Draw and explain cellular transmitter.
1 Answer

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Phase modulator section and mixer:

It is a low power FM unit operating in the frequency range of 824 to 849 MHz. The Carrier is obtained from frequency synthesizer and is applied to phase Modulator along with the amplified voice signal. Modulator o/p converted using a mixer to final transmitter frequency. Mixer o/p is applied to Class C amplifier. Carrier i/p for the phase modulator & the local oscillator frequency signal for mixer are produced by frequency synthesizer.

Final Amplifier:

The final amplifier stage is designed to supply 3W to antenna.

Automatic Power Control Circuit and DC amplifier:

The automatic power control circuit controls the o/p power of the transmitter automatically, with the help of power o/p detector & DC amplifier. Receiver picks up the special control signals & sends to APC that sets transmitter o/p power level. Transmitter o/p is fed to duplexer. Transmitter o/p power is controlled by cell site & MTSO.

Directional coupler:

It taps off an accurate sample of the transmitter output power & rectifies it in to a proportional dc signal. This signal is used in the APC circuit & is transmitted back to cell site.

Duplexer :

It allows the transmitter and receiver to use the same antenna. Since cellular units are full duplex operation the transmitter and receiver will operate simultaneously.

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