UNIT-III AM & FM RECEIVERS

Learn the theory, components, and operation of AM transmitters, receivers, and superheterodyne receivers in analog communication. Understand the functions and classifications of radio receivers.

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am transmitter
superheterodyne receiver
rf amplifier
modulation
communication receiver
fm receiver
signal rejection
demodulation

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UNIT-III AM & FM RECEIVERS

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UNIT-III AM & FM RECEIVERS Subject:-Analog Communication By Shweta Suryawanshi.
Objectives • Draw a simplified block diagram of an AM tuned radio frequency receiver. • Explain the theory of superheterodyne. • Draw & identify the parts of a simple superherterodyne block diagram. • Know the reasons for including an RF amp in a receiver. • Define the term selectivity, sensitivity, Image frequency • Understand the theory and operation of a communication receiver. • Draw a simplified block diagram of an FM receiver.
AM Transmitter • AM transmitters are used in medium wave (MW) and short wave (SW) frequency bands for AM broadcast. The two types of AM transmitters that are used based on their transmitting powers are: • High Level • Low Level • Choice between the two modulation schemes depends on the transmitting power of the AM transmitter. In broadcast transmitters, where the transmitting power may be of the order of kilowatts, high level modulation is employed. In low power transmitters, where only a few watts of transmitting power are required , low level modulation is used.
High Level Transmitter Figure: High Level Transmitter
High Level Transmitter contd.. Carrier oscillator • Generates the carrier signal in the RF range. • The carrier oscillator generates a sub multiple of the required carrier frequency which is then multiplied by the frequency multiplier stage to get the required carrier frequency. • This is done to achieve frequency stability
High Level Transmitter contd.. Buffer Amplifier • The purpose of the buffer amplifier is two fold. It first matches the output impedance of the carrier oscillator with the input impedance of the frequency multiplier, the next stage of the carrier oscillator. It then isolates the carrier oscillator and frequency multiplier. • This is required so that the multiplier does not draw a large current from the carrier oscillator. If this occurs, the frequency of the carrier oscillator will not remain stable.
High Level Transmitter contd.. Frequency Multiplier • This stage is also known as harmonic generator. • It generates higher harmonics of carrier oscillator frequency. • The frequency multiplier is a tuned circuit that can be tuned to get the requisite carrier frequency that is to be transmitted.
High Level Transmitter contd.. Power Amplifier • The power of the carrier signal is then amplified in the power amplifier stage. A class C power amplifier gives high power current pulses of the carrier signal at its output. Audio Chain • The audio signal to be transmitted is obtained from the microphone, as shown in figure. The audio driver amplifier amplifies the voltage of this signal. A class A or a class B power amplifier amplifies the power of the audio signal.
High Level Transmitter contd.. Modulated Class C Amplifier • This is the output stage of the transmitter. The modulating audio signal and the carrier signal, after power amplification, are applied to this modulating stage. • Modulation takes place at this stage. The class C amplifier also amplifies the power of the AM signal to the required transmitting power.
Low Level Transmitter Figure: Low Level Transmitter
Low Level Transmitter contd.. • It is similar to a high-level transmitter, except that the power of the carrier and audio signals are not amplified. These two signals are directly applied to the modulated class C power amplifier. • Modulation takes place at this stage, and the power of the modulated signal is amplified to the required transmitting power level. • The transmitting antenna then transmits the signal.
RECEIVERS Receiver is an electronic equipment which pick ups the desired signal, reject the unwanted signal and demodulate the carrier signal to get back the original modulating signal.
Function of Receivers • Intercept the incoming modulated signal • Select desired signal and reject unwanted signals • Amplify selected R.F signal • Detect modulated signal to get back original modulating signal • Amplify modulating frequency signal
Classification of Radio Receivers Depending upon application • AM Receivers - receive broadcast of speech or music from AM transmitters which operate on long wave, medium wave or short wave bands. • FM Receivers – receive broadcast programs from FM transmitters which operate in VHF or UHF bands.
Communication Receivers - used for reception of telegraph and short wave telephone signals. Television Receivers - used to receive television broadcast in VHF or UHF bands. Radar Receivers – used to receive radio detection and ranging signals.
Depending upon fundamental aspects Tuned Radio Frequency (TRF)Receivers Super-heterodyne Receivers
Tuned Radio Frequency (TRF) Receiver: Composed of RF amplifiers and detectors. No frequency conversion It is not often used. Difficult to design tunable RF stages. Difficult to obtain high gain RF amplifiers Super-hetrodyne Receiver Downconvert RF signal to lower IF frequency Main amplifixcation takes place at IF Communication Receiver Downconvert RF signal to two IF frequency RECEIVER TYPES
TRF (Tuned Radio frequency) RECEIVER Speaker Power amplifier 2nd RF Amplifier Detector 1st RF Amplifier Audio amplifier Ganged Tuning

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