Frequency-Shift Keying

Frequency-Shift Keying, also known as FSK is a type of digital frequency modulation. It is also often called as binary frequency shift keying or BFSK

Similar to analog FM, it is a constant-amplitude angle modulation.

This presentation will discuss the concepts behind FSK

1. FREQUENCY SHIFT-KEYING
ALSO KNOWN AS FSK

2. •Similar to the analog FM, it is a constant-amplitude angle
modulation
•The modulating signal (fm) is binary.
•Often called binary frequency shift-keying (BPSK).
FREQUENCY SHIFT-KEYING
Digital Data Modulation | Frequency Shift-Keying
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3. Digital Data Modulation | Frequency Shift-Keying
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Data
Carrier
Modulated
Signal

4. •General expression for FSK
�
??????????????????�=�
�cos{2????????????
�+�
??????�??????
��}
Such that:
�
�= Carrier Voltage�
??????= Modulating Voltage
??????
�= Carrier Frequency??????
�= Frequency Deviation
GENERAL EXPRESSIONS
Digital Data Modulation | Frequency Shift-Keying
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5. •The modulating signal is a normalized binary waveform where:
•For logic 1, �
??????= +1V
�
??????????????????�=�
�cos{2????????????
�+??????
��}
•For logic 0, �
??????= -1V
�
??????????????????�=�
�cos{2????????????
�−??????
��}
GENERAL EXPRESSIONS
Digital Data Modulation | Frequency Shift-Keying
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6. Digital Data Modulation | Frequency Shift-Keying
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SPACE AND MARK FREQUENCIES
0
Space
1
Mark

7. SPACE AND MARK FREQUENCIES
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•The binary 1 has a specific
frequency called the
“mark frequency (f1)
•Likewise, binary 0 has a
specific frequency called the
“space frequency (f0)”

8. SPACE AND MARK FREQUENCIES
•The space and mark frequencies
are determined through the
modulated signal’s frequency
deviation (??????
�)
•Space frequency is -??????
�away
from the carrier frequency (fc)
•Mark frequency is +??????
�away
from the carrier frequency (fc)
Digital Data Modulation | Frequency Shift-Keying
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f1f0 fc
??????
�
??????
�

9. BIT RATE, BIT TIME AND BAUD
•Bit rate (fb): number of bits delivered or received per second.
•Has a unit of bps or bits per second
•Bit time (T): the reciprocal of bitrate
•Baud: number of times a signal changes per second
????????????�??????=
??????
�
??????
Where: N = no. of bits per signal period/symbol | fb = bit rate
Digital Data Modulation | Frequency Shift-Keying
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10. Digital Data Modulation | Frequency Shift-Keying
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Graph of data
sent in 1 second.
1 division is
equivalent to a
symbol or signal
period.
Bit rate: 5 bps
Baud: 5 baud
1 1 1
0 0
1 symbol

11. Digital Data Modulation | Frequency Shift-Keying
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Graph of data
sent in 1 second.

12. FSK BIT RATE, BAUD & BANDWIDTH
•Baud: Because BFSK uses binary, N = 1
????????????�??????=
??????
�
??????
=
??????
�
1
=??????
�
Where: N = no. of bits per signal period | fb = bit rate
•Minimum Bandwidth:
??????�=2(??????
�+??????
�)
Digital Data Modulation | Frequency Shift-Keying
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13. MODULATION INDEX
•In analog frequency modulation (FM):
??????=
??????
�
??????
??????
Where ??????
�= freq. deviation | ??????
??????= modulating freq.
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14. FSK MODULATION INDEX
•However in FSK:
??????=
∆
??????
??????
??????
or ∆
�(T)
Where ∆
�= freq. deviation between ??????
0and ??????
1
??????
�= bit rate
T = bit time: reciprocal of bitrate
Digital Data Modulation | Frequency Shift-Keying
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15. FULL DUPLEX OPERATION
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16. FSK MODULATION
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Oscillator 1
Oscillator 2
Digital Modulation Signal
FSK Output
1
0

17. PROBLEM
“Glitches” or Phase Discontiuities
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1
0 1 10 0
Caused by:
•Arbitrarily choosing mark and
space frequencies
•Abrupt changes between signal.

18. SOLUTION
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➢Using mark and space frequencies that are coherent and are integral
multiples of the bit rate.
•Coherent FSK: A type of modulation where the mark and space
frequencies start and stop at zero crossing points.
•use less bandwidth and perform better in the presence of noise.

19. CONTINUOUS-PHASE FSK
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1
0
1 10 0
Carrier Frequency
VCO
Digital Modulation
Signal

20. MINIMUM-SHIFT KEYING (MSK)
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•An improved variation of CPFSK.
•MSK further improve spectral efficiency by using a low modulation index.
•This type of modulation generally specifies:
m = 0.5

21. GAUSSIAN FILTERED MSK (GMSK)
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•An improved form of MSK.
•Gaussian low-pass filter
•removes some of the higher-level harmonics that are responsible for the added sidebands
and wider bandwidth.
•Rounds the edges of the signal lengthening the rise and fall times.
•Reduces harmonic content and decreases overall signal bandwidth.

22. GAUSSIAN FILTERED MSK (GMSK)
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23. NON-COHERENT FSK DEMODULATOR
Digital Data Modulation | Frequency Shift-Keying
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24. PLL-FSK DEMODULATOR
Digital Data Modulation | Frequency Shift-Keying
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25. AUDIO FSK
•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.
•A higher tone for marks (1)
•A lower tone for space(0)
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26. ADVANTAGES AND DISADVANTAGES
Advantage
•It has lower probability of error (Pe).
•It provides high SNR (Signal to Noise Ratio).
•It has higher immunity to noise due to constant envelope. Hence it is robust
against variation in attenuation through channel.
•FSK transmitter and FSK receiver implementations are simple for low data
rate application.
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27. ADVANTAGES AND DISADVANTAGES
Disadvantage
•It uses larger bandwidth compare to other modulation techniques such as ASK
and PSK. Hence it is not bandwidth efficient.
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28. APPLICATIONS
•Caller ID on Telephone Systems
•Amateur Radio
•Early Telephone-Line Modems.
•Emergency Broadcast Systems
•Modems
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29. BELL 103 AND BELL 202
•Introduced on 1965, being the first
commercial computer modem.
•Provided full-duplex service at 300bit/s
and 300 baud over normal phone lines.
•Originating:
•Mark tone of 1270 Hz, Space ton of 1070 Hz
•Answering:
•Mark tone of 2225 Hz, Space ton of 2025 Hz
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Image of AT&T’s (formerly Bell Labs) Data Phone
that utilizes the Bell 103 modem

30. BELL 202
•Introduced on 1976
•Encode and transfer data at a rate
of 1200 bits per second, half-
duplex.
•Mark tone of 1200 Hz, Space tone
of 2200 Hz
•Currently used for Caller ID systems
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Image of RACAL VADIC’s modem that
utilizes the Bell 202 modem

31. •Japan has the Emergency Warning System (EWS)
•known locally as KinkyuKeihouHousou(緊急警報放送 )
•Utilizes a multiplexed digital alarm signal on the
audio carrier wave.
•Transmitted at 64 bits/s.
•Signal contains area division, and date-time along
with the start and end signals.
•Mark tone of 1024 Hz | Space tone: 640 Hz
EMERGENCY BROADCAST SYSTEM
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32. •The U.S. has the Emergency Alert System (EAS)
•FSK is used as a header tone for the warning
called the “SAME” header.
•Uses alternating tones of 853 Hz and 960 Hz
EMERGENCY BROADCAST SYSTEM
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33. THANK YOU!