Subject : Principle of Communication Engineering

Topic : Radio Transmitters and Receivers

Difficulty : Medium

i. Sensitivity

• Sensitivity of a receiver is its ability to identify and amplify weak signals at the receiver output.

• It is often defined in terms of voltage that must be applied to the input terminals of the receiver to produce a standard output power which is measured at the output terminals.

• The higher value of receiver gain ensures smaller input signal necessary to produce the desired output power.Thus a receiver with good sensitivity will detect minimum RF signal at the input and still produce utilizable demodulated signal.
• Sensitivity is also known as receiver threshold. It is expressed in microvolts or decibels.
• Sensitivity of the receiver mostly depends on the gain of IF amplifier. It can be improved by reducing the noise level and bandwidth of the receiver.

• Sensitivity can be graphically represented as a curve shown in Fig2. Below, which depicts that sensitivity varies over the tuning band.

  

  Fig2. Sensitivity curve

ii) Image frequency:

• Image frequency is the problem of receiving two radio station simultaneously at the same point of the receiver dial.

  Image frequency(fsi)=fs+2IF

• For example, suppose the receiver is tuned to pick up a signal on a frequency of 600 kHz. The local oscillator will be operating at a frequency of 1,055 kHz. The received and local oscillator signals are mixed, or heterodyned, in the converter stage and one of the frequencies resulting from this mixing action is the difference between the two signals, is 455 kHz, the I-F frequency.

• However, if there is a station operating on a frequency of 1,510 kHz, and this signal passes through the rather broad tuned input circuit and appears on the grid of the converter tube, it too will mix with the local oscillator and produce a frequency of 455 kHz (1,510 - 1,055 = 455). This signal will also be accepted by the I-F amplifier stage and passed on, thus both signals will be heard in the output of the receive.

iii. Selectively

• The selectivity of an AM receiver is defined as its ability to accept or select the desired band of frequency and reject all other unwanted frequencies which can be interfering signals.

• Adjacent channel rejection of the receiver can be obtained from the selectivity parameter. Response of IF section, mixer and RF section considerably contribute towards selectivity.

• The signal bandwidth should be narrow for better selectivity.

• Graphically selectivity can be represented as a curve shown in Fig1. below, which depicts the attenuation offered to the unwanted signals around the tuned frequency.

iv. Fidelity

• Fidelity of a receiver is its ability to reproduce the exact replica of the transmitted signals at the receiver output.
• For better fidelity, the amplifier must pass high bandwidth signals to amplify the frequencies of the outermost sidebands, while for better selectivity the signal should have narrow bandwidth. Thus a trade off is made between selectivity and fidelity.
• Low frequency response of IF amplifier determines fidelity at the lower modulating frequencies while high frequency response of the IF amplifier determines fidelity at the higher modulating frequencies.

v. Double spotting

• Double spotting is a condition where the same desired signal is detected at two nearby points on the receiver tuning dial.

• One point is the desired point while the other is called the spurious or image point. It can be used to determine the IF of an unknown receiver.

• Poor front-end selectivity and inadequate image frequency rejection leads to double spotting.

• Double spotting is undesirable since the strong signal might mask and overpower the weak signal at the spurious point in the frequency spectrum.

• Double spotting can be counter acted by improving the selectivity of RF amplifier and increasing the value of IF.

• Consider an incoming strong signal of 1000 kHz and local oscillator tuned at 1455 kHz. Thus a signal of 455 kHz is produced at the output of the mixer which is the IF frequency.

• Now consider the same signal but with 545kHz tuned local oscillator. Again we get 455 kHz signal at the output.

• Therefore the same 1000 kHz signal will appear at 1455 kHz as well as 545 kHz on the receiver dial and the image will not get rejected. This is known as Double spotting phenomenon.

• It is also known as Adjacent channel selectivity.

vi. Image frequency rejection

• The capability of a receiver to reject image frequency is decided by its image frequency rejection ratio.

• It is defined as the ratio of gain at signal frequency to the gain at image frequency.

  α=Gain at fs/ Gaian of fsi

• The image frequency rejection depends on front end selectivity of receiver. For better rejection, the gain at image frequency should be less & this is achieved by increasing selectivity of RF stage.

Noise triangle in FM

• Noise triangle is Unwanted deviation of the carrier frequency. Noise signal produces amplitude and phase modulation in FM.

• The magnitude of this unwanted frequency deviation depends on the relative amplitude of the noise with respect to the carrier. Let us consider the noise signal and carrier signal vectorially.

  Fig1. Vector effect of noise on carrier

• When noise signal is superimposed on carrier, the amplitude of noise is added with carrier amplitude produces amplitude modulation in FM. The maximum deviation of amplitude carrier = Vn, as shown in Fig1.

• At the same time, noise vector is constantly changing phase angle w.r.t carrier signal Vc which will change the phase deviation (θ) of FM wave i.e. phase deviation θ of FM changes due to noise. Thus, noise modulates the carrier in terms of amplitude as well as phase.

  Fig.2 Noise triangle

  • From noise triangle in the Fig2., it is seen that noise in AM and PM remains constant for entire audio range because the modulation index due to signal are independent on modulating frequency but in FM the effect of noise will be increased with the increase in modulating frequency.
  • Thus, noise has more effect on higher frequencies in FM. The triangular distribution of noise in FM is called as FM noise triangle.

i. Sensitivity

• Sensitivity of a receiver is its ability to identify and amplify weak signals at the receiver output.

• It is often defined in terms of voltage that must be applied to the input terminals of the receiver to produce a standard output power which is measured at the output terminals.

• The higher value of receiver gain ensures smaller input signal necessary to produce the desired output power.Thus a receiver with good sensitivity will detect minimum RF signal at the input and still produce utilizable demodulated signal.
• Sensitivity is also known as receiver threshold. It is expressed in microvolts or decibels.
• Sensitivity of the receiver mostly depends on the gain of IF amplifier. It can be improved by reducing the noise level and bandwidth of the receiver.

• Sensitivity can be graphically represented as a curve shown in Fig2. Below, which depicts that sensitivity varies over the tuning band.

  

  Fig2. Sensitivity curve

ii) Image frequency:

• Image frequency is the problem of receiving two radio station simultaneously at the same point of the receiver dial.

  Image frequency(fsi)=fs+2IF

• For example, suppose the receiver is tuned to pick up a signal on a frequency of 600 kHz. The local oscillator will be operating at a frequency of 1,055 kHz. The received and local oscillator signals are mixed, or heterodyned, in the converter stage and one of the frequencies resulting from this mixing action is the difference between the two signals, is 455 kHz, the I-F frequency.

• However, if there is a station operating on a frequency of 1,510 kHz, and this signal passes through the rather broad tuned input circuit and appears on the grid of the converter tube, it too will mix with the local oscillator and produce a frequency of 455 kHz (1,510 - 1,055 = 455). This signal will also be accepted by the I-F amplifier stage and passed on, thus both signals will be heard in the output of the receive.

iii. Selectively

• The selectivity of an AM receiver is defined as its ability to accept or select the desired band of frequency and reject all other unwanted frequencies which can be interfering signals.

• Adjacent channel rejection of the receiver can be obtained from the selectivity parameter. Response of IF section, mixer and RF section considerably contribute towards selectivity.

• The signal bandwidth should be narrow for better selectivity.

• Graphically selectivity can be represented as a curve shown in Fig1. below, which depicts the attenuation offered to the unwanted signals around the tuned frequency.

iv. Fidelity

• Fidelity of a receiver is its ability to reproduce the exact replica of the transmitted signals at the receiver output.
• For better fidelity, the amplifier must pass high bandwidth signals to amplify the frequencies of the outermost sidebands, while for better selectivity the signal should have narrow bandwidth. Thus a trade off is made between selectivity and fidelity.
• Low frequency response of IF amplifier determines fidelity at the lower modulating frequencies while high frequency response of the IF amplifier determines fidelity at the higher modulating frequencies.

v. Double spotting

• Double spotting is a condition where the same desired signal is detected at two nearby points on the receiver tuning dial.

• One point is the desired point while the other is called the spurious or image point. It can be used to determine the IF of an unknown receiver.

• Poor front-end selectivity and inadequate image frequency rejection leads to double spotting.

• Double spotting is undesirable since the strong signal might mask and overpower the weak signal at the spurious point in the frequency spectrum.

• Double spotting can be counter acted by improving the selectivity of RF amplifier and increasing the value of IF.

• Consider an incoming strong signal of 1000 kHz and local oscillator tuned at 1455 kHz. Thus a signal of 455 kHz is produced at the output of the mixer which is the IF frequency.

• Now consider the same signal but with 545kHz tuned local oscillator. Again we get 455 kHz signal at the output.

• Therefore the same 1000 kHz signal will appear at 1455 kHz as well as 545 kHz on the receiver dial and the image will not get rejected. This is known as Double spotting phenomenon.

• It is also known as Adjacent channel selectivity.

vi. Image frequency rejection

• The capability of a receiver to reject image frequency is decided by its image frequency rejection ratio.

• It is defined as the ratio of gain at signal frequency to the gain at image frequency.

  α=Gain at fs/ Gaian of fsi

• The image frequency rejection depends on front end selectivity of receiver. For better rejection, the gain at image frequency should be less & this is achieved by increasing selectivity of RF stage.

Noise triangle in FM

• Noise triangle is Unwanted deviation of the carrier frequency. Noise signal produces amplitude and phase modulation in FM.

• The magnitude of this unwanted frequency deviation depends on the relative amplitude of the noise with respect to the carrier. Let us consider the noise signal and carrier signal vectorially.

  Fig1. Vector effect of noise on carrier

• When noise signal is superimposed on carrier, the amplitude of noise is added with carrier amplitude produces amplitude modulation in FM. The maximum deviation of amplitude carrier = Vn, as shown in Fig1.

• At the same time, noise vector is constantly changing phase angle w.r.t carrier signal Vc which will change the phase deviation (θ) of FM wave i.e. phase deviation θ of FM changes due to noise. Thus, noise modulates the carrier in terms of amplitude as well as phase.

  Fig.2 Noise triangle

  • From noise triangle in the Fig2., it is seen that noise in AM and PM remains constant for entire audio range because the modulation index due to signal are independent on modulating frequency but in FM the effect of noise will be increased with the increase in modulating frequency.
  • Thus, noise has more effect on higher frequencies in FM. The triangular distribution of noise in FM is called as FM noise triangle.