Frequency-Shift Keying

FREQUENCY SHIFT-KEYING
ALSO KNOWN AS FSK

• 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
2

3
Data
Carrier
Modulated
Signal

• General expression for FSK
𝑉𝑓𝑠𝑘 𝑡 = 𝑉𝑐cos{2𝜋 𝑓𝑐 + 𝑣 𝑚 𝑡 𝑓𝑑 𝑡}
Such that:
𝑉𝑐 = Carrier Voltage 𝑣 𝑚 = Modulating Voltage
𝑓𝑐 = Carrier Frequency 𝑓𝑑 = Frequency Deviation
GENERAL EXPRESSIONS
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• The modulating signal is a normalized binary waveform where:
• For logic 1, 𝑣 𝑚 = +1V
𝑉𝑓𝑠𝑘 𝑡 = 𝑉𝑐cos{2𝜋 𝑓𝑐 + 𝑓𝑑 𝑡}
• For logic 0, 𝑣 𝑚 = -1V
𝑉𝑓𝑠𝑘 𝑡 = 𝑉𝑐cos{2𝜋 𝑓𝑐 − 𝑓𝑑 𝑡}
GENERAL EXPRESSIONS
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SPACE AND MARK FREQUENCIES
0
Space
1
Mark

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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)”

• The space and mark frequencies
are determined through the
modulated signal’s frequency
deviation (𝑓𝑑)
• Space frequency is -𝑓𝑑 away
from the carrier frequency (fc)
• Mark frequency is + 𝑓𝑑 away
from the carrier frequency (fc)
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f1f0 fc
𝑓𝑑 𝑓𝑑

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
𝐵𝑎𝑢𝑑 =
𝑓𝑏
𝑁
Where: N = no. of bits per signal period/symbol | fb = bit rate
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10
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

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Graph of data
sent in 1 second.

FSK BIT RATE, BAUD & BANDWIDTH
• Baud: Because BFSK uses binary, N = 1
𝐵𝑎𝑢𝑑 =
𝑓𝑏
𝑁
=
𝑓𝑏
1
= 𝑓𝑏
Where: N = no. of bits per signal period | fb = bit rate
• Minimum Bandwidth:
𝐵𝑊 = 2(𝑓𝑑 + 𝑓𝑏)
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MODULATION INDEX
• In analog frequency modulation (FM):
𝑚 =
𝑓𝑑
𝑓𝑚
Where 𝑓𝑑 = freq. deviation | 𝑓𝑚 = modulating freq.
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FSK MODULATION INDEX
• However in FSK:
𝑚 =
∆ 𝑑
𝑓 𝑏
or ∆ 𝑑(T)
Where ∆ 𝑑 = freq. deviation between 𝑓0 and 𝑓1
𝑓𝑏 = bit rate
T = bit time: reciprocal of bitrate
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FULL DUPLEX OPERATION
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FSK MODULATION
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Oscillator 1
Oscillator 2
Digital Modulation Signal
FSK Output
1
0

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.

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.

CONTINUOUS-PHASE FSK
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1
0
1 10 0
Carrier Frequency
VCODigital Modulation
Signal

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

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.

GAUSSIAN FILTERED MSK (GMSK)
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NON-COHERENT FSK DEMODULATOR
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PLL-FSK DEMODULATOR
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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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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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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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APPLICATIONS
• Caller ID on Telephone Systems
• Amateur Radio
• Early Telephone-Line Modems.
• Emergency Broadcast Systems
• Modems
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BELL 103 AND BELL 202
• Introduced on 1965, being the first
commercial computer modem.
• Provided full-duplex service at 300 bit/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

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

• Japan has the Emergency Warning System (EWS)
• known locally as Kinkyu Keihou Housou (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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• 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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Frequency-Shift Keying

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