![Phase Modulation
• In phase modulation, phase of the carrier is varied in
accordance with the instantaneous value of the
amplitude of the modulating signal keeping the
amplitude of the carrier fixed.
• Vpm = Accos[wct + pd]
where pd is phase difference](https://image.slidesharecdn.com/fmppt-170829150709/85/Frequency-Modulation-4-320.jpg)
![Frequency Modulation
• Let :
f(t) = Vmcoswmt Modulating signal
and
vc(t) = Accoswct Carrier signal
Then Frequency modulated signal will be :
Vpm(t) = Accos[wct + kmf∫coswmt]
where mp = modulation index
Ac = Amplitude of carrier](https://image.slidesharecdn.com/fmppt-170829150709/85/Frequency-Modulation-6-320.jpg)
Frequency Modulation
- 1. GOVT. ENGINEERING COLLEGE, AJMER
- 2. Frequency Modulation A PRESENTATION ON: By : Neeraj Kumar (15EC58) To : Dr. Rekha Mehra
- 3. Angle Modulation • It is basically of two types: • 1) Phase Modulation(PM) • 2) Frequency Modulation(FM) • Frequency range for angle modulation is 88MHz to 108MHz.
- 4. Phase Modulation • In phase modulation, phase of the carrier is varied in accordance with the instantaneous value of the amplitude of the modulating signal keeping the amplitude of the carrier fixed. • Vpm = Accos[wct + pd] where pd is phase difference
- 5. Frequency Modulation • In frequency modulation, frequency of the carrier is varied in accordance with the instantaneous value of the amplitude of the modulating signal keeping the amplitude of the carrier fixed.
- 6. Frequency Modulation • Let : f(t) = Vmcoswmt Modulating signal and vc(t) = Accoswct Carrier signal Then Frequency modulated signal will be : Vpm(t) = Accos[wct + kmf∫coswmt] where mp = modulation index Ac = Amplitude of carrier
- 7. Features of FM • FM system is hi-fi (high fidelity). • Since FM signal has constant amplitude signal, the signal to noise ratio for such system is much higher. • The frequency spectrum of FM is same as that of AM in case of NBFM. But in general, the number of sidebands and carrier amplitude varies according to modulation index mf.
- 8. Parameters of FM • FM requires infinite amount of bandwidth for transmission since the number of sidebands are infinite in FM signal. BW = 2(∆ + fm) • Modulation index : mf = max. dev./ mod. freq. = ∆/mf • Power of FM : It is always constant in FM. • P = (Ac^2)/2
- 9. Classification of FM • On the basis of modulation index, FM is classified in two parts : • 1) If m<1, then FM is said to be Narrow Band FM (NBFM). • 2) If m>1, then FM is said to be Wide Band FM (WBFM).
- 10. NBFM and WBFM • In NBFM the maximum modulating frequency is 3KHz and maximum frequency deviation is 75KHz. • In WBFM the maximum modulating frequency is 30Hz to 15KHz and maximum frequency deviation is 75KHz. • Bandwidth of WBFM is 15 times of that of NBFM.
- 11. Pre-emphasis and De-emphasis • If freq. ↑ then amplitude ↓. • Due to this signal decreases hence decreasing the signal to noise ratio(SNR). • Therefore, the high frequency component which have low SNR are boosted or emphasized prior to the transmission of the signal. • This is done by using a pre-emphasis circuit and is used at the transmitting end just before the modulation takes place.
- 12. Pre-emphasis and De-emphasis • Now, since the relative SNR for various frequency components has been disturbed then those frequency components which were initially boosted or emphasized are now brought down to the same level to keep the same quality of the signal. • This is done by using a de-emphasis circuit and is used at the receiving end.
- 13. Generation of NBFM NBFM Modulator:
- 14. Generation of WBFM • There are two basic methods for generating FM signals known as direct and indirect methods. The direct method makes use of a device called voltage controlled oscillator (VCO) whose oscillation frequency depends linearly on the modulation voltage.
- 15. Generation of WBFM • By Valactor Method :
- 16. Generation of WBFM Indirect Method (Armstrong’s Method):