User's Manual

SRM9000X8 800MHz SERVICE MANUAL

3.2.2

Power Control

The final output power is stabilised by a power control feedback loop. A printed circuit transmission line, L561,

resistor R560, diodes D561A and D561B, and other associated components, form the power detector. Comparator

U551A and associated components provide the power setting and power control functions. Forward and reverse

power is sampled by the power detector and applied as a DC voltage to the inverting input of the comparator. The

TX_PWR set voltage is a DC voltage proportional to the programmed Tx power setting and is applied to the non-

inverting input of the comparator.

The TX_PWR calibration voltage originates from the PLA as a PWM signal and is integrated for application to the

comparator.

The PA module output level variation due to variations of supply voltage, output load or temperature, is detected

and applied to the comparator. The comparator proportionally adjusts the PA module bias supply and the bias

supply for the PA driver (Q531A) and thus the PA drive level. High temperature protection is provided by thermistor

R557, that progressively reduces the power level if the PA module temperature becomes excessive.

3.2.3

Antenna Changeover and Harmonic Filter

The antenna changeover circuit consisting of pin diodes D581, D582 and D583, is switched by transistors Q571,

Q572 and Q573 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 C585. The short

circuit at the receiver input is transformed to an effective open circuit at D581, by a lumped transmission line

(L591), 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 loss. The harmonic reject, low pass

filter comprises L592, L593, L594 and associated capacitors.

3.2.4

Transmitter Audio Processing

The microphone audio input signal is applied to the radio microphone input (AUDIO_IN1) and is derived from an

external microphone and pre-amplifier that provides a typical speech signal level of 40mV RMS. U801A is a control

gate that switches between AUDIO_IN1 and OPTION_AUDIO1 to provide external audio options and data input.

U801C 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 DSP after A-D conversion by CODEC (U800). The processed

transmitter audio/data from the CODEC output at VOUTR, is applied to the VCO as a modulation signal with a

level of approximately 200mV P/P via AF Switch (U801B).