P.J. PARDESHI Asst. Professor MITCOE FM TRANSMISSION P.J. PARDESHI Asst. Professor MITCOE

Types of FM Narrow band FM Narrow band FM is defined as the situation where the modulation index  is small. From the table of Bessel functions it may be seen that for small , (  0.3) there is only the carrier and significant sidebands, i.e. BW = 2fm. FM with   0.3 is referred to as narrowband FM (NBFM). Maximum modulating frequency is usually 3kHz maximum frequency deviation is =75 kHz.

Types of FM Wide band FM  Wideband FM is defined as the situation where the modulation index  is larger. For  > 0.3 there are more than 2 significant sidebands. As  increases the number of sidebands increases. This is referred to as wideband FM (WBFM). Modulation frequencies extend from 30 Hz to 15 kHz. Maximum permissible deviation is=75 kHz. Wideband FM system need large bandwidth, typically 15 times that of narrowband FM system.

Generation of FM using PM Modulating Wave x(t) FM Wave Integrator Phase Modulator Carrier Oscillator

Relation between FM & PM

METHODS OF FM GENERATION DIRECT METHOD REACTANCE MODULATOR VARACTOR MODULATOR INDIRECT METHOD ARMSTRONG METHOD

DIRECT METHOD- Frequency of oscillator of carrier In direct method, the modulating (base band) signal directly modulates the carrier .The carrier signal is generated using a LC oscillator circuit. Frequency of oscillator of carrier 𝜔 𝐶 = 1 𝐿𝐶

The instantaneous frequency of the carrier wave is directly varied in accordance with the message signal by means of an voltage controlled oscillator. The frequency determining network in the oscillator is chosen with high quality factor (Q-factor) and the oscillator is controlled by the incremental variation of the reactive components in the tank circuit of the oscillator.

Varactor diode Generation A varactor diode is a semiconductor diode whose junction capacitance varies linearly with applied voltage when the diode is reverse biased.

FM Transmitters Crystal oscillator phase modulator frequency multiplier Power amplifier Audio source NBFM WBFM ANTENNA

Frequency multiplier- Crystal oscillator- Crystal oscillator generates the stable carrier signal. Phase modulator- The phase modulator modulates the carrier signal and the massage signal in the low power range to generate a narrowband FM. Frequency multiplier- The frequency multiplier is used to increase the frequency deviation and carrier signal frequency to a desired level.

Power amplifier- Antenna- The power amplifier gives the required power level to the signal which passes through the antenna. Antenna- Antenna is a device which is used for sending and receiving the information.

Indirect Method- Armstrong Method

Imp links https://www.elprocus.com/voltage-controlled-oscillator- working-application/ http://www.radio-electronics.com/info/rf-technology- design/pm-phase-modulation/what-is-pm-tutorial.php http://www.ques10.com/p/11412/how-can-you-use-a- varactor-diode-in-the-generation/