User's Manual

SRM9000X8 800MHz SERVICE MANUAL

3. TECHNICAL DESCRIPTION

3.1 RECEIVER

Refer to Figure 3-1.

3.1.1

Front End Filters and RF Amplifier

The receiver input signal from the antenna passes through the harmonic filter and antenna T/R switch. With the

mobile in receive mode, diodes D580, D582 and D583 in the antenna switch are reverse biased allowing the

receiver input signal to be coupled through to the receiver front-end with minimal loss. The overall insertion loss of

the harmonic filter and switch is approximately 0.8dB.

The signal is then fed through SAW bandpass filter (Z400) to the input of the RF amplifier (Q404). The SAW filter

bandpass covers 851MHz to 870MHz. The RF amplifier stage comprises a low noise transistor amplifier (Q404)

that is compensated to maintain good linearity and low noise matching; this provides excellent intermodulation and

blocking performance across the full operating range. The overall gain of the front-end is approximately 9dB.The

RF amplifier has constant current bias controlled by Q402. The output of the RF amplifier is coupled through a

varactor-tuned bandpass filter comprising of two ceramic resonators (Z430 and Z431). The varactors have

individual PWM tuning voltages, TUNE 1 and TUNE 2, that are derived from PLA (U300). The tuning voltages

values for the filter varactors are controlled by the alignment data stored in the radio. The DSP processes these

data to optimise the filter tuning for each of the programmed channel frequencies.

A negative bias supply originates from the DSP/PLA as a PWM signal for the two filter tuning voltages for the

specific channel frequency selected. The PWM signal, which is dependent on channel frequency and tuning,

passes through level shifting transistors Q451 to Q454, where it is converted to a negative voltage in the range

-0.5V to -11.5V. The -12.0V rail for the level translators is generated by U904E/F, with D903 to D906 providing the

required voltage multiplication.

3.1.2

First Mixer and IF Section

The output of the ceramic resonator pair is then fed into U441, a high performance passive mixer that converts the

RF signal to a IF of 45MHz. The first local oscillator injection level is typically +8dBm with high side injection.

Following the mixer is a IF amplifier (Q461) that provides approximately 15dB of gain and, in association with its

output circuitry, presents the required load conditions to the 4 pole 45MHz crystal filters Z471A/Z471B.

The crystal filters provide part the total required selectivity for adjacent channel operation with the remaining

selectivity provided by a DSP bandpass filter algorithm.

3.1.3

Quadrature Demodulator

Additional IF gain of approximately 44dB occurs in U481, which is a dedicated IF AGC Amplifier and Quadrature

Demodulator. The AGC voltage for U481 is derived from the RSSI function of the DSP. The onset of AGC

operation occurs when RF input signal level at the antenna connector exceeds -90dBm and can reduce the gain

by approximately 100dB for strong signals.

Conversion of the 45MHz IF signal to I and Q baseband signals is carried out by the demodulator section of U481.

The 90MHz second local oscillator signal for U481 is generated by VCO Q730, which is phase locked by the PLL

CPIF output of U721, via feedback signal FINIF.