Frequency modulation, Generation of NBFM and WBFM

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):