Maxon Telecom SP210 - Manual (Page 35)

SP200/210 Maxon SP200/210 Radio
Issue 1.0 DETAILED FUNCTIONAL DESCRIPTION
03/01 Page 5-7
5.6 Transmitter
The transmitter comprises:
Figure 5-3 - Transmitter Block Diagram
Buffer
The RF output level from the VCO is 0dBm. This is amplified to +17dBm by the buffer amplifier
consisting of Q16, Q17, Q3 and associated components. All stages are simple common-emitter
amplifiers with resistive biasing and tuned collectors. A pi-type attenuator (R31, 32 and 34) is used
between the two stages.
PA module
The PA module consists of a hybrid amplifier. Pin 3 is permanently connected to the battery voltage
and pin 2 is connected to the supply when switched by Q22, which is controlled via the power control
circit by the Tx enable line. The RF input is on pin 1 and the output is on pin 4.
Low pass filter
L7, L8, L11, C73, C74, C75 and C76 form a 7th order Chebyshev low pass filter. Unwanted
harmonics are reduced by 65dB.
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 L13 look open circuit (1/4 wave tuned stub). This prevents the
Tx signal from passing to the receiver stage.
In receive, the diodes D5 and D6 are reverse biased. L13 is now in circuit, passing the signal from the
antenna to the front end without signal loss.
Automatic power control (APC) circuits
The APC circuit consists of IC3B, Q19, Q21, Q22 and associated components. The PA supply current
passes through R109. Variations in power output, produce variations in supply current, which is
amplified by IC3A and Q19.
The voltage on the wiper of RV1 is passed to IC3B and compared with the fixed reference voltage
developed by the potential divider R45 and R46. The output of IC3B, pin 7, controls Q21 and Q22 to
provide supply voltage to the PA driver transistors Q501 and Q502. RV3 controls the low power
setting and is switched into circuit by Q23.
A change in PA output will result in an inverse change in driver supply voltage maintaining a constant
RF output.