section consists of U40 and U33. U40 is a
driving amplifier for U33. The output of U40 is fed to hybrid power module U33. The power
control of U33 is controlled by the power control regulator Q3 and U44A.
The RF output of U33 is fed through a directional coupler to the antenna switching network.
ANTENNA SWITCH
The antenna switch consists of PIN diodes and a filter network. In the receive mode, PIN
diodes are unbiased and effectively disconnected from the circuit. Under this circumstance,
the received signal is free to pass to the input of helical resonator Z1 through a low pass
filter.
During the transmit mode, the PIN diodes are biased ON by the Q19. When the diodes are
conducting, one diode provides a low impedance path for the transmit signal to the
ANTENNA
connector and the other diode shorts out the capacitor in the switching circuit. With the
capacitor shorted, the filter network is equivalent to a quarter wave transmission line with no
RF current flowing through the inductor.
DIRECTIONAL COUPLER
The directional coupler is used during diagnostic measurements of forward and reflected
power at the connector. The directional coupler consists of an in-line transmission
ANTENNA
line section that conducts RF energy from power amplifier U33 to the antenna switch.
Coupled line sections located immediately adjacent to, and either side of the transmission
line, receive a sample of the RF energy passing through the coupler. Rectifiers mounted on
each end of coupled line sections produce DC voltages. These DC voltages are proportional
to the power levels appearing on the coupled lines.
The directional coupler provides a forward power measurement that can be displayed through
local diagnostics when the radio has the Remote Maintenance Module installed.
KEYLINE AND CONTROL CIRCUITS
There are three push-to-talk (PTT) inputs to the keyline control circuit. Three negative inputs
and one positive going input. The negative going inputs come from J1-16, the modem, and
the audio processing board. These are inverted and added to the positive going input coming
from J1 pin 14. This signal drives the keyline inverter U19A.
RADIO DISABLE
J1-Pin 12, is the input to the radio disable circuit. The radio disable circuit minimizes power
consumption by shutting off all of the transceiver circuits except for Q15 and CR32B. This
allows the user to inhibit transceiver functions with an open-collector interface circuit.
J1-Pin 12 should be left open to permit the transceiver to function normally.
MDS 05-2415A01, Rev. A THEORY OF OPERATION 5-3