Radio Receiver | Communication System - Electronics and Communication Engineering (ECE) PDF Download

Introduction

• The function of the radio receiver is to receive the signal and perform demodulation to recover the original message signal. The radio transmitter sends the signal at the initial stage. The antenna present at the transmitter side radiates the signal, which is captured by the other antenna present at the radio receiver.
• We have already discussed the process of transmission using a radio transmitter. The modulation process is the main principle in radio transmitters, where the signal is transmitted through the communication channel to the receiver. The main principle of the receiver is demodulation. Let's discuss the process of signal reception and recovery in the radio receiver.

AM demodulation

• The demodulation process of AM is similar to that of FM (Frequency Modulation) and other types of modulation. The only difference is the change in the demodulation block of the receiver. The demodulation process of the radio receiver involves processing the received signal to recover the baseband signal, which is also known as the message signal.
• We assume that the signal has suffered great attenuation while transmitting through the communication channel. Hence, the amplification of the received signal is necessary to improve the attenuation.

The block diagram of the radio receiver is shown below:

• The received signal carrier is known as the RF (Radio Frequency) carrier with the operating Frequency of Fr. The function of the RF amplifier is to amplify the received signal to remove any attenuation in the signal, which is present as the beginning block of the radio receiver. After amplification, it passes the signal to the mixer. The RF carrier signal is multiplied with a sinusoidal waveform provided by the local oscillator operating at the Frequency of Fo. It helps in converting the carrier frequency to the baseband frequency. The demodulation process is just the opposite of the modulation process. In modulation, the baseband frequency is converted to the carrier frequency, while in demodulation, the carrier frequency is converted back to the baseband frequency.
• The process of mixing two signals is known as heterodyning. If the selected oscillator frequency is above the RF frequency, the process of mixing is also known as Superheteroyne.
• The multiplication of carrier signal with the sinusoidal waveform produces two output frequencies, which is the sum and difference of the two frequencies of these signals. The sum frequency is Fo + Fr, and the difference frequency is Fo - Fr.
• The mixer implicitly contains the filter that rejects the sum frequencies and passes the difference frequencies (Fo - Fr) to the IF (Intermediate Frequency) carrier. A RF carrier is replaced by the IF carrier to produce the intermediate frequency range at the output. The output of the IF carrier is applied to the IF amplifier. The output is further passed to the demodulator and finally to the baseband filter, which recovers the baseband signal. Thus, the main function of the receiver was to perform the conversion from the carrier frequency to the baseband frequency. If the signal is strong enough for demodulation, filters and amplifiers can be avoided. The carrier input signal is directly applied to the mixer in such cases.
• In the case of the synchronous demodulation method, we need to use an asynchronous carrier source.
• The RF amplifiers can possess' several amplification stages depending on the requirements and the signal strength.
• The main advantage of the Superheterodyne principle is the tuning of the receiver to different signals. Here, we do not need a separate amplification stage and separate tuning. It makes the transmission process more difficult. Using the Superheterodyne principle, we only need to change the frequency of the local oscillator to go from one RF frequency to other.

AGC (Automatic Gain Control)

The voltage gain at the receiver in several amplification stages is very high. It is required when the input is of very low frequency, and the required output is of high frequency. The high gain converts the low-frequency signals to the high frequency. It helps in the transmission of very weak signals. But, if the input signal is of high frequency, the high gain at the receiver would not be an advantage and may cause distortion. AGC automatically adjusts the gain by detecting the strength of the signal. Otherwise, the constant adjustment in the system is required for efficient transmission, which becomes difficult.

Functions of a radio receiver

The functions of a radio receiver are as follows:

1. Amplification

• The amplification is the first essential part of the reception at the radio receiver. The incoming radio signal is generally attenuated. The amplifier helps in removing the attenuation from the signal. The other function of the amplifiers is to increase the amplitude of the input radio signals. It uses power from the batteries or plugs to increase the amplitude. Today, most devices uses the transistor for amplification purpose.
• The amplifiers are used at both the transmitting and the receiving end. At the first stage, it is used to make the signal suitable for modulation. At the receiving end, it is used to make the signal free from noise to send it to the receiver (for example, speaker).

2. Demodulation

The signal passes from many modulators, mixer, and amplifier stages. At the receiver, the signal is demodulated to separate the original signal from the modulated carrier signal. It is done with the help of a demodulator. Every type of receiver requires a different demodulation process.
For example:

• DSBSC (Double Sideband Suppress Carrier) requires a coherent detection method for demodulation
• SSBC (Single Sideband with carrier) requires an envelope detector method for demodulation
• Fm receiver uses the FM type demodulator

3. Bandpass filtering

Various transmitters transmit the radio waves at different frequencies to prevent any interference between the signals. Each transmitter has a respective receiver that selects its signal based on the frequency. Bandpass filters are used to filter out the desired radio signal for the respective transmitter. It filters out the desired signal and blocks other signals present at other frequencies. It helps to detect the desired signal and ground all other radio signals at resonant frequencies. It may also contain tuned circuits between the antenna and the ground.

Types of radio receiver

The radio receivers are classified as:

• Superheteroyne receiver
• Regenerative receiver
• Super regenerative receiver
• Direct conversion receiver
• Tuned radio frequency receiver

1. Superheteroyne receiver

• The above discussed receiver is a Superheteroyne receiver. It uses frequency mixing to convert the frequencies to the intermediate frequency (IF). It was invented by an American inventor and electrical engineer named Edwin Armstrong. But, due to the early patent, the credit of invention was credited to the French radio manufacturer named Lucien Lavy. Most of the receivers used in the data transmission process are the Superheteroyne receivers. Some receivers are also based on direct sampling.
• At the beginning of the era of radio receivers, TRF (Tuned Radio Frequency) receivers were commonly used due to their low cost and easy operation. These receivers were less popular due to the high cost and skilled labor required for its operation. After the 1920s, Superheterodyne receivers were created based on the IF frequency, also known as IF transformers. But, it was replaced by the vacuum tube radio receivers invented around the 1930s.

2. Regenerative receiver

The regenerative receivers are generally used to increase the gain of the amplifiers. It was invented and patented in 1914 by Edwin Armstrong. The receivers were used between 1915 and World War II due to their better sensitivity and selectivity. The principle of such receivers is the positive feedback that works as a regeneration process. The output is applied again to the input to increase its amplification. By the 1930s, these receivers were replaced by the TRF and Superheterodyne receivers due to their disadvantage of radiation interference. But, regenerative receivers are widely used in amplifiers and oscillators.

3. Super regenerative receiver

It is a regenerative receiver with a large type of regeneration to achieve high amplification. Edwin Armstrong also invented it in 1922. It is used in various devices, such as walkie-talkies and wireless networking. It works well for AM (Amplitude Modulation) and wideband FM (Frequency Modulation), while regenerative receivers work well for narrowband FM. Super regenerative receivers cannot properly detect the SSB 9Single Sideband Signals) because it always self oscillates. It can receive the strongest signals, as it works best for the frequency bands free from any interference.

4. Direct conversion receiver

The function of DCR (Direct Conversion Receiver) is similar to that of Superheteroyne receiver, except the conversion of frequency to IF (intermediate Frequency). DCR demodulates the incoming radio signal using the synchronous detection driven by the local oscillator. The frequency is closely equivalent to the carrier frequency. It does not involve the complexity of two frequency conversions like Superheteroyne receiver. It uses only one frequency converter. If a synchronous detector following the IF stage is used in the Superheteroyne receiver, the demodulated output would be the same as the Direct Conversion Receiver.

5. Tuned radio frequency receiver

The TRF (Tuned Radio Frequency) uses one or more Radio Frequency (RF) amplifiers to extract an audio signal from an incoming radio signal. The concept of using more than one RF amplifier was to amplify the incoming signal at each successive stage, which helps in removing interference. The operation of early invented receivers was complex due to the separate tuning of the frequency to the station's frequency. But, later models were operated using a single knob to control the frequency. TRF was replaced by the Superheterodyne receivers invented by Edwin Armstrong around the 1930s.

History

In 1887, a German physicist named Heinrich Hertz identified the first radio waves using the series of his experiments based on the electromagnetic (EM) theory. The invention was based on various types of antennas including spark excited dipole antennas. But, they could detect only the transmission upto 100 feet from the transmitter. He also discovered spark gas transmitter in the same year.

• These transmitters were popular between 1887 and 1917. But, the information transmitted by these spark transmitters was noisy and was not suitable for audio transmission.
• Thus, the first invented radio receivers could only detect radio waves and the receiving device was called detector. There were no amplifiers at that time to amplify the signal.
• In 1895, G Marconi developed the first radio communication system.
• By 1897, Marconi and other researchers have accepted the use of tuned circuits in the radio wave transmission. It also behaves as a bandpass filter by passing the desired range of frequencies and rejecting the other when connected between the antenna and a detector.
• Around 1900, radios began to be used commercially across the globe.
• The coherent detectors were used for the radio transmission. It was used in the early radio receiver for upto 10 years.
• In 1907, coherent detectors were replaced by crystal detectors.
• Till 1920, various detectors were discovered, such as electrolytic detectors, and magnetic detectors.
• In 1920, the invention of vacuum tube detector replaced all other detectors discovered before the 1920s. During this era, the detector was renamed as a demodulator.
• The demodulator was a device that could extract audio signals from the radio signal.
• In 1924, the invention of the dynamic core loudspeaker improved the audio frequency response of the system as compared to previous invented speakers.
• After that, various types of radio receivers were invented.
• In 1947, the transistor era came and found various radio transmission applications.
• After 1970s, digital technology created another revolution and translated the entire receiver circuits into the chip.