Unit 6 | unit 6 noise

Analog Communication

Unit 6

Analog communication

6.1 Noise: Different types of Noise

A signal may be contaminated along the path by noise. Noise may be defined as any unwanted introduction of energy into the desired signal.

In radio receivers noise may produce hiss in the loud speaker output. Hence noise is random and unpredictable.

Noise is produced in both external and internal to the system.

External noise includes

Atmospheric noise (lightening)
Galactic noise(thermal radiation)
Industrial noise (from motors, ignition)

We can minimize or eliminate external noise by proper system design.

Internal noise is generated inside the system.It is resulted due to random motion of charged particles in resistors, conductors and electronic devices.

With proper design it can be minimized but never can be eliminated.

Types of Noise

White Noise
Thermal noise
Shot noise
Partition noise
Low frequency or flicker noise
Burst noise
Avalanche noise

White Noise

One of the important random processes is the white noise process. Noises in many practical situations are approximated by the white noise process.

White noise plays an important role in modelling of WSS signals. A white noise process W(t) is a random process that has constant power spectral density at all frequencies.

Thus

S w(w) = N0/2 ----------------(1) - ∞<w<∞

Where No is real constant and called the intensity of the white noise. The corresponding autocorrelation is given by

Rw( = N/2 ( where ( is the Dirac delta.

The average power of white noise

P wg = E W 2 = 1/2π dw -> ∞

The autocorrelation function and the PSD of white noise process is shown in Figure.

Fig.1: Autocorrelation function

Thermal Noise

Thermal noise is the electronic noise generated by thermal agitation of charge carriers inside an electrical conductor in equilibrium, which happens regardless of any applied voltage.

Movement of electrons will form kinetic energy in the conductor related to temperature in the conductor.

When the temperature increases the movement of electrons increases and current flows through the conductor.

Current flows due to free electrons which create noise voltage n(t).

Noise voltage is influenced by temperature hence it is called thermal noise.

Q. Calculate the thermal noise power available from any resistor at room temperature (290K) for a bandwidth of 1Mhz . Calculate also the corresponding noise voltage given that R = 50 Ω.

Pn = 1.38 x 10 -23 x 290 x 10 6

= 4 x 10 -15.

En2 = 4 x 50 x 1.38 x 10 -23 x 290

= 0.895 µ V

Shot Noise

Shot noise arises because the current consists of a vast number of discrete charges.
The continuous flow of these discrete pulses gives rise to almost white noise. There is a cut-off frequency by the time it takes for the electron or other charge carrier to travel through the conductor.
Unlike thermal noise, this noise is dependent upon the current flowing and has no relationship to the temperature at which the system is operating.
Shot noise is more apparent in devices such as PN junctions. The electrons are transmitted randomly and independently of each other.

Partition Noise

When the circuit is to divide in between two or more paths then the noise generated is known as Partition noise. The reason for this generation is random fluctuation in the division.

Flicker Noise

Flicker noise or modulation noise is the one appearing in transistors operating at low audio frequencies. Flicker noise is proportional to the emitter current and junction temperature. However, this noise is inversely proportional to the frequency.

Burst Noise

Burst noise consists of step-like transitions suddenly between two or more discrete voltages or current levels.

Each shift in offset voltage or current lasts for millisecond to seconds . It is also known as popcorn noise because of the popping or crackling sound it produces in audio circuits.

Avalanche Noise

Avalanche noise is the noise produced when a junction diode is operated at the onset of avalanche break down , a semiconductor junction phenomenon in which the carriers having high voltage gradient develop sufficient energy to dislodge additional carriers through physical impact creating ragged current flow.

Key Takeaways:

The average power of white noise

P wg = E W 2 = 1/2π dw -> ∞

2. Noise may be defined as any unwanted introduction of energy into the desired signal.

6.2 SNR in AM, FM and PM System and use of emphasis Circuit in FM for SNR optimization

Signal to Noise Ratio

Signal-to-Noise Ratio (SNR) is the ratio of the signal power to noise power. The higher the value of SNR, the greater will be the quality of the received output.

Figure of Merit

The ratio of output SNR and input SNR can be termed as Figure of Merit. It is denoted by F. It describes the performance of a device.

F=(SNR)O/(SNR)I

SNR for AM

Consider the following receiver model of AM system to analyze noise.

Fig.2: Receiver model of AM system

Channel signal to noise ratio is given by

Where W is the bandwidth and is modulation index

Output signal-to-noise ratio (of AM receiver) is given by

SNR for FM

Transmitted power of an FM waveform:

From

SNR for PM

Numericals:

The diagram below represents the first three stages of a typical AM or FM receiver. Find the following quantities

(a)

(b)

(c)

(d)

2. The signal power at the input to a receiver is 6.2nW and the noise power at the input to that receiver is 1.8nW. Find SNR and SNRdB

Use of emphasis Circuit in FM for SNR optimization

Comparison: FM and AM

Let us compare the result for sinusoidal 100% modulation

FM is better if or more. But this comes at the cost of higher bandwidth as

For and bandwidth of AM system is

Several authors to make the comparison not on the basis of equal power but rather on the basis of equal signal power measured when the modulation In this case, as it can be easily verified, we find that the above equation can be replaced by