SUPER HETERODYNE RECEIVER in Analog Communications

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Super Heterodyne Receiver
  • fs is the carrier frequency of the tuned radio station.
  • After receiving a signal from the antenna, an RF amplifier boosts the signal strength and the carrier frequency is fs.
  • In a super heterodyne receiver, the mixer is used to down-convert the RF signal into IF(Intermediate Frequency). The IF is constant and the value of IF depends upon the application. The IF used in AM radio is 455 KHz. So the local oscillator frequency is adjusted so that fl-fs is 455 KHz. This process is called tuning.
  • The IF amplifier is a tuned amplifier that is always tuned to 455 KHz. The Quality factor is 45.5 so the bandwidth is 10 KHz.
Quality Factor and Bandwidth
  • The Demodulator converts the IF signal into AF. AF and power amplifiers increase the signal strength to the required level.
  • Loudspeakers convert electrical signals into audio.

Example: Consider a receiver with IF=500 KHz and three signals received from the antenna having frequencies 600 KHz, 800 KHz, and 1000 KHz.

Case-1: Assume that the receiver is tuned to 600 KHz. The local oscillator frequency is adjusted to 1100 KHz to down-convert the signal to 500 KHz. This signal is exactly within the pass band because of fr=500 KHz.

800 KHz signal is down-converted to 300 KHz.

1000 KHz signal is down-converted to 100 KHz.

So these two signals are not within the pass band.

Case-2: Assume the receiver is tuned to an 800 KHz radio station. The local oscillator frequency is adjusted to 1300 KHz to down-convert the signal to 500 KHz.

IMAGE FREQUENCY AND ITS SUPPRESSION:

Image Frequency and supression
  • Consider a receiver with IF=500 KHz and assume that the receiver is tuned to a 600 KHz radio station. The local oscillator frequency is adjusted to 1100 KHz to down-convert the signal to 500 KHz.
  • Assume that another signal is received from the antenna having a frequency of 1600 KHz. This signal is also down-converted to 500 KHz and causes interference to the required signal. The signal which is causing interference is called the image frequency.

 fsi=fs+2IF

  • The image frequency signal is considered the noise because it is causing interference with the required signal. The image frequency suppression is not possible after the mixer. A tuned circuit is used at the input of the mixer to suppress the image frequency.

The frequency response of the tuned circuit is shown in the below figure:

Tuned Circuit-Frequency Response

The gain of the tuned circuit at image frequency should be as minimal as possible.

To determine the suppression factor Image Rejection Ratio(IRR) is used.

Image Rejection Ratio

Q=Quality factor

For optimal investment performance, a high IRR is unequivocally essential.

To increase the IRR the tuned circuits are connected in cascade.

Csacading Circuits

Implementing a tuned circuit in the RF amplifier stage is an effective strategy to enhance IRR.

Problems Related to Super Heterodyne Receiver:

1.      A receiver without an RF amplification stage is tuned to a frequency of 555 KHz. The local oscillator frequency is adjusted to 1010 KHz and the quality factor is 50. Calculate Intermediate Frequency, Image Frequency, and Image Rejection Ratio(IRR).

Solution:

fs= 555 KHz, fl= 1010 KHz, Q=50

Intermediate frequency (IF) = fl-fs = 1010 KHz – 555 KHz = 455 KHz.

 fsi= fs +2 (IF) = 1465 KHz

IRR=113

2.  A receiver having no RF amplifier is tuned to 100 KHz. The IF is 455 KHz. Q is 100. Calculate the image frequency and IRR and repeat the calculations if the receiver is tuned to 25 MHz.

Solution:

fs= 1000 KHz, Q=100, IF=455 KHz

fsi= fs +2 (IF) = 1910 KHz

IRR =138

fs= 25 MHz

fsi= fs +2 (IF) = 25.91 MHz

IRR =7.07

To improve the IRR either Q or IF should be increased.

If Q is increased, the Bandwidth of the tuned circuit decreases and the gain at fsi also decreases. So the IRR increases. This method is not used practically because the main bandwidth of the tuned circuit should be 10 KHz.

Increasing the intermediate frequency (IF) leads to a greater separation between the image frequency and the desired signal frequency (fs). The gain at image frequency decreases, so IRR increases.

If the carrier frequencies are in the KHz range, intermediate frequency (IF) should also be in the same range. So 455 KHz is suitable for Amplitude Modulation radio only. In Frequency Modulation (FM) radio carrier frequencies will be in the range of 88 MHz-108 MHz. so the intermediate frequency (IF) used in the FM radio is 10.7 MHz.

Choice of choosing Intermediate Frequency (IF):

  • The amplitude modulation system operates within a frequency range of 550 KHz to 1650 KHz.
  • If the Intermediate Frequency is chosen moderately as 455 KHz, then the quality factor is given by
  • Q=fr/Bandwidth = 455 K/10K =45.5
  • It is simple to design the filter with this Quality Factor.
  • Selecting an extremely high intermediate frequency (IF), such as 10MHz, results in a required Q factor of 1000, which poses significant challenges for practical circuit design due to the difficulty in achieving such high Q values.
  • Choosing a reduced value for the Intermediate Frequency (IF) causes the image frequency to move closer to the actual signal frequency (fs).
  • The reduced frequency separation between the image and signal frequencies significantly hinders the RF amplifier’s ability to suppress the unwanted image signal.
  • Image
  • If the intermediate frequency is reduced, the spectrum as image frequency will come closer to the original signal frequency fs, and rejection of image frequency will become difficult.
Choosing Intermediate Frequency

Applications of SUPER HETERODYNE RECEIVER

  1. Broadcast Radio Reception: Superheterodyne receivers are the backbone of AM and FM radio broadcasting, enabling clear and reliable reception of your favorite stations.
  2. Television Broadcasting: The superheterodyne principle is essential in TV receivers, allowing you to tune into different channels and enjoy high-quality video and audio content.
  3. Two-Way Radio Communication: Walkie-talkies, CB radios, and other forms of two-way communication rely on superheterodyne receivers to enable clear communication over specific frequencies.
  4. Mobile Phone Communication: Your smartphone uses a superheterodyne receiver to pick up signals from cell towers, ensuring you stay connected wherever you go.
  5. Satellite Communication: Superheterodyne receivers are used in satellite dishes to receive signals from orbiting satellites, enabling satellite TV, internet, and other forms of communication.

Q1. What is a superheterodyne receiver?

  • A superheterodyne receiver is a radio receiver that converts received signals to a fixed intermediate frequency (IF) for amplification and demodulation, improving selectivity and sensitivity.

Q2. How does a superheterodyne receiver work?

  • The receiver mixes the incoming radio frequency (RF) signal with a local oscillator (LO) signal to produce the IF signal. The IF signal is amplified, filtered, and demodulated to recover the original audio or data.

Q3. What are the advantages of a superheterodyne receiver?

  • Superior selectivity (ability to reject unwanted signals)
  • High sensitivity (ability to receive weak signals)
  • Good frequency stability

Q4. What are the applications of superheterodyne receivers?

  • AM/FM radio broadcasting
  • Television broadcasting
  • Two-way radio communication (e.g., walkie-talkies, CB radios)
  • Mobile phone communication
  • Satellite communication

Q5. What is the image frequency problem in superheterodyne receivers?

  • The image frequency is an undesired input frequency that can produce the same IF as the desired signal, leading to interference. It is typically addressed using RF filtering or image rejection techniques.
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