Digital Modulation
Digital Communications
Engr. Rose Anne Reano, PUP-STB
Objectives
■ Define and describe digital amplitude modulation
■ Define and describe frequency-shift keying
Amplitude-Shift Keying
Digital Modulation
Amplitude Shift Keying
A binary information signal directly
modulates the amplitude of the analog
carrier.
Amplitude Shift Keying
It is sometimes called digital amplitude
modulation (DAM).
Mathematically,
Amplitude Shift Keying
For logic 1 input, For logic 0 input,
(+1 V) (-1 V)
Amplitude Shift Keying
The carrier is either “on” or “off”, which is
why ASK is sometimes referred to as on-off
keying (OOK)
Amplitude Shift Keying
• It is important to note that for every
change in the input binary data stream,
there is one change in the ASK waveform,
and the time of one bit (tb) equals the time
of one analog signalling element (ts).
Amplitude Shift Keying
• It is also important to note that for the
entire time the binary input is high, the
output is a constant-amplitude, constant-
frequency signal, and for the entire time
the binary input is low, the carrier is off.
Amplitude Shift Keying
• The bit time is the reciprocal of the bit rate
and the time of one signaling element is
the reciprocal of the baud.
Amplitude Shift Keying
• The rate of change of the ASK waveform
(baud) is the same as the rate of change
of the binary input (bps); thus, the bit rate
equals the baud.
Amplitude Shift Keying
• With ASK, the bit rate is also equal to the
minimum Nyquist bandwidth.
Amplitude-Shift Keying
EXAMPLE:
Determine the baud and minimum
bandwidth necessary to pass a 10kbps
binary signal using amplitude shift keying.
Amplitude-Shift Keying
SOLUTION:
For ASK, N = 1.
Amplitude-Shift Keying
The use of amplitude-modulated analog
carriers to transport digital information is a
relatively low-quality, low-cost type of digital
modulation and, therefore, is seldom used
except for very low speed telemetry circuits.
Frequency-Shift Keying
Digital Modulation
Frequency-Shift Keying
FSK is a form of constant-amplitude angle
modulation similar to standard frequency
modulation (FM) except the modulating
signal is a binary signal that varies between
two discrete voltage levels rather than a
continuously changing analog waveform.
Frequency-Shift Keying
FSK is sometimes called binary FSK
(BFSK).
Frequency-Shift Keying
For logic 1 (+ 1V)
For logic 0 (- 1V)
Frequency-Shift Keying
• With binary FSK, the
carrier center
frequency is shifted
(deviated) up and
down in the
frequency domain by
the binary input
signal.
Frequency-Shift Keying
• As the binary input
signal changes from a
logic 0 to a logic 1 and
vice versa, the output
frequency shifts
between two
frequencies: a mark, or
logic 1 frequency and a
space, or logic 0
frequency.
Frequency-Shift Keying
• The mark and space
frequencies are
separated from the
carrier frequency by the
peak frequency
deviation.
Frequency-Shift Keying
• With FSK, frequency deviation is defined as
the difference between either the mark or
space frequency and the center frequency, or
half the difference between the mark and
space frequencies.
Frequency-Shift Keying
• the mark frequency is
the higher frequency
and the space
frequency is the lower
frequency.
Frequency-Shift Keying
• The time of one bit (tb) is the same as the time
the FSK output is a mark or space frequency.
Frequency-Shift Keying
• Thus, the bit time equals the time of an FSK
signalling element, and the bit rate equals the
baud.
Frequency-Shift Keying
• The baud for binary FSK can also be
determined by substituting N = 1.
Frequency-Shift Keying
• The minimum bandwidth for FSK is given as
Frequency-Shift Keying
• It resembles Carson’s rule for determining the
approximate bandwidth for an FM wave.
Frequency-Shift Keying
EXAMPLE:
Determine (a) the peak frequency deviation, (b)
minimum bandwidth, and (c) baud for a binary
FSK signal with a mark frequency of 49 kHz, a
space frequency of 51 kHz, and an input bit rate
of 2 kbps.
Frequency-Shift Keying
SOLUTION:
a. The peak frequency deviation is
Frequency-Shift Keying
SOLUTION:
b. The minimum bandwidth is
Frequency-Shift Keying
SOLUTION:
c. For FSK, N 1, and the baud is
Frequency-Shift Keying
• Bessel functions can also be used to
determine the approximate bandwidth for an
FSK wave.
Frequency-Shift Keying
• Since it takes a high and a low to produce a
cycle, the highest fundamental frequency
present in a square wave equals the repetition
rate of the square wave, which with a binary
signal is equal to half the bit rate.
Frequency-Shift Keying
• The formula used for modulation index in FM
is also valid for FSK:
Frequency-Shift Keying
• The worst-case modulation index (deviation
ratio) is that which yields the widest bandwidth.
Frequency-Shift Keying
• The worst-case or widest bandwidth occurs
when both the frequency deviation and the
modulating-signal frequency are at their
maximum values.
Frequency-Shift Keying
• The peak frequency
deviation in FSK is constant
and always at its maximum
value, and the highest
fundamental frequency is
equal to half the incoming
bit rate.
Frequency-Shift Keying
• The peak frequency
deviation in FSK is constant
and always at its maximum
value, and the highest
fundamental frequency is
equal to half the incoming
bit rate.
Frequency-Shift Keying
EXAMPLE:
Using a Bessel table, determine the minimum
bandwidth for the same FSK signal described
in Example 1 with a mark frequency of 49 kHz,
a space frequency of 51 kHz, and an input bit
rate of 2 kbps.
Frequency-Shift Keying
SOLUTION:
The modulation index is found by
Frequency-Shift Keying
SOLUTION:
From a Bessel table, three sets of significant
sidebands are produced for a modulation index
of one.
Frequency-Shift Keying
Transmitter and Receiver
Digital Modulation
FSK Transmitter
• Figure shows a simplified binary FSK
modulator,
FSK Transmitter
• The center frequency (fc) is chosen such
that it falls halfway between the mark and
space frequencies.
FSK Transmitter
• A VCO FSK modulator can be operated in
the sweep mode where the peak frequency
deviation is simply the product of the binary
input voltage and the deviation sensitivity of
the VCO.
FSK Transmitter
• With the sweep mode of modulation, the
frequency deviation is expressed
mathematically as
FSK Transmitter
• Frequency deviation is simply plus or minus
the peak voltage of the binary signal times
the deviation sensitivity of the VCO.
FSK Transmitter
• Since the peak voltage is the same for a
logic 1 as it is for a logic 0, the magnitude of
the frequency deviation is also the same for
a logic 1 as it is for a logic 0.
FSK Receiver
• The FSK input
signal is
simultaneously
applied to the
inputs of both
bandpass filters
(BPFs) through
a power splitter.
FSK Receiver
• The respective
filter passes
only the mark or
only the space
frequency on to
its respective
envelope
detector.
FSK Receiver
• The envelope
detectors, in turn,
indicate the total
power in each
passband, and the
comparator
responds to the
largest of the two
powers.
FSK Receiver
• This type of FSK
detection is
referred to as
noncoherent
detection;
FSK Receiver
• There is no
frequency involved
in the
demodulation
process that is
synchronized
either in phase,
frequency, or both
with the incoming
FSK signal.
FSK Receiver
• For a coherent
FSK receiver, the
incoming FSK
signal is multiplied
by a recovered
carrier signal that
has the exact
same frequency
and phase as the
transmitter
reference.
FSK Receiver
• However, the two
transmitted
frequencies (the
mark and space
frequencies) are
not generally
continuous;
FSK Receiver
• it is not practical to
reproduce a local
reference that is
coherent with both
of them.
FSK Receiver
• Consequently,
coherent FSK
detection is
seldom used.
FSK Receiver
• The most common circuit used for
demodulating binary FSK signals is the phase
locked loop (PLL)
FSK Receiver
• As the input to the PLL shifts between the mark
and space frequencies, the dc error voltage at
the output of the phase comparator follows the
frequency shift.
FSK Receiver
• Because there are only two input frequencies
(mark and space), there are also only two
output error voltages.
FSK Receiver
• Generally, the natural frequency of the PLL is
made equal to the center frequency of the FSK
modulator. As a result, the changes in the dc
error voltage follow the changes in the analog
input frequency and are symmetrical around 0
V.
FSK Receiver
• Binary FSK has a poorer error performance
than PSK or QAM and, consequently, is seldom
used for high-performance digital radio
systems.
FSK Receiver
• Its use is restricted to low-performance, low-
cost, asynchronous data modems that are used
for data communications over analog, voice-
band telephone lines.
Reference Book:
Advanced Electronic
Communications Systems
by Wayne Tomasi
Thank you for listening! ^_^
Objectives
■ Define phase-shift keying
■ Explain binary phase-shift keying
■ Explain quaternary phase-shift keying
Objectives
■ Describe 8- and 16-PSK
■ Describe quadrature-amplitude modulation
■ Explain 8-QAM
■ Explain 16-QAM
■ Define bandwidth efficiency
■ Explain carrier recovery
■ Explain clock recovery
Objectives
■ Define and describe differential phase-shift
keying
■ Define and explain trellis-code modulation
■ Define probability of error and bit error rate
■ Develop error performance equations for FSK,
PSK, and QAM
