X-2 Technical Service Manual Technical Description 4-3
Publication No: 15-02047
4.1.3 Receive/Transmit Switching
104-02907 sheet 3
The microcontroller IC403
[F6]
controls whether the path through the
transceiver is set up for receiving or transmitting. Serial data (I
2
C bus) from
the microcontroller feeds a 16 bit output expander IC405
[E8]
on the
Rx/Exciter PCB.
104-02907 sheet 1
Output pin 15, which is the transmit/receive select line, connects to cascaded
NAND gates IC7/D
[A6]
and IC7/C to operate the appropriate mixers via
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
[B2]
and V8
[B7]
. These supply the
+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]
and V9
[B8]
. These switches supply the +10 V to switch on the transmit
mixers IC2
[D3]
and IC9
[D7]
.
Three additional functions of switch V9 in the transmit mode are:
• DC is applied from the collector via D9
[C9]
and R54 to pin 7 of IC13.
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
[C9]
pin 5. With a
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
[C8]
. This
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
[D3]
by providing a DC bias
current via R312 to pin 5.
104-02908
Serial data from the microcontroller is also applied to IC1
[C4]
on the PA
assembly. When transmit is selected, a ground on pin 11 energises the
transmit/receive relay K7
[D8]
. This completes the path from the PA output via
the filters to the antenna socket.
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