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UHF (403-470MHz) Transmitter Power Amplifier (PA) 25 W

3.5

Antenna Switch

The antenna switch consists of two PIN diodes, D4471 and D4472. In the receive mode, both diodes

are off. Signals applied at the antenna jack J4401 are routed, via the harmonic ﬁlter, through

network L4472, C4474 and C4475, to the receiver input. In the transmit mode, K9V1 turns on Q4471

which enables current sink Q4472, set to 96 mA by R4473 and VR4471. This completes a DC path

from PASUPVLTG, through L4437, D4471, L4472, D4472, L4471, R4474 and the current sink, to

ground. Both diodes are forward biased into conduction. The transmitter RF from the directional

coupler is routed via D4471 to the harmonic ﬁlter and antenna jack. D4472 also conducts, shunting

RF power and preventing it from reaching the receiver port (RXIN). L4472 is selected to appear as a

broadband lambda/4 wave transmission line, making the short circuit presented by D4472 appear as

an open circuit at the junction of D4472 and the receiver path.

3.6

Harmonic Filter

Components L4491-L4493 and L4472, C4491, C4496-98 form a Butterworth low-pass ﬁlter to

attenuate harmonic energy of the transmitter to speciﬁcations level. R4491 is used to drain

electrostatic charge that might otherwise build up on the antenna. The harmonic ﬁlter also prevents

high level RF signals above the receiver passband from reaching the receiver circuits, improving

spurious response rejection.

3.7

Power Control

The transmitter uses the Power Control IC (PCIC, U4501) to control the power output of the radio. A

portion of the forward RF power from the transmitter is sampled by the directional coupler and

rectiﬁed, to provide a DC voltage to the RFIN port of the PCIC (pin 1) which is proportional to the

sampled RF power.

The ASFIC (U0221) has internal digital to analog converters (DACs) which provide a reference

voltage of the control loop to the PCIC via R4505. The reference voltage level is programmable

through the SPI line of the PCIC. This reference voltage is proportional to the desired power setting

of the transmitter, and is factory programmed at several points across the frequency range of the

transmitter to offset frequency response variations of the transmitter's power detector circuit.

The PCIC provides a DC output voltage at pin 4 (INT) which sets the drain current of the ﬁrst

(U4401) and second (Q4421) transmitter stage via current control op-amps U3402-1 and U3402-2.

This adjusts the transmitter power output to the intended value. Variations in forward transmitter

power cause the DC voltage at pin 1 to change, and the PCIC adjusts the control voltage above or

below its nominal value to raise or lower output power.

Capacitors C4502-4, in conjunction with resistors and integrators within the PCIC, control the

transmitter power-rise (key-up) and power-decay (de-key) characteristic to minimize splatter into

adjacent channels.

U4502 is a temperature-sensing device, which monitors the circuit board temperature in the vicinity

of the transmitter driver and ﬁnal devices, and provides a dc voltage to the PCIC (TEMP, pin 30)

proportional to temperature. If the DC voltage produced exceeds the set threshold in the PCIC, the

transmitter output power will be reduced so as to reduce the transmitter temperature.

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