The Fig. below shows the block diagram of cellular transmitter. It is a low power FM transmitter operating in the frequency range of 825 to 845 MHz. There are 666 3D-KHz transmit channels. Channel 1 is 825.03 MHz, channel 2 is 825.06 MHz and so on up to channel 666 on 844.98 MHz. As shown in the Fig. 15.68, the phase modulator modulates the preamplified voice signal and the carrier input from the frequency synthesizer. It produces a deviation of ± 12 KHz. The modulated output is translated upto final transmitter frequency with the help• of mixer whose second input also comes from the frequency synthesizer.
The mixer output is fed to class C power amplifier stages where the output signal is generated. The final amplifier stage is designed to supply about 500 mW to 3W power to the antenna. A unique feature of the high power transmitter is that its output is controllable by the cell site and MTSO. In such phones, a special control signals picked up by the receiver are sent to an automatic power control (APC) circuit that sets the transmitter to one of 8 power output levels.
The directional coupler samples the transmitter output power and rectifies it into a proportional de signal. This signal is used in the APC circuit and is transmitted back to the cell site permitting the MTSO to know the present power level. This automatic power control feature permits optimum cell site reception with minimal power. It also helps to minimize interference from other stations in the same or adjacent cells.
The transmitter output is then fed to a duplexer or isolator. It allows the transmitter and receiver to share the same antenna. Since cellular telephone units operate in full-duplex mode, the transmitter and receivers operate simultaneously. For proper isolation the transmit and receive frequencies are spaced 45 MHz apart from each other. However, to minimize the interference it is necessary to keep the transmitter power out of the sensitive receiver. To keep proper isolation, duplexer consists of two very sharp bandpass filter, one for the transmitter and one for the receiver.
In cellular phone, dual-conversion superheterodyne receiver is used. An RF amplifier boosts the level of the received cell signal. The receiver frequency range is 870.03 to 889.98 MHz. The first mixer translates the incoming signal down to first IF of 82.2 MHz. The local oscillator frequency is derived from the frequency synthesizer. The local oscillator frequency sets the receiver channel.
The signal from the output of the mixer is amplified by an IF amplifier and then passed to the second mixer. The second mixer is controlled by a crystal controlled local oscillator. The second IF is usually either 10.7 MHz or 455 kHz. The signal is then demodulated, de-emphasized, filtered, and amplified before being applied to the output speaker in the handset.
The output of the FM demodulator is also sent to band pass filter circuit that selects the control audio tones and digital control data stream. This information is used to set and control both the transmitter and receiver. The demodulator output is amplified and used as a receiver signal strength indicator (RSSI).
The RSSI signal is sent back to the cell site so that the MTSO can monitor the receiver signal from the cell and make decision about switching to another cell if signal from current cell is weak.
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