Pn sequence generator USING MAT LAB
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PN SEQUENCE GENERATOR
USING MAT LAB
WHAT IS PN SEQUENCE GENERATOR
Pseudo random Noise sequence generator is a circuit which generates pseudo random noise sequences.
A pseudo- noise sequence is a binary sequence with a noise like wave form that usually generated by means of a Linear feedback shift registers.
Block Diagram Of Linear Feedback Shift Register
WHAT IS LINEAR FEEDBACK SHIFT
A Feedback shift register consists of an ordinary shift register made up of m Flip-Flops .
The flip-flops in the shift register are regulated by a single timing clock.
At each pulse of the clock, the state of each flip-flop is shifted to the next one i.e. the contents of the register is shifted right by one position at each clock cycle.
The feedback from the left most register are XOR- ed together, to give a random sequence.
But this binary sequence is not totally random, because it does repeat after many cycles, thatâ„¢s why it is so called as pseudo random noise sequence generator.
Shift Register Length:
This is often referred to as the degree, and in general, the longer the shift register, the longer the duration of the pn sequence before it repeats.
The combination of taps and their location is often referred to as a polynomial, and expressed as:
Where the leading Ëœ1â„¢ represents x^0, which is the output of the last register, x^2 is the output of register stage 3 and x^3 the output of XOR.
A maximal sequence for a shift register of length L is referred as m-sequence, and is defined as:
These pn sequence generators are widely used in Cmos testing process as to generate test patterns.
And also used to generate spreading codes in CDMA,FDMA and PCM.
CMOS TESTING PROCESS
In VLSI designing process we will spend 25% for manufacturing, and 75% for testing.
By detecting a malfunctioning chip at an earlier level, the manufacturing cost may be kept low.
In each period of a maximal â€œ length sequence, the number of 1â„¢s is always one more than the number of 0â„¢s.This property is called as the Balance property.
Among the runs of 1â„¢s and of 0â„¢s in each period of a maximal-length sequence, one half the runs of each kind are of length one, one fourth are of a length two, one- eighth are of length three, and so on as long as these fractions represent meaningful numbers of runs. This property is called the Run property.
The autocorrelation function of a maximal-length sequence is periodic and binary value. This property is called as correlation property.
PN sequences have single autocorrelation peak, this feature permits the transmission to be asynchronous.
ADVANTAGES AND DISADVANTAGES :
The sequences generated by an LFSR can be nearly random with long periods, which aids in making the spread signal appear noise like.
Easy of implementation.
Output of lfsr is periodic.
For any given size of LFSR, a maximum of different unique m-sequences can be generated .
Cross correlation properties are good for more than two users. Thatâ„¢s why we go for gold sequences.
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