User's Manual

Buffer
When the radio is in transmit mode the diode D2 is forward biases enabling the modulated RF signal
from the VCO to pass to the buffer/pre-amplifier Q21 and Q22 and associated components.
The output signal is passed from Q22 to IC5 via a matching network consisting of Inductor L7 and
C73.
PA module
The signal is then amplified for transmission by IC5, which is a power amplifier module.
Low pass filter
The amplified RF signal is passed through the stripline coupler and is fed to the harmonic low pass
filter, comprising L12 to L15 and C94-C98 and then to the antenna connector (ANT). The stripline
coupler provides a sample of the RF signal for the automatic power control.
Antenna Switch
When transmitting, the diodes D5 and D6 are forward biased, allowing the RF to pass to the antenna.
D6 is shorted to ground which makes L11 look open circuit (1/4 wave tuned stub). This prevents the
TX signal from passing to the receiver stage.
Automatic power control (APC) circuits
The automatic power control contains the stripline coupler, diode D4, variable resistor RV1, U4A
and transistors Q19 and Q23.
The RF signal present in the coupler is rectified by D4, to produce a DC voltage. This DC voltage is
passed to one input of IC4A, which is a differential amplifier. In transmit mode a DC reference level
for U4A is supplied by the potential divider R34/R35.
The reference level and the detected level from D4 are compared a difference signal is produced. The
difference signal drives Q19, which then drives Q23. Q23 controls the supply voltage to the first
amplifier stage in IC5. This control loop produces a constant power output at the antenna connector
( ANT ).
RV1 is used to adjust the voltage that is fed back from D4 which defines the output RF power level.
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