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# FSK modulation demodulation Essay

1.1 ABSTRACT:

2 .1 BLOCK DIAGRAM:

Fig 2.1 Block diagram for FSK modulation and de-modulation

2.2.1 PIN DIAGRAM- IC 565 :

Fig 2.2 Pin diagram for IC 565

2.2.2 PIN DIAGRAM -IC 555:

Fig 2.3 Pin diagram for IC 555

3.1 DESIGN CALCULATION:
3.1.1 FSK MODULATOR:
ON time TH=0.693RBC
OFF time TL=0.693(RA+RB)C
Total time T=TH+TL=0.693(RA+2RB)C

1
f1= ————————– → (1)
0.69(RA+2RB) C1

1
f2= ———————————— →(2)
0.69(RA+2RB) C 1C2
———–
C1+C2

Duty cycle D=ON time/Total time=(RB)/(RA+2RB)=0.3
RB= 0.3RA+0.6RB
RB=0.75RA
Let f1= 1050 Hz, f2= 1250 Hz, C1=0.01μf

From (1)

1
1050 = ———————————
0.69(RA+2*0.75RA) 0.01μf

1
=———————————
0.69*2.5RA*0.01μf

RA=55.2KΩ
RB=0.75RA
RB=41.4KΩ

From (2)

1
1250 = ————————————————
0.69(55.2KΩ+2(41.4KΩ)) 0.01μf*C2
—————-
0.01μf+C2

0.01μf+C2 C2 = ———————————–
0.69(138KΩ)1250*0.01μf

0.01μf+C2 C2 = —————
1.19

1.19C2 – C2 = 0.01μf
0.19C2 = 0.01μf
C2 = 52.63 nf

3.1.2 FSK DEMODULATOR:

Upper cut off frequency of RC ladder circuit fH=1/(2πRC)
Assume R2=R3=R4=R
C2=C3=C4=C
fH=(key in frequency+2 maximum frequency)/2

=(150+2(1250))/2=1325Hz
Let C=0.02μf then R=1/(2πCfH)=1/(2π*1325Hz*0.02μf )=7K Ω f0=0.3/(R1C1)
f0=(f1+f2)/2=(1050+1250)/2=1150Hz
Let C1=0.01μf
R1=0.3/(1150*0.01μf)=26K Ω

flock=8f0/10=(8*1150)/10=920Hz

fcapture should be less than flock.

Choose fcapture =400Hz

fcapture =(1/2π)(2π*flock)/(R0C0)
R0=internal resistance=3.6K Ω

C0=(2πflock)/( R0)=920/(2π (400)2*3.6K Ω)=254nf

4.1 CIRCUIT DIAGRAM:
4.1.1 MODULATOR:

Fig 4.1 Circuit diagram for FSK modulation

4.1.2 DE-MODULATOR:

Fig 4.2 Circuit diagram for FSK de-modulation

5.1 WORKING PRINCIPLE:

6.1 OUTPUT

Fig 6.1 Output for FSK modulation and de-modulation

6.2 OBSERVATION:
INPUT:
Amplitude = V
ON time TH = ms
OFF time TL= ms
Frequency f = Hz

FSK MODULATOR:
For positive half cycle
Amplitude = V
ON time TH = ms
OFF time TL= ms
Frequency f = Hz

For negative half cycle
Amplitude = V
ON time TH = ms
OFF time TL= ms
Frequency f = Hz

FSK DEMODULATOR:
Amplitude = V
ON time TH = ms
OFF time TL= ms
Frequency f = Hz

7.1 CONCLUSION:

7.2 APPLICATIONS:
7.2.1 USED IN THE GSM MOBILE STANDARD
Minimum frequency-shift keying or minimum-shift keying (MSK) is a particularly spectrally efficient form of coherent FSK. In MSK the difference between the higher and lower frequency is identical to half the bit rate. Consequently, the waveforms used to represent a 0 and a 1 bit differ by exactly half a carrier period. This is the smallest FSK modulation index that can be chosen such that the waveforms for 0 and 1 are orthogonal. A variant of MSK called GMSK is used in the GSM mobile phone standard. 7.2.2 USED IN CALLER ID

FSK is commonly used in Caller ID and remote metering applications. Audio frequency-shift keying (AFSK) is a modulation technique by which digital data is represented by changes in the frequency (pitch) of an audio tone, yielding an encoded signal suitable for transmission via radio or telephone. Normally, the transmitted audio alternates between two tones: one, the “mark”, represents a binary one; the other, the “space”, represents a binary zero. 7.2.3 USED TO MODULATE AN RF CARRIER FOR TRANSMISSION

AFSK differs from regular frequency-shift keying in performing the modulation at baseband frequencies. In radio applications, the AFSK-modulated signal normally is being used to modulate an RF carrier (using a conventional technique, such as AM or FM) for transmission.