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Quadrature Phase Shift Keying

Quadrature Phase Shift Keying (QPSK) is a form of Phase Shift Keying in which two bits are taken at a time and modulated, selecting one of four possible carrier phase shifts (0, 90, 180, or 270 degrees). QPSK allows the signal to carry twice much information than ordinary PSK, using the same bandwidth. QPSK is used for satellite transmission of MPEG2 video, cable modems, videoconferencing, cellular phone systems, and other forms of digital communication over an RF carrier.

HARDWARE IMPLEMENTATION

Design a 555 clock generator at a frequency of 12 kHz. Construct a divide by 4 Johnson counter using JK flip flops to generate four phases of square waves. The four phases are given to four second order LPFs to obtain two sine waves (+sine, -sine) and two cosine waves (+cosine, -cosine). Finally, those four signals are given to the 4:1 multiplexer (4052). Design a 3-bit PRBS generator using 4013-D Flip Flops. The clock for this data generator is from Q1 of Johnson counter as shown in the block diagram. The select lines of 4052 are connected to the PRBS generator. The 4052output is the desired QPSK signal.

BLOCK DIAGRAM

S1

S0

Y

CARRIER PHASE

0

0

X0

SINE

0

1

X1

-SINE

1

0

X2

COS

1

1

X3

-COS

CLOCK FROM IC 555

OUTPUT WAVEFORM

The clock signal can be generated by IC 555, operating in the Astable mode. The frequency of that clock signal can be calculated by

f= 1/t

t=0.69 RC

t=0.69 (10k) (0.01uf)

f=14.492 kHz

JOHNSON COUNTER (DIVIDE BY FOUR)

The Johnson counter (÷ by 4) can be designed using JK Flip Flop as shown in figure to generate the four phases of square waves.

OUTPUT WAVEFORM

CMOS JK FLIP FLOP (4027)

CD4027BMS is a single monolithic chip integrated circuit containing two identical complementary-symmetry J-K master slave flip-flops. Each flip-flop has provisions for individual J, K, Set Reset, and Clock input signals. Buffered Q and ̅Q̅̅ signals are available as outputs. The CD4027BMS is supplied in these 16-lead outline packages:

SECOND ORDER LOW PASS FILTER

The second order Low pass filter can be constructed using op-amp as shown in figure, to convert the square wave into sine waveform. We need to convert the square wave of four different phases, so, we can use LM 324 (Quad op-amp), which consists of four op-amps within a single DIP chip.

QUAD OP AMP (LM324)

The LM324 series has quad op-amps with true differential inputs. The quad amplifier can operate at supply voltages as low as 3.0 V or as high as 32 V.

FEATURES

· Short Circuited Protected Outputs

· True Differential Input Stage

· Single Supply Operation: 3.0 V to 32 V (LM224, LM324, LM324A) or + 16V

· Four Amplifiers Per Package

PINOUT OF LM324

OUTPUT WAVEFORM OF LM324 (PIN NUMBER 1 AND 7)

OUTPUT WAVEFORM OF LM324 (PIN NUMBER 8 AND 14)

OUTPUT WAVEFORM

PRBS DATA GENERATOR

OUTPUT WAVEFORM OF CLOCK AND PRBS

CMOS 4:1 MULTIPLEXER (4052)

The CD4052 analog multiplexers demultiplexers are digitally controlled analog switches. Control of analog signals up to 15Vp-p can be achieved by digital signal amplitudes of 3−15V. For example, if VDD = 5V, VSS = 0V and VEE = −5V, analog signals from −5V to +5V can be controlled by digital inputs of 0−5V. When a logical “1” is present at the inhibit input terminal all channels are “OFF”. CD4052BC is a differential 4-channel multiplexer having two binary control inputs, A and B, and an inhibit input. The two binary input signals select 1 or 4 pairs of channels to be turned on and connect the differential analog inputs to the differential outputs.

Features:

· Wide range of digital and analog signal levels: digital 3 to 15V, analog to 15Vp-p

( +7.5V).

· Logic level conversion for digital addressing signals of 3 – 15V (VDD − VSS = 3 – 15V) to switch analog signals to 15 Vp-p (VDD − VEE = 15V).

· Very low quiescent power dissipation under all digital-control input and supply conditions: 1 μ W (typ.) at VDD − VSS = VDD − VEE = 10V.

· Binary address decoding on chip.

Based on the selection lines the IC 4052 selects the appropriate input as shown in the waveform.

COMPLETE CIRCUIT DIAGRAM

QAM

Quadrature Amplitude Modulation, QAM utilizes both amplitude and phase components to provide a form of modulation that is able to provide high levels of spectrum usage efficiency.

QAM, Quadrature Amplitude Modulation is in analogue transmissions, AM stereo transmissions, and data applications. It is able to provide a highly effective data compression and it is used in cellular phones, Wi-Fi and all high speed data communications systems.

QAM is a signal in which two carriers shifted in phase by 90 degrees (i.e. sine and cosine) are modulated and combined. As a result of their 90° phase difference they are in quadrature and this gives rise to the name. Often one signal is called the In-phase or “I” signal, and the other is the quadrature or “Q” signal.

The resultant overall signal consisting of the combination of both I and Q carriers contains of both amplitude and phase variations. In view of the fact that both amplitude and phase variations are present it may also be considered as a mixture of amplitude and phase modulation.

HARDWARE IMPLEMENTATION

Refer the Block diagram. First of all, we have to obtain four different amplitude levels of sine signal by applying the same input signal to the 4 op amps with different gain output. The op amps provides signal with four different amplitude levels and the same is applied to op amps in next stage to get 90 degree phase shift of those 4 signals. All the 8 signals are applied to the 8:1 multiplexer. The select lines are connected with PRBS data generator for selecting the inputs randomly. The clock signal for the PRBS is generated by 555 timer, operating in Astable mode. Finally we will get the output as 4 different amplitudes as well as 2 different phase shifts.

S2

S1

S0

Y

0

0

0

X0

0

0

1

X1

0

1

0

X2

0

1

1

X3

1

0

0

X4

1

0

1

X5

1

1

0

X6

1

1

1

X7

To generate the sine wave with four different amplitudes, we have to use four op-amps with different gains. The gain of an op-amp can be adjusted by varying the feedback resistance RF.

GAIN = RF / RIN and RP = RIN // RF

OP AMP WITH GAIN =1

OP AMP WITH GAIN =2

OP AMP WITH GAIN =3

OP AMP WITH GAIN =4

OUTPUT WAVEFORMS OF FOUR OP-AMPS

Instead of using four number of IC 741, we can use LM 324, which is having four op-amps in one single chip. So that we can reduce number of IC pins as well as component occupying space.

OUTPUT WAVEFORMS OF FOUR OP-AMPS OPERATED AS PHASE SHIFTER

OP AMP AS PHASE SHIFTER

Finally, all the 8 sine signals are applied to the 8:1 multiplexer(IC 4051), and select lines are connected from PRBS to achieve QAM output.

QAM WAVEFORM

QAM applications

QAM is in many radio communications and data delivery applications. However some specific variants of QAM are used in some specific applications and standards.

For domestic broadcast applications for example, 64 QAM and 256 QAM are often used in digital cable television and cable modem applications. In the UK, 16 QAM and 64 QAM are currently used for digital terrestrial television using DVB - Digital Video Broadcasting. In the US, 64 QAM and 256 QAM are the mandated modulation schemes for digital cable as standardized by the SCTE in the standard ANSI/SCTE 07 2000.

In addition to this, variants of QAM are also used for many wireless and cellular technology applications. Here the link conditions can vary and accordingly the order of the QAM modulation used can normally be altered dynamically with the level of error correction to achieved the best throughput. This means balancing the QAM order with the level of error correction against the prevailing link conditions. As data rates have risen and the demands on spectrum efficiency have increased, so too has the complexity of the link adaptation technology. Data channels are carried on the cellular radio signal to enable fast adaptation of the link to meet the prevailing link quality and ensure the optimum data throughput, balancing transmitter power, QAM order, and forward error correction, etc.

Minimum Shift Keying 

Minimum shift keying, MSK, is a form frequency modulation, based on a system called continuous-phase frequency-shift keying. Minimum shift keying (MSK) offers advantages in terms of spectral efficiency when compared to other similar modes, and it also enables power amplifiers to operate in saturation enabling them to provide high levels of efficiency.

It is found that PSK waveform has sharp phase changes as shown in figure. These transitions potentially create signals that have sidebands extending out a long way from the carrier, and this creates problems for many radio communications systems, as any sidebands outside the allowed bandwidth cause interference to adjacent channels and any radio communications links that may be using them.

MSK, minimum shift keying has the feature that there are no phase discontinuities and this significantly reduces th

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