FM TRANSMITTER Punjab Edusat Society

FM TRANSMITTERS Frequency modulation technique is used. In FM frequency of the carrier is varied in proportion with the amplitude of the modulating signal keeping its amplitude constant.

Punjab Edusat Society FM TRANSMITTERS Used in radio, TV sound broadcasting & police wireless transmission. In FM transmitter the FM signal is transmitted by a transmitter. The information is contained in its frequency variation.

Punjab Edusat Society FM TRANSMITTERS The FCC has assigned a band of 20 MHz to the commercial FM broad cast service. This band extends from 88 MHz to 108 MHz.

Punjab Edusat Society FM TRANSMITTERS This 20 MHz band is divided in 100 channels, each having a bandwidth of 200 KHz. For providing high quality reliable music the maximum frequency deviation allowed is 75 KHz, with a maximum modulating signal frequency of 15 KHz.

Punjab Edusat Society METHODS OF FM GENERATION Indirect Methods Direct Methods Methods of FM Generation

Punjab Edusat Society DIRECT FM In direct FM generation the frequency of the carrier is changed directly in proportion with the modulating signal amplitude. Types of Direct FM Reactance modulator Varactor diode modulator

Punjab Edusat Society DIRECT FM These methods use a varactor diode or a reactance transistor for presenting a variable reactance across the frequency determining circuit of an oscillator.

Punjab Edusat Society DIRECT FM Oscillator Tank Circuit Modulating Signal Variable reactance device

Punjab Edusat Society DIRECT FM When the variable reactance of the device varies with the modulating signal the oscillator generator the corresponding FM signal.

Punjab Edusat Society DIRECT MODULATORS Few other direct modulators are: Varactor diode modulator. Reactance modulator. V.C.O. modulator. Stabilized reactance modulator. Cross by direct FM transmitters.

Punjab Edusat Society REACTANCE MODULATOR In the reactance modulator, a transistor or FET is operated as a variable reactance (inductive or capacitive) device. This device is connected across the tuned circuit of an oscillator. As the instantaneous value of modulating voltage changes, the reactance offered by the transistor or FET will change proportionally.

Punjab Edusat Society VARACTOR DIODE Here the voltage applied across the varactor diode varies in proportion with the modulating voltage. This will vary the junction capacitance of the varactor diode. The varactor diode appears in parallel with the oscillator tuned circuit. Hence the oscillator frequency will change with change in varactor diode capacitance and FM wave is produced.

Punjab Edusat Society ADVANTAGES OF DIRECT FM MODULATION The main advantage of direct FM generation is the simplicity of the modulators and their low cost.

Punjab Edusat Society DISADVANTAGES OF DIRECT METHOD In the direct method of FM generation we have to use the LC oscillator. The LC oscillator frequency is not stable. Therefore its not possible to use such oscillator for communication or broadcast purpose.

Punjab Edusat Society DIRECT FM Therefore we have to use a scheme in which we can use the crystal oscillator to control the carrier frequency. Therefore we have to use the automatic frequency control system.

Punjab Edusat Society INDIRECT METHOD ( ARMSTRONG METHOD) Here FM is obtained through phase modulation. A Crystal oscillator is used and hence the frequency stability is very high. A narrow band PM signal is generated via DSB- SC signal and a 90 ° phase shifted sub carrier signal from a crystal oscillator.

Punjab Edusat Society INDIRECT METHOD ( ARMSTRONG METHOD) The integration of the modulating signal makes the NBPM signal to NBFM signal. This NBFM signal is applied to a harmonic generator (frequency multiplier) which will increase both the carrier frequency and the maximum deviation to the required.

Punjab Edusat Society COMPARISION OF AM AND FM BROADCASTING AM Broadcasting It requires smaller transmission bandwidth. It can be operated in low, medium and high frequency bands. It has wider coverage. FM Broadcasting It requires larger bandwidth. It needs to be operated in very high and frequency bands. Its range is restricted to 50 km.

Punjab Edusat Society COMPARISION OF AM AND FM BROADCASTING The demodulation is simple. The stereophonic transmission is not possible. The system has poor noise performance. The process of demodulation is complex. In this, stereophonic transmission is possible. It has an improved noise performance.