Types of Radio Receivers: Working, Characteristics, and Applications

Introduction

A radio receiver, also known simply as a receiver or radio, is an electronic device that receives radio waves and converts the information carried by them into a usable form. It works with an antenna, which intercepts radio frequency electromagnetic waves and converts them into small alternating currents. These currents are processed by the receiver to extract the desired information.

The receiver uses electronic filters to separate the required radio signal from all other signals captured by the antenna. It then amplifies the signal and demodulates it to recover audio, video, or digital data. Radio receivers exist as standalone devices or embedded circuits inside equipment such as televisions, mobile phones, wireless modems, and communication systems.

Types of Radio Receivers

Radio receivers are classified into the following types:

• Superheterodyne Receiver
• Regenerative Receiver
• Super-Regenerative Receiver
• Direct Conversion Receiver
• Tuned Radio Frequency (TRF) Receiver

1. Superheterodyne Receiver

A superheterodyne receiver converts incoming frequencies to an intermediate frequency (IF) using the process of frequency mixing. It was invented by Edwin Armstrong, though early patent credit was given to Lucien Levy. Today, superheterodyne receivers are widely used in most communication systems.

In the early stages of radio development, TRF receivers were widely used because of their simplicity and low cost.
However, due to complex tuning and lack of stability, they were gradually replaced by superheterodyne receivers in the 1920s and 1930s.

2. Regenerative Receiver

A regenerative (regen) receiver introduces positive feedback into the circuit, significantly increasing gain and selectivity. A portion of the RF amplifier output is fed back to the input in phase, allowing signals to be repeatedly amplified—sometimes up to 1000 times or more.

The increased selectivity is due to the tuned circuit inside the amplifier. Positive feedback effectively increases the
Q-factor of the coil, improving selectivity and reducing resistance. The regenerative receiver offers better performance than basic TRF receivers and can approach the performance level of a superheterodyne receiver.

3. Super-Regenerative Receiver

Invented by Edwin Armstrong in 1922, the super-regenerative receiver is a high-gain version of the regenerative receiver. It is commonly used in walkie-talkies, remote controls, and wireless networking equipment. It works well with AM and wideband FM signals but does not support narrowband FM or SSB (Single Sideband) because it self-oscillates.

4. Direct Conversion Receiver

A direct conversion receiver converts the received signal directly to baseband frequency without using intermediate frequencies. Originally used for AM, CW (Morse), and SSB reception, it is now widely used in digital communication systems. Modern IQ demodulators make it suitable for phase-based modulation schemes such as PSK and QAM.

5. Tuned Radio Frequency (TRF) Receiver

A TRF receiver uses one or more RF amplifier stages to extract the audio signal from the incoming radio signal.
Multiple RF amplifiers improve signal strength and reduce interference. Early TRF receivers required separate tuning for each stage, making operation complicated. Later versions introduced single-knob tuning. TRF receivers were eventually replaced by superheterodyne receivers.

Characteristics of a Radio Receiver

• Sensitivity
• Selectivity
• Fidelity
• Double Spotting
• Tracking

1. Sensitivity

Sensitivity refers to the voltage required at the receiver input to produce a standard output power. It is usually measured in microvolts or dB below one volt. Higher sensitivity means the receiver can detect weaker signals.

Professional receivers express sensitivity based on the minimum signal power needed for acceptable output signal quality.
Factors affecting sensitivity include:

• Gain of the IF amplifier
• Gain of the RF amplifier
• Noise figure of the receiver

2. Selectivity

Selectivity is the ability of a receiver to separate the desired signal from nearby unwanted signals. It is measured by varying the frequency of the input signal and observing how much unwanted signals are attenuated.

Selectivity depends mainly on the response of the IF amplifier, with minor contributions from the mixer and RF amplifier. Better selectivity improves adjacent channel rejection.

3. Fidelity

Fidelity refers to the receiver’s ability to reproduce audio signals accurately without distortion. It depends primarily on the frequency response of the AF amplifier. High-fidelity receivers require a flat response across a wide audio frequency range.

4. Double Spotting

Double spotting occurs when a weak station appears at two points on the tuning dial. This is caused by poor image frequency rejection due to inadequate front-end selectivity. Improving image rejection reduces the occurrence of double spotting.

5. Tracking

Tracking ensures that in a superheterodyne receiver, the local oscillator frequency maintains a constant frequency difference (usually 455 kHz for AM systems) above the incoming signal frequency. This constant difference ensures proper mixing and stable IF output.