Design Of A QPSK Carrier Generation By RC Phase Shift Oscillator

By :Shilpi Kumar 112389Ruchika Agarwal 112375Priyanka Bhattacharjee

1123128Rituparna Ghosh 1123129

OBJECTIVE :To study the principle of operation of a QPSK carrier generation by RC Phase shift Oscillator by generating four carriers S0, S1, S2 and S3 to be used in QPSK modulator. Carriers should have 90 degree phase shift between themselves. Their amplitudes should be the same .SPECIFCATIONS : Carriers of same amplitude more than 1 V. Frequency of each carriers is 10 KHz each.

OVERVIEW

What Is QPSK ?

In telecommunications, Quadrature Phase Shift Keying modulation refers to a technique for varying the phase of a carrier wave -- a wave of fixed amplitude and frequency -- by applying a digital signal, so that it can carry a signal in radio or television transmissions.

Phase Shift :

In its simplest form, digital phase modulation, or phase shift keying modulation, changes the phase of the carrier wave by using digital data to switch between two signals of the same frequency, but opposite phase. However, the number of phase shifts is not limited to just two states and in QPSK modulation the carrier wave undergoes four changes in phase, corresponding to the 0, 90, 180 and 270 degree position within the waveform.

Bandwidth :

In PSK, in each time period the phase of the carrier wave can change only once while the amplitude remains constant. However, in QPSK, four phases -- with each finite phase change representing unique digital data -- are possible, so two binary digits, or “bits," of information can be transmitted within each time period. In other words, the rate of change of the signal in QPSK allows the carrier wave to transmit two bits of information rather than one and effectively doubles the bandwidth, or transmission capacity, of the carrier wave. QPSK transmits twice the data rate in a given bandwidth compared to BPSK - at the same BER.

Gray Coding :

QPSK modulation takes input bits, two at a time, and creates a symbol that represents one of four phases. However, the performance of QPSK modulation can be enhanced by the use of a technique known as Gray coding. Gray coding maps every two input bits to one of four unique symbols in such a way that the pairs of bits vary by just a single bit from symbol to symbol. If a symbol is received in error, it will contain just one erroneous bit if it was received in error to an adjacent symbol.

Constellation Diagram Depicting Gray Coding

PROCESS OF

GENERATION

Block Diagram Of A QPSK Transmitter

In fig shows a block diagram of a typical QPSK transmitter. The unipolar binary message (data) first converted into a bipolar non-return-to-zero (NRZ) sequence using a unipolar to bipolar converter. The bit stream is then split into two bit streams I(in-phase) and Q(Quadrature) .The bit stream in-phase(I) is called the “even” stream and Quadrature(Q) is called “Odd” stream.

The input data go to the Serial to Parallel Converter then it is split up into two. The two bit stream fed to the Low pass filter (LPF).Then the two bit stream after filtering fed to the modulator. The filter at the output of the modulator confines the power spectrum of the QPSK signal within the allocated band. The two modulator bit stream are summed and fed to the band pass filter (BPF) and produce the QPSK output.

Conceptual transmitter structure for QPSK. The binary data stream is split into the in-phase and Quadrature-phase components. These are then separately modulated onto two orthogonal basis functions. In this implementation, two sinusoids are used. Afterwards, the two signals are superimposed, and the resulting signal is the QPSK signal.

Timing diagram for QPSKThe binary data that is conveyed by this waveform is: 1 1 0 0 0 1 1 0.The odd bits, highlighted here, contribute to the in-phase component: 1 1 0 0 0 1 1 0The even bits, highlighted here, contribute to the Quadrature-phase component: 1 1 0 0 0 1 1 0

Jumps in phase can be seen as the PSK changes the phase on each component at the start of each bit-period.

Applications :

QPSK modulation is one of the most popular digital modulation techniques for satellite communication, including Digital Video Broadcast - Satellite, and cable networks because it is resilient, easy to implement and less susceptible to noise than other modulation techniques.. QPSK is also widely used in code-division multiple access -- a digital technology used in cellular phones -- and other forms of digital communication over a radio frequency carrier wave.

LIMITATION

Differential phase-shift keying (DPSK)

Differential phase shift keying (DPSK) is a common form of phase modulation that conveys data by changing the phase of the carrier wave. In BPSK and QPSK there is an ambiguity of phase if the constellation is rotated by some effect in the communication channel through which the signal passes. This problem can be overcome by using the data to change rather than set the phase.

For a signal that has been differentially encoded, there is an obvious alternative method of demodulation. Instead of demodulating as usual and ignoring carrier-phase ambiguity, the phase between two successive received symbols is compared and used to determine what the data must have been. When differential encoding is used in this manner, the scheme is known as differential phase-shift keying (DPSK). This is subtly different to just differentially encoded PSK since, upon reception, the received symbols are not decoded one-by-one to constellation points but are instead compared directly to one another.

RC PHASE SHIFT OSCILLATOR

90 Degree Phase Shifter

The figure shows a phase shifter which shifts place of output from input by 90 degree. Here, two phase shifters are identical and have 45 degree phase shift each.

The voltage follower acts as a buffer between two phase shifters.

At frequency where w=1/CR and phase lag = 45 degree, attenuation is 1/sqrt(2).

When two such phase shifters are in cascade, net phase lag is 90 degree and attenuation is ½ .

360 Degree Phase Shifter

When four such 90 degree phase shifters are connected, the resultant phase shift is 360 (or 0) degree. Figure shows a phase shift oscillator utilizing four such phase shifter in the feedback network.

The amplifier gives minimum gain of 2x2 ^4 = 32 (multiplier 2 arises due to attenuation of ½ at the non inverting input of the amplifier). The amplifiers at each phase shifter putput makes amplitudes of S0, S1, S2 and S3 equal.

OUTPUT AND CIRCUIT DIAGRAM

FIG: First quarter of circuit

FIG: Full Circuit

Veroboard Implementation:

FIG: Front view of veroboard with four

outputsFIG: Front view of

veroboard showing IC bases

Phase-Shifted Output

FIG: 90˚ and 270˚ phase shifted output

FIG: 180˚ and 360˚ phase shifted output

REMARKS :

The aim was to implement RC phase shift oscillator circuit.

We were successful in getting all the four outputs at 90degree phase differences.

Necessary data has been recorded for documentation.

THANK YOU !