Introduction To Analog filters Design file · Web viewSuch group is called a digital Word. ... One...

Non-Coherent Amplitude Shift Keying Chapter 2

CHAPTER 2

2.1 Theory of Digital Modulation

A digital communication system is one which sends number of signals from

one place to another in order to convey information.

Information can be presented as groups of (usually binary) digits. Such group

is called a digital Word. It is usually convenient to send the digit serially (one after the

other) and to put them together again as words at the receiving end.

Analog information, such as the voltage signal from a telephone, can be

converted to digital from, sent over a digital communication channel, and recovered to

analog form at the receiver.

In order to transmit a signal it is often modulated, and with digital signal the

modulation is referred to as “keying”. The amplitude shift keying (ASK) is considered

as the simplest way of shifting the frequency spectrum of a signal from base band to

some other band of frequencies.

The use of a higher frequency range reduces antenna size. In the modulation

process, the baseband signals constitute the modulating signal and the high-frequency

carrier signal is a sinusiodal waveform. There are three basic ways of modulating a

sine wave carrier. For binary digital modulation, they are called [ binary amplitude-

shift keying (BASK), binary frequency-shift keying (BFSK) and binary phaseshift

keying (BPSK)].

7


Modulation also leads to the possibility of frequency multiplexing. In a

frequency-multiplexed system, individual signals are transmitted over adjacent,

nonoverlapping frequency bands. They are therefore transmitted in parallel and

simultaneously in time. If we operate at higher carrier frequencies, more bandwidth is

available for frequency-multiplexing more signals.

Transmission of data across a noisy communications channel requires some

manner of separating the valid data from the background noise. The most common

way to accomplish this is to modulate the data at the transmission side and to

demodulate the data on the reception side, the end result being that the data coming

from the receiver are the same as the data being presented to the transmitter. The

efficiency of the modulation/demodulation process determines the accuracy of the

data coming from the receiver. Therefore, careful consideration must be given to the

selection of an appropriate modulation-demodulation scheme.

Figure 2-1…Different digital modulation techniques

2.2 Amplitude Shift Keying [ASK]

Amplitude shift keying -ASK- as shown in figure 19 in the context of digital

communications is a modulation process which imparts to a sinusoid two or more

8


discrete amplitude levels (Also called on-off keying - OOK). These are related to the

number of levels adopted by the digital message.

The digital data to be transmitted is the binary number . Two amplitudes are

used to directly represent the data, either 0 or 1. In this case, the modulation is called

binary amplitude shift keying or BASK

Figure 2-2…Binary Amplitude-Shift Keying (BASK)

A binary amplitude-shift keying (BASK) signal can be defined by

Equation 2-1…Characteristic of ASK

where A is a constant , fc is the carrier frequency, and Tb is the bit duration.

Figure 2-3 shows the BASK signal sequence generated by the binary

sequence 0 1 0 1 0 0 1. The amplitude of a carrier is switched or keyed by the binary

signal m(t). This is sometimes called on-off keying (OOK).

9


Figure 2-3…(a) Binary modulating signal (b) ASK signal.

There are sharp discontinuities shown at the transition points. These result in

the signal having an unnecessarily wide bandwidth. Bandlimiting is generally

introduced before transmission, in which case these discontinuities would be 'rounded

off'. The band-limiting may be applied to the digital message, or the modulated signal

itself.

It is a special case of amplitude modulation (AM). Amplitude modulation has

the property of translating the spectrum of the modulation to the carrier frequency.

The bandwidth of the signal remains unchanged.

The fact that AM simply shifts the signal spectrum is often used to convert the

carrier frequency to a more suitable value without altering the modulation. This

process is known variously as mixing, up-conversion or down-conversion. Some form

of conversion will always be present when the channel carrier occupies a frequency

range outside the modulation frequency range.

One of the disadvantages of ASK, compared with FSK and PSK, for example,

is that it has not got a constant envelope. This makes its processing (eg, power

10


amplification) more difficult, since linearity becomes an important factor. However, it

does make for ease of demodulation with an envelope detector.

A block diagram of a basic non coherent ASK generator is shown in

Figure 2-4.

Figure 2-4…Transmitter Of ASK

While in figure 2-4 we can see a coherent ASK generator, the main difference

between the coherent systems and non-coherent systems that the coherent systems

needs carrier synchronization while this is not required in non coherent systems.

Figure 2-5…Coherent ASK transmitter

11


2.2.1 Bandwidth modification

As already indicated, the sharp discontinuities in the ASK waveform of

Figure2-3 imply a wide bandwidth. A significant reduction can be accepted before

errors at the receiver increase unacceptably. This can be brought about by

bandlimiting (pulse shaping) the message before modulation, or bandlimiting the ASK

signal itself after generation, Both these options are illustrated in Figure22, which

shows one of the generators we will be modeling.

2.2.2 Using dual analog switch

The simplest method for binary ASK is to use a switch to gate the carrier on

and off, driven by the data signal as shown in figure 2-4 earlier while we can use any

mean of multiplication to achieve the goal which is modulating the carrier with the

data signal.

2.2.3 Symmetry in ASK

Spectrum of an ASK signal can be determined from its baseband data stream if

the ASK modulation process if seen as a multiplication or mixing of the baseband

symbol stream with the carrier wave.

Consider a single frequency cos(wmt) from within the baseband spectrum and

perform the mathematical multiplication with the carrier cos(wct) ...

2.2.4 Demodulation methods

For demodulation and detection purpose two techniques can be used.

Non-coherent (Threshold) detection to detect presence or absence of a carrier

signal.

12


Coherent demodulation; multiplying the modulated signal by the carrier

signal to recover the baseband data signal. This detection is better for a noisy

environment.

2.2.4.1 Non-Coherent Detection

In the coherent detector, the exact reproduction of the carrier is needed (i.e.

requires carrier or phase synchronization).

Figure 2-6…Non coherent Detection

The diagram in figure 2-6 is a non-coherent detector which does not require carrier or

phase synchronization.

Both asynchronous and synchronous demodulation methods are used for the

demodulation of ASK signals. It is apparent that the ASK signal has a well defined

envelope. Thus it is amenable to demodulation by an envelope detector. A

synchronous demodulator would also be appropriate. We note that: Envelope

detection circuitry is simple. Synchronous demodulation requires a phase-Iocked local

carrier and therefore carrier acquisition circuitry.

With band limiting of the transmitted ASK neither of these demodulation

methods would recover the original binary sequence; instead, their outputs would be a

band limited version. Thus further processing - by some sort of decision-making

circuitry for example - would be necessary.

13


Thus demodulation is a two-stage process:

Recovery of the bandlimited bit stream

Regeneration of the binary bit stream

Envelope demodulation

Having a very definite envelope, an envelope detector can be used as the first

step in recovering the original sequence. Further processing can be employed to

regenerate the true binary waveform this is can be.

Figure 2-7…Envelope detector operation

2.2.4.2 Coherent ASK Detection

A coherent detector figure 2-7 operates by mixing the incoming data signal

with a locally generated carrier reference and selecting the difference component from

the mixer output.

Figure 2-8…Coherent detection of ASK

14


If the modulated data signal is a(t).cos(wct) and the reference carrier

cos(wct + q) where q is the phase error between the source and reference carriers, the

mixer output becomes:

a(t).cos(wct).cos(wct + q) = 0.5a(t).cos(q) + 0.5a(t).cos (2wct + q)

If q = 0 (reference carrier phase coherent) output is proportional to a(t)

Then coherent detection has better noise rejection

2.2.5 Coherent ASK Vs non-coherent ASK

In non-coherent detection, V is amplitude of the signal

Coherent ASK is more resistive to noise than non-coherent ASK, coherent

ASK is more complicated than non-coherent ASK due to the need of a local carrier in

coherent ASK receiver that is synchronized with the transmitted carrier (i.e. having

the same frequency and phase as the transmitted carrier).

2.3 Frequency Shift Keying [ FSK ]

Frequency-shift keying (FSK) is a method of transmitting digital signals. The

two binary states, logic 0 (low) and 1 (high), are each represented by an analog

waveform. Logic 0 is represented by a wave at a specific frequency, and logic 1 is

represented by a wave at a different frequency. A modem converts the binary data

from a computer to FSK for transmission over telephone lines, cables, optical fiber, or

wireless media. The modem also converts incoming FSK signals to digital low and

high states, which the computer can "understand."

15

http://searchnetworking.techtarget.com/sDefinition/0,,sid7_gci212716,00.html

http://searchsystemsmanagement.techtarget.com/sDefinition/0,,sid20_gci212583,00.html

http://whatis.techtarget.com/definition/0,,sid9_gci212160,00.html






Figure 2-9…Frequency shift keying

FSK is simplified a form of Frequency Modulation (FM). For good noise

performance and high bandwidth operation, FSK is the modulation technique of

choice. In true FM, an analog signal is represented with a linear frequency deviation

from center. FSK is a binary form of frequency modulation which uses hard shifts

between deviant frequencies to represent the data originally impressed on the carrier.

The magnitude of frequency shift is directly related to the magnitude of the

modulation source voltage.The modulation source is allowed two states: “on” and

“off”. When the modulation source is “off ” , the carrier frequency is shifted down

from the center frequency. When the modulation source is “on”, the carrier frequency

is shifted up from the center frequency. The amount that the carrier frequency is

shifted is referred to as the frequency deviation.

2.3.1 Modulation Process

In FSK, the instantaneous frequency of the carrier is switched between 2 or

more levels according to the baseband digital data, for example at logic 0 frequency a

wave of Acos wct is sent while when logic 1 data come, the system will produce a

wave of Acos n wct (where n = integer)

16


Figure 2-10…Generated waveform of FSK

Also in FSK systems we can use two different methods of modulation, either

we use non-Coherent scheme or a coherent scheme.

A non-coherent FSK transmitter is shown in figure 2-11 where the simply data

source is connected directly to a voltage controlled oscillator that can give two

different frequencies, one sent at logic 1 and the other is sent at logic 0 inorder to

generate and FSK signal.

Figure 2-11…Non-coherent FSK transmitter

Unlike ASK, a carrier is always present with FSK modulation. This affords the

designer several benefits. First, the carrier will load the receiver at all times providing

greatly increased noise immunity. Secondly, the strength (or amplitude) of the carrier

can be used to determine the quality of the incoming signal.

17


As illustrated in figure 2-12 the coherent FSK system is more complex than

non-coherent one, but as said earlier the coherent systems have more immunity to

nose but they are more expensive, the same idea of non-coherent FSK is applied here.

Figure 2-12…Coherent FSK transmitter

2.3.2 Demodulation process

Demodulation of FSK depends on the modulation used either coherent or non-

coherent and below we can see the both receiver diagrams.

2.3.2.1 Non-Coherent FSK receiver

Figure 2-13 shown a non-coherent FSK receiver where the received signal

enters the system through two band pass filters so that we can separate each frequency

of the FSK signal either fc+fd or fc-fd after getting each frequency separated we pass

the signal top an envelope detector that will reconstruct the level of the data (the peak

voltage of the signal) the resultant is passed to a summer that will give us the final

value of the voltage the value will be compared with a reference voltage so that we

can regenerate the original data.

18


Figure 2-13…Non-coherent FSK receiver

2.3.2.2 Coherent FSK receiver

Coherent FSK method uses a local carrier generator so that we can reconstruct

the data sent, this can be achieved by multiplying the entering signal by a carrier

signal as shown in figure 2-14.

Figure 2-14…Coherent FSK receiver

A drawback of the continuous carrier is that the transmitter is always drawing

power and generating an output. Therefore, the transmitter will ultimately require a

higher supply current than ASK-based systems. In addition, the output power cannot

be legally increased in countries

FSK is a Non Return to Zero modulation method. This means that the non-

modulated condition is between the “off” and “on” condition. In other words, the

19


carrier should never be at the center frequency when modulation is present. The

benefit here is noise immunity. Hysteresis can be applied to the detector, eliminating

the effect of spurious frequency modulation generated from sources other than the

data stream.

Since FSK relies on frequency change, and not amplitude change, to indicate

data states, an FSK receiver is inherently immune to amplitude noise. This is of great

importance in bands which are extremely crowded and have a high potential for near-

band interference. This increased noise immunity suggests a potential for higher data

rates.

FSK modulation for the transmission of data has many features and limitations

to consider.

2.3.3 Advantages of FSK are:

Higher data rates

Continuous carrier presence

High noise immunity

2.3.4 Limitations of FSK are:

Higher cost

High power consumption

Larger size of equipments

FSK modulation should be used for applications where data rate, noise

immunity, and using one channel are of primary concern.

20


2.4 Phase Shift Keying [PSK]

Phase-shift keying (PSK) is a method of transmitting and receiving digital

signals in which the phase of a transmitted signal is varied to convey information.

There are several schemes that can be used to accomplish PSK. The simplest

method uses only two signal phases: 0 degrees and 180 degrees. The digital signal is

broken up timewise into individual bits (binary digits). The state of each bit is

determined according to the state of the preceding bit. If the phase of the wave does

not change, then the signal state stays the same (low or high). If the phase of the wave

changes by 180 degrees -- that is, if the phase reverses -- then the signal state changes

(from low to high, or from high to low). Because there are two possible wave phases,

this form of PSK is sometimes called bi-phase modulation.

Figure 2-15…Phase shift keying

More complex forms of PSK employ four or eight wave phases. This allows

binary data to be transmitted at a faster rate per phase change than is possible with

biphase modulation. In four-phase modulation, the possible phase angles are 0, +90 ,

-90, and 180 degrees; each phase shift can represent two signal elements. In eight-

21


phase modulation, the possible phase angles are 0, +45, -45, +90, -90, +135, -135, and

180 degrees; each phase shift can represent four signal elements.

Equation 2-2…Characteristic of PSK

Carrier frequency and amplitude remains same only the phase of the carrier is

shifted by 180 degree.For demodulation only Coherent technique can be used. In this

case the phase of received signal must be compared with a reference signal which is

the synchronised local carrier signal

Normally, PSK signals are transmitted by generating an audio signal which is

sent using SSB modulation. It is also possible to directly phase-key a radio frequency

carrier.

The technology used for receiving PSK signals is extremely complex with

compensation required for spreading of the signal energy between symbols (inter-

symbol interference) and for removing the effects of multipath in the HF environment

This is very much flavour of the moment in the amateur radio community at

present. This an experimental mode used by radio amateurs as a teleprinter signal

using extremely narrow bandwidths. This has the advantage that the mode is resistant

to noise and requires comparatively low power levels for successful communications.

Phase shift keying is a technique which shifts the period of a wave as we said

earlier ,this wave has a period of p. Also the start of the wave's period is at 0 as in

figure 2-16.

22


Figure 2-16…Wave of period p and starts at 0

In figure 2-17 the same wave as the first, but its phase has been shifted. Notice

that the period starts at the wave's highest point (1).

Figure 2-17… Wave of period p shifted

It just so happens that we have shifted this wave by one quarter of the wave's

full period. We can shift it another quarter, if we wanted to, so the original wave

would be shifted by half it's period. And we could do it one more time, so that it

would be shifted three quarters of it's original period.

This means we have 4 separate waves. So why not let each wave stand for

some binary value? Since there are 4, we can let each wave signify 2 bits

(00,01,10,11) this can be summarized in table 2-1.

Bit value Amount of shift

00 None

01 ¼

23


10 ½

11 ¾

Table 2-1…Phase shift of each bit value

2.4.1 Modulation Process:

In PSK, the phase of the carrier signal is switched between 2 or more values in

response to the baseband digital data the waveform is shown in figure 2-18. There are

two methods of generating PSK signals one of them is the coherent PSK and the other

is the differentially coherent PSK, the differentially coherent PSK is simpler than the

coherent PSK.

Figure 2-18… PSK signal waveform

24


2.4.1.1 Differentially Coherent PSK Transmitter

Figure 2-19… Differentially Coherent PSK Transmitter

2.4.1.2 Coherent PSK transmitter

Figure 2-20…Coherent PSK transmitter

25


2.4.2 Demodulation Process

2.4.2.1 Differentially coherent PSK Receiver

Figure 2-21… Differentially coherent PSK Receiver

2.4.2.2 Coherent PSK receiver

Figure 2-22…Coherent PSK Receiver

2.4.3 Summery of Digital modulation techniques

Modulation Type Data Transmitted signal

Amplitude Shift Keying0 0

1

Frequency Shift Keying0

1

Phase Shift Keying0

1

26


2.4.4 Compression between digital modulation methods

Modulation Type Resistivitiy to noise Technical complexity

Amplitude Shift KeyingLess Resistivitiy to noise than

other systemsSimple hardware

Frequency Shift KeyingMore resistive to noise than ASK

and less than PSK

Complex hardware with

respect to ASK and simple

with respect to PSK

Phase Shift KeyingThe most resistive to noise with

respect to the other systems

Complex hardware with

respect to the other systems

27

Introduction To Analog filters Design file · Web viewSuch group is called a digital Word. ... One...

Documents

Transcript of Introduction To Analog filters Design file · Web viewSuch group is called a digital Word. ... One...