Radio Receivers

In this lecture, we are going to learn about radio receivers, the function of receivers, different types of radio receivers, and their uses. So let’s start with an introduction to radio receivers and then we will see the details of it.

Introduction of Radio Receiver

• We know that in a communication system, a radio transmitter radiates or transmits a modulated carrier signal. This modulated carrier signal travels down the channel. i.e. transmission medium and reaches at the input of radio receiver. This means that the modulated carrier signal is picked up by the antenna of the radio receiver.

• This modulated signal so received is generally very weak. therefore, inside the receiver, this weak signal is first amplified in an R.F. amplifier stage of the radio receiver. Also since the received modulated signal contains a lot of noise or unwanted signals at adjacent frequencies, it must be selected and the noise must be rejected.

• Finally, in the receiver, the R.F. carrier or modulated signal must be demodulated to get back the original, modulating or baseband signal (i.i. audio signal in case of the broadcast receiver) is generally weak, it has to amplify in one or more stage of an audio amplifier.

Function of Receiver

1. Intercept the incoming modulated signal (i.e. electromagnetic waves) by the receiving antenna.
2. Select the desired signal and reject the unwanted signals.
3. Amplify this selected R.F. signal.
4. Detect the modulated signal to get back the original modulating or baseband signal.
5. Amplify the modulating frequency signal.

• From the above discussion, we can summarize the main function of radio receivers as:

This means that a radio receiver is electromagnetic equipment that picks up the desired signal, rejects the unwanted signals, amplifies the desired signal, and demodulates the modulated signal to get back the original modulating frequency signal.

Classification of Radio Receiver

• We can classify the radio receivers in two ways:

(A) Depending upon the applications, the radio receivers may be classified as follows:

1. Amplitude modulation (A.M.) Broadcast Receivers: These receivers are used to receive the broadcast of speech or music transmitted from amplitude modulation broadcast transmitters which operate on longwave, mediumwave and short wavebands.
2. Frequency Modulation (F.M.) Broadcast Receivers: These receivers are used to receive the broadcast programs of F.M. broadcast transmitters which operate in VHF or UHF bands.
3. Communication Receivers: Communication receivers are used for the reception of telegraphs and short-wave telephone signals. This means that communication receivers are used for various purposes other than broadcast services.
4. Television Receivers: Television receivers are used to receive television broadcasts in VHF or in UHF bands.
5. Radar Receivers: Radar receivers are used to receive Radar signals.

(B) Depending upon the fundamental aspects, the radio receivers may also be classified as under:

1. Tuned Radio Frequency (TRF) Receivers
2. Superheterodyne Receiver

• In fact, various forms of receivers have been proposed from time to time. However, only two of them became popular for commercial applications. These are the Tuned Radio Frequency (TRF) receiver and the superheterodyne receiver. Presently, the superheterodyne receiver is the most popular and most widely used. The TRF receiver was used earlier in the 1940s. The TRF receiver had some inherent drawbacks which were removed in the superheterodyne receiver. To know more about the Tuned radio receiver and superheterodyne receiver click on each receiver highlighted.

Characteristics of Radio Receiver

• In this section let us discuss various superheterodyne receiver characteristics. They are as under:

1. Sensitivity
2. Selectivity
3. Fidelity
4. Double Spotting
5. Tracking

• Let us discuss each characteristic one by one in detail.

1. Sensitivity of Receiver:

The sensitivity of a radio receiver may be defined as its ability to amplify weak signals.

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

• sensitivity is expressed in microvolts or in decibels below 1 volt and is measured at three points along with the tuning range when a production receiver is lined up.

• However, for professional receivers, the sensitivity is generally quoted in terms of signal power required to produce a minimum acceptable output signal with a minimum acceptable output noise level.

• A few factors determining the sensitivity of a receiver are as under:

• The gain of the IF amplifier.
• The gain of the RF amplifier.
• The noise figure of the receiver.

2. Selectivity of Receiver

The selectivity of a receiver may be defined as the ability to reject unwanted signals.

• It is also expressed the attenuation that the receiver offers to signal at frequencies adjacent to the one to which it is tuned.

• In selectivity measurement, the frequency of the generators is varied to either side of the frequency to which the receiver is tuned. Naturally, the output of the receiver falls since the input frequency is not incorrect. Thus the input voltage must be increased until the output is the same as it was originally.

• The selectivity of the receiver depends upon the following factors:

1. Selectivity varies with receiving frequency and becomes somewhat worse when the receiving frequency is raised.
2. In general, it is mainly determined by the response of the IF section, with the mixer and RF amplifier input circuits playing a small but significant part.
3. Selectivity is the main factor that determines the adjacent channel rejection of a receiver.

3. Fidelity in Communication

Fidelity is the ability of a receiver to reproduce all the modulating frequencies equally.

• Fidelity basically depends on the frequency response of the AF amplifier.

• Higher fidelity is essential in order to reproduce good quality music faithfully i.e. without introducing any distortion. For this, it is essential to have a flat frequency response over a wide range of audio frequencies.

4. Double Spotting

When a receiver picks up the same short wave station at two nearby points on the receiver dial, the double spotting phenomenon takes place.

• The main cause for double spotting is poor front-end electricity, i.e., inadequate image frequency rejection. The front end of the receiver does not select different adjacent signals very well.

• The adverse effect of double spotting is that a weak station may be marked by the reception of a nearby strong strong station at the spurious point of the dial.

• On the other hand, double spotting may be used to calculate the IF of an unknown receiver.

• If image frequency rejection is improved, then certainly there will be a corresponding decrease in the double spotting occurrence.

5. Tracking or Tunning of a Reciever

• In a superheterodyne receiver, the local oscillator frequency is made to track with the tuned circuits which are tuned to the incoming signal frequency in order to make a constant frequency difference at the output of the mixer.

• For a general, AM broadcast system, the intermediate frequency (I.F.) is 455kHz. This indicates that the local oscillator should always be set at a frequency that is 455kHz above the incoming signal frequency.

Frequently Asked Questions on Radio Receivers