Effect of Noise, Analog Filtering and Digital Filtering

Effect of noise:

In electronics,noise is a random fluctuation in an electrical signal, a characteristic of all electronic circuits. Noise generated by electronic devices varies greatly, as it can be produced by several different effects.Thermal noise is unavoidable at a non-zero temperature (see fluctuation-dissipation theorem) while other types depend mostly on device type (such as shot noise ,which needs steep potential barrier) or manufacturing quality and semiconductor defects, such as conductance fluctuations, including 1/f noise.

In communication systems, noise is an error or undesired random disturbance of a useful information signal in a communication channel. The noise is a summation of unwanted or disturbing energy from natural and sometimes man-made sources. Noise is, however, typically distinguished from interference, (e.g.cross-talk, deliberate jamming or other unwanted electromagnetic interference from specific transmitters), for example in the signal-to-noise ratio(SNR),signal-to-interference ratio(SIR) signal-to-noise plus interference ratio(SNIR) measures. Noise is also typically distinguished from distortion, which is an unwanted systematic alteration of the signal waveform by the communication equipment, for example in the signal-to-noise and distortion ratio(SINAD). In a carrier-modulated passband analog communication system, a certain carrier-to-noise ratio(CNR) at the radio receiver input would result in a certain signal-to-noise ratio in the detected message signal. In a digital communications system, a certain Eb/ No(normalized signal-to-noise ratio) would result in a certain bit error rate(BER).While noise is generally unwanted, it can serve a useful purpose in some applications, such as random number generation or dithering.

Types of noise:

1.Thermal noise:

When charge carriers are thermally generated then a potential drop may arise at the two terminals of the device causing thermal noise. As the amount of thermal noise generated depends on upon the temperature of the circuit, very sensitive circuits such as preamplifier sin radio telescopes sometimes cooled in liquid nitrogen to reduce the noise level.
2.Shot noise:

If electrons flow across a barrier, then they have discrete arrival times. Those discrete arrivals exhibit hot noise. The output of a shot noise generator is easily set by the current. Typically, the barrier in a diode is used.Shot noise in electronic devices results from unavoidable random statistical fluctuations of the electric current when the charge carriers (such as electrons) traverses a gap. The current is a flow of discrete charges, and the fluctuation in the arrivals of those charges creates shot noise. Shot noise is similar to the noise created by rain falling on a tin roof. The flow of rain may be relatively constant, but the raindrops arrive discretely.

3.Partition noise:

When the partition of net current occurs in the semiconductor devices then partition noise occurs. So, this noise arises due to the current division.

4.Carrier generation and Recombination noise:

In a semiconductor , the generation of the carrier and its recombination occurs when the device is biased. Due to recombination, neutralization of holes and electrons produces a class of noise known as .Carrier generation and Recombination noise.

5. Burst noise:

Burst noise consists of sudden step-like transitions between two or more levels (non-Gaussian), as high as several hundred microvolts, at random and unpredictable times. Each shift in offset voltage or current lasts for several milliseconds and the intervals between pulses tend to be in the audio range (less than 100Hz), leading to the term popcorn noise for the popping or crackling sounds it produces in audio circuits.

Filters:

Filters are networks that process signals in a frequency-dependent manner.They are frequency selective network which passes the desirable frequency but blocks the unwanted frequency and this process is called Filtering.Signal ï¬Âltering consists of processing a signal to remove a certain band of frequencies within it. The band of frequencies removed can be either at the low-frequency end of the frequency spectrum, at the high-frequency end, at both ends, or in the middle of the spectrum. Filters to perform each of these operations are known respectively as low-pass ï¬Âlters, high-pass ï¬Âlters, band-pass ï¬Âlters and band-stop ï¬Âlters (also known as notch ï¬Âlters). All such ï¬Âltering operations can be carried out by either analog or digital methods.

1.Analog filtering:

Passive linear electronic analog filters are those filters which can be described withLiner differential equation; they are composed of capacitors, inductors and, sometimes,resistors(passive) and are designed to operate on continuously varying (analog) signals.

Analog filters have many practical applications. A simple, single-pole, low-pass filter (the integrator) is often used to stabilize amplifiers by rolling off the gain at higher frequencies where excessive phase shift may cause oscillations. Also,A simple, single-pole, high-pass filter can be used to block dc offset in high gain amplifiers or single supply circuits. Filters can be used to separate signals, passing those of interest, and attenuating the unwanted frequencies.

Basically, there are 4 types of analog filters :

1.Low-pass filter

2.High-pass filter

3.Band-pass filter

4.Band-stop(Band-reject) filter

Digital filtering:

They are widely used in adaptive filtering applications in communications such as echo cancellation in modems, noise cancellation, and speech recognition.Depending upon the impulse response , there are two types of digital response. They are:

1.Finite impulse response filter(FIR)

2.Infinite impulse response filter(IIR)

They are briefly explained below:

1.Finite impulse response filter(FIR):

By varying the weight of the coefficients and the number of filter taps, virtually any frequency response characteristic can be realized with an FIR filter. FIR filters can achieve performance levels which are not possible with analog filter techniques (such as perfect linear phase response). However, high-performance FIR filters generally require a large number of multiply-accumulates and therefore require fast and efficient DSPs.

Characteristics:

1.Impulse Response has a Finite Duration (N Cycles)

2.Linear Phase, Constant Group Delay (N Must be Odd)

3.No Analog Equivalent

4.Always Stable

5.Can be Adaptive

6.Computational Advantages when Decimating Output

7.Easy to Understand and Design

Infinite impulse response filter(IIR):

Infinite impulse response filters get their name because their impulse response extends for an infinite period of time. This is because they are recursive, i.e., they utilize feedback. Although they can be implemented with fewer computations than FIR filters, IIR filters do not match the performance achievable with FIR filters and do not have linear phase. Also, there is no computational advantage achieved when the output of an IIR filter is decimated because each output value must always be calculated.

Characteristics:

1.Users Feedback (Recursion)

2.Impulse Response has an Infinite Duration

3.Potentially Unstable

4.Non-Linear Phase

5.More Efficient than FIR Filters

6.No Computational Advantage when Decimating Output

7.Usually Designed to Duplicate Analog Filter Response

8.Usually Implemented as Cascaded Second-Order Sections (Biquads)

References

1.https://en.wikipedia.org/wiki/Noise_(signal_processing)

2.Vyacheslav Tuzlukov (2010), Signal Processing Noise, Electrical Engineering and Applied Signal Processing Series, CRC Press.