Noise in Conventional AM

Conventional amplitude modulation (AM) systems are also susceptible to various types of noise that can degrade the quality of the received signal. The most common types of noise in conventional AM systems are thermal noise, atmospheric noise, and intermodulation distortion.

1. Thermal noise: As mentioned earlier, thermal noise is generated by the random motion of electrons in a conductor at a finite temperature. In AM systems, thermal noise is present in the receiver components such as amplifiers and mixers, and it can degrade the quality of the received signal.

2. Atmospheric noise: Atmospheric noise, also known as static, is caused by natural sources such as lightning and can interfere with the received signal. Atmospheric noise can cause distortion and make the received signal difficult to decode.

3.Intermodulation distortion: Intermodulation distortion (IMD) is a type of distortion that occurs when two or more signals are mixed in a nonlinear device. In AM systems, IMD can be caused by nonlinearities in the receiver components such as mixers, amplifiers, and filters. IMD can create unwanted signals that interfere with the received signal and degrade its quality.

To minimize the effect of noise and distortion in AM systems, various techniques such as filtering, equalization, and modulation schemes can be used. For example, using a high-Q filter can help minimize the effect of atmospheric noise by rejecting unwanted signals. Using a low noise amplifier (LNA) at the beginning of the receiver chain can help amplify the received signal and minimize the effect of thermal noise. Additionally, using a high-quality carrier recovery circuit can help extract the carrier signal and improve the receiver's overall performance.

 

  •     In practical applications, the modulation index a is in the range of 0.8-0.9.
  •     Power content of the normalized message process depends on the message source.
  •     Speech signals : Large dynamic range, PM is about 0.1.
  •     The overall loss in SNR, when compared to a baseband system, is a factor of
  •     0.075 or equivalent to a loss of 11 dB.


The reason for this loss is that a large part of the transmitter power is used to send the carrier component of the modulated signal and not the desired signal. To analyze the envelope-detector performance in the presence of noise, we must use certain approximations. This is a result of the nonlinear structure of an envelope detector, which makes an exact analysis difficult
In this case, the demodulator detects the envelope of the received signal and the noise process.The input to the envelope detector is
r(t)= [𝐴𝑐[1 + 𝑎𝑚𝑛(t)] + 𝑛𝑐(t)] ) cos(2𝜋𝑓𝑐t)- 𝑛𝑠(𝑡)sin(2𝜋𝑓𝑐t)
Therefore, the envelope of r ( t ) is given by


Now we assume that the signal component in r ( t ) is much stronger than the noise component. Then

We observe that, at the demodulator output, the signal and the noise components are no longer additive. In fact, the signal component is multiplied by noise and is no longer distinguishable. In this case, no meaningful SNR can be defined. We say that this system is operating below the threshold.The subject of threshold and its effect on the performance of a communication system will be covered in more detail when we discuss the noise performance in angle modulation.