Superheterodyne Receiver

In this lecture, we are going to learn about the most important radio receiver called Superheterodyne Receiver. We will the basic function of it, a block diagram of the Superheterodyne Receiver, advantages of the Superheterodyne Receiver. So let’s discuss getting the basic concept of a Superheterodyne Receiver.

Introduction of Superheterodyne Receiver

• All the drawbacks in the TRF receiver have been removed in Superheterodyne Receiver. This basic Superheterodyne Receiver is most widely used in most communication.

• This means that the Superheterodyne principle is used in all types of receivers like television receivers, radar receivers, etc.

Superheterodyne Receiver Block Diagram

• The below figure shows the Superheterodyne Receiver Block Diagram.

• In a superheterodyne receiver, the incoming RF signal frequency is combined with the local oscillator signal frequency through a mixer and is converted into a signal of a lower fixed frequency. This lower fixed frequency is known as an intermediate frequency (I.F.).

• However, the intermediate frequency signal contains the same modulation as the original signal. This intermediate frequency signal is now amplified and demodulated to reproduce the original signal.

• The word heterodyne stands for mixing. Here we have mixed the incoming signal frequency with the local oscillator frequency. Therefore this receiver is called the superheterodyne receiver.

• Thus, in a superheterodyne receiver, a constant frequency difference is maintained between the local oscillator signal frequency and incoming RF signals frequency through capacitance tunning in which the capacitances are ganged together and operated by a common control knob The intermediate frequency (IF) amplifier generally contains a number of transformers each consisting of a pair of mutually coupled tuned circuits. Thus, with this large number of double-tuned circuits, operating at a specially chosen frequency, the IF amplifier provides most of the gain (i.e. sensitivity) and bandwidth requirements (i.e. selectivity) of the receiver. This means that the IF amplifier determines the sensitivity and selectivity of the superheterodyne receiver.

• Also, since the characteristics of the IF amplifier are independent of the incoming frequency to which the receiver is tuned, the selectivity and sensitivity of the superheterodyne receiver are quite uniform throughout its tunning range and not subject to the variations like a TRF receiver. Further, since the I.F. amplifier works at a fixed I.F. frequency, the design of this system is quite easy to provide high gain and constant bandwidth.

• Because of its narrow bandwidth, the I.F. amplifier rejects all other frequencies except intermediate frequency (I.F.). Actually, this rejection process reduces the risk of interference from other stations or sources. In fact, this selection process is the key to the superheterodyne receiver’s exceptional performance.

• After the I.F. amplifier, the signal is applied at the input of the demodulator which extracts the original modulating signal (i.e. audio signal). This audio signal is amplified by an audio amplifier to get a particular voltage level. This amplified audio signal is further amplified by a power amplifier to get a specified power level so that it may activate the loudspeaker. The loudspeaker is a transducer that converts this audio electrical signal into an audio sound signal and thus the original signal is reproduced i.e. the original transmission is received.

Characteristics of Superheterodyne 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:

1. The gain of the IF amplifier.
2. The gain of the RF amplifier.
3. 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 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.

Advantages of Superheterodyne Receiver

• The advantages of the superheterodyne receiver make it the most suitable for the majority of radio receiver applications like AM, FM, communications, single-sideband, television, and even radar receivers; all use the superheterodyne principle. This means that it can be considered today’s standard form of a radio receiver.

1. No variation in bandwidth. The BW remains constant over the entire operating range.
2. High sensitivity and selectivity.
3. High adjacent channel rejection.

Frequency Parameters of AM Superheterodyne Receiver

• The AM receiver has the following frequency parameters:

• Two frequency bands: Medium wave (MW) band and short wave (SW) band
• RF carrier range (MW) range: 535 kHz to 1650 kHz (SW band) : 5 to 15MHz
• Intermediate frequency IF: 455kHz
• IF bandwidth B: 10 kHz

Frequently Asked Questions on Superheterodyne Receiver