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

2. 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.

3. 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.

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

5. Relation between FM & PM

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

7. 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

8. 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.

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

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

11. 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.

12. 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.

13. Indirect Method- Armstrong Method

14. Imp links
working-application/
design/pm-phase-modulation/what-is-pm-tutorial.php
varactor-diode-in-the-generation/