DRAFT D
TNM-M-E-0001 Page 17
3.2.2 Power Control
Output power is stabilised by a power control feedback loop. L1, R54, a printed circuit
transmission line, D6 and associated components comprise the power detector with Q3/10, U3
and associated components providing the power setting and control sections. Forward and
reverse power is sampled by the power detector and applied as a DC voltage to the inverting
input of comparator U3A. The TX PWR SET voltage which is a DC voltage proportional to the
programmed TX power setting is applied to the non-inverting input of the comparator. PA
module output level changes due to supply voltage, load or temperature variations are detected
and applied to the comparator which proportionally adjusts the PA driver (Q12) supply, and
therefore the PA drive level, via Q10/Q3. High temperature protection is provided by thermistor
R452 which progressively reduces the power level if the PA module temperature becomes
excessive. Q15 and Q16 provide for dual power control time constants necessary for good
power ramp and decay characteristics.
3.2.3 Antenna Changeover and Harmonic Filter
The antenna changeover circuit consisting of pin diodes D3/D4/D5 is switched by Q4/Q8/Q11
and associated circuitry allowing the transmitter output to be coupled to the antenna while
providing isolation for the receiver input. With the transmitter switched on, the diodes are
forward biased allowing power to be coupled through to the antenna and isolating the receiver
by grounding its input at C28. The short circuit at the receiver input is transformed to an effective
open circuit at D3 by L13, which minimises transmitter loading. With the transmitter switched off
the diodes are reverse biased allowing the receiver input signal to reach the receiver front end
with minimal loading and loss.
The harmonic rejection low pass filter comprises L10/11/12 and associated capacitors.
3.2.4 Transmitter Audio Processing
Microphone audio input signals of 40mV RMS with a source impedance of 470 ohms are
provided at the microphone input (AUD IN1) by an external microphone unit comprising an
electret microphone insert and a preamplifier with a gain of 18dB. U108 is a control gate for the
microphone audio signals.
AUD IN2 is the external audio options and data input which is controlled by gate U107. Inverter
Q20 ensures that the data or audio options signals are muted when the mic. audio gate is
active. The AUD IN2 input level and source impedance is the same as the microphone input.
Q812 is a unity gain amplifier which provides buffering of the audio and data signals. U103B
provides CODEC input switching which selects either the receiver I signal or transmitter
audio/data signals depending on the TX/RX mode. All pre-emphasis, filtering, compression and
limiting processes for narrow and wideband operation are carried out in the CODEC (U800)
under the control of the DSP. The processed transmitter audio/data from the CODEC output at
VOUTL is applied to the VCO as a modulation signal with a level of approximately 200mV P/P.
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