Receive/Transmit Switching - Codan X-2 Service Manual

X-2 Technical Service Manual
4.1.3

Receive/Transmit Switching

104-02907 sheet 3
The microcontroller IC403
transceiver is set up for receiving or transmitting. Serial data (I
the microcontroller feeds a 16 bit output expander IC405
Rx/Exciter PCB.
104-02907 sheet 1
Output pin 15, which is the transmit/receive select line, connects to cascaded
NAND gates IC7/D
switching transistors in the receive and transmit modes.
Receive Mode
In the receive mode Pin 15 of IC405 is high (+10 V). This sets output pin 11
of IC7/D low and forward bias transistors V1
+10 V to switch on the receive mixers IC1 and IC8 respectively.
Transmit Mode
In the transmit mode pin 15 of IC405 is low (0 V). This sets output pin 11
of IC7/D high, switching off the receive mixers. Because pin 11 is connected
to a second NAND gate (pin 9), output pin 10 is now low and forward biases
V2 [B4]
mixers IC2
Three additional functions of switch V9 in the transmit mode are:
•
•
•
104-02908
Serial data from the microcontroller is also applied to IC1
assembly. When transmit is selected, a ground on pin 11 energises the
transmit/receive relay K7
the filters to the antenna socket.
Publication No: 15-02047
and IC7/C to operate the appropriate mixers via
[A6]
and V9 [B8] . These switches supply the +10 V to switch on the transmit
[D3] and IC9 [D7] .
DC is applied from the collector via D9
This sets the noise limiter IF amplifier to minimum gain and inhibits the
noise limiter from operating in the transmit mode.
104-02907 sheet 2
The collector is connected to analogue switch IC305/B
high on pin 5 of IC305/B the analogue switch is closed applying a
ground to differential input pin 5 of AGC amplifier IC307/B
sets the AGC to 0 V, switching off the receive 455 kHz IF amplifier
V301/V302 [B5] , avoiding the necessity to inhibit the demodulator IC302
in transmit mode.
V9 enables the transmit modulator IC301
current via R312 to pin 5.
[D8]
[F6] controls whether the path through the
[B2]
and R54 to pin 7 of IC13.
[C9]
[D3]
. This completes the path from the PA output via
Technical Description 4-3
2
C bus) from
on the
[E8]
and V8 . These supply the
[B7]
pin 5. With a
[C9]
[C8] . This
by providing a DC bias
[C4] on the PA