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AM receiver is an electronic system designed to capture radio frequency (RF)
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- 2. AM Receivers • Introductionto AM Receivers • An Amplitude Modulation (AM) receiver is an electronic system that: • Captures radio frequency (RF) signals transmitted through space. • Selects the desired station among many. • Amplifies the weak received signals. • Demodulates the carrier wave to recover the original information (audio). • Converts the electrical audio signal to sound. • The process involves tuning, amplification, detection, and audio reproduction.
- 3. AM Receivers Function Description Reception Theantenna intercepts the electromagnetic wave and converts it into an electrical signal. Selection Tuning circuits select the desired frequency while rejecting others. Amplification Multiple amplifier stages increase signal strength. Detection Demodulation process extracts the audio information from the carrier. Reproduction Audio amplifiers and loudspeakers convert it into audible sound. Main Functions of a Receiver
- 4. AM Receivers • Classificationof AM Receivers • AM receivers are classified into two major types based on their architecture: 1.Tuned Radio Frequency (TRF) Receiver 2.Superheterodyne Receiver
- 5. Tuned Radio FrequencyReceiver • Tuned Radio Frequency (TRF) Receiver • The TRF receiver directly amplifies the incoming modulated RF signal using multiple tuned RF amplifier stages before demodulation. • Each stage is tuned to the carrier frequency of the desired station, so the receiver amplifies only that frequency and its sidebands.
- 6. Tuned Radio FrequencyReceiver • Block Diagram • Each RF amplifier has a tuned LC circuit that must resonate at the desired frequency.
- 7. Tuned Radio FrequencyReceiver • Circuit Description 1.Antenna & RF Stage 1. The antenna picks up many AM signals simultaneously. 2. The first LC tuned circuit selects one carrier frequency fc using the resonance condition: 3. The RF amplifier (usually transistor-based) boosts this signal. 2.Tuned RF Amplifier Stages 1. Additional stages provide higher gain and better selectivity. 2. Each has its own LC circuit; they are ganged together so they track the same frequency when tuning.
- 8. Tuned Radio FrequencyReceiver • Detector Stage • The amplified RF is demodulated using a diode envelope detector, producing the original modulating signal ( ). 𝑚 𝑡 • Audio Amplifier & Speaker • The recovered audio signal is amplified to drive the speaker
- 9. Tuned Radio FrequencyReceiver • Frequency Response • Each tuned circuit has a bandwidth BBB determined by: where Q is the quality factor. • For AM signals, the bandwidth must be at least twice the maximum modulating frequency (e.g. 2×5 kHz=10 kHz). • However, in TRF receivers, Q varies with frequency, so selectivity changes as you tune across the band.
- 10. Tuned Radio FrequencyReceiver • Limitations i 1.Tracking Problem: Each LC circuit must tune to exactly the same frequency; small differences cause distortion or weak reception. 2.Variable Bandwidth: The bandwidth increases with frequency, causing poor selectivity at high frequencies. 3.Instability: At high RF gains, feedback between stages may cause oscillations (regeneration). 4.Nonuniform Sensitivity: Amplification changes with frequency due to inconsistent tuning. 5.Difficult to Tune: Multi-stage tuning requires mechanically linked capacitors that must track precisely.
- 11. Tuned Radio FrequencyReceiver • Physical Meaning of Q • A high Q means: • Narrow bandwidth • Sharp resonance peak • High selectivity • Low energy loss • A low Q means: • Broad resonance • Poor selectivity • Higher losses
- 12. Tuned Radio FrequencyReceiver • Advantages • Simple design and easy to understand. • Fewer components, lower cost. • Works for strong local signals. • No frequency conversion stage needed.
- 13. Superheterodyne receiver • Thesuperheterodyne receiver overcomes TRF limitations by converting the incoming RF signal to a fixed lower frequency, called the Intermediate Frequency (IF). • At the IF stage: • Amplifiers and filters can be precisely designed for high gain and narrow bandwidth. • Selectivity and sensitivity become independent of the tuned station frequency.
- 14. Superheterodyne receiver • Principleof Heterodyning • The process of mixing two frequencies in a nonlinear device to produce sum and difference frequencies is called heterodyning. • If: • Then, the mixer output contains: • The difference frequency is selected as intermediate frequency fIFis and amplified.
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- 16. Superheterodyne receiver • 1.RF Amplifier and RF filter • Amplifies the selected signal and reduces noise. • Provides partial image rejection. • Uses a tuned LC filter to select the station. • 2. Local Oscillator (LO) • Generates a frequency that tracks the tuned signal so that:
- 17. Superheterodyne receiver • 3.Mixer • Nonlinear device (e.g., transistor or diode). • Produces fsumand fdiff . • IF filter selects fIF= f ∣ LO−fs . ∣ • 4. IF Amplifier • Provides high, stable gain at fixed fIF(typically 455 kHz). • Defines receiver selectivity and bandwidth. • Usually consists of multiple tuned stages with high Q.
- 18. Superheterodyne receiver • 5.Detector (Demodulator) • Recovers the modulating audio signal using envelope detection: • 6. Audio Amplifier • Amplifies recovered audio to drive the speaker. • May include tone and volume controls.
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- 20. Superheterodyne receiver • Selectivityand Sensitivity • Selectivity: Ability to reject adjacent channels. Determined by the IF filter bandwidth. • Sensitivity: Minimum input signal required for satisfactory output (typically 1–5 µV). Depends on the gain and noise figure of the RF and IF amplifiers.
- 21. Superheterodyne receiver • AutomaticGain Control (AGC) • When the received signal strength changes, the output volume would fluctuate. AGC applies a DC bias voltage (derived from detector output) to control the gain of earlier amplifier stages automatically. • AGC ensures: • Constant output level. • Protection from overload by strong sign
- 22. Superheterodyne receiver • DoubleConversion Receiver • Used in advanced communication receivers: • Two IF stages: a high first IF for good image rejection and a low second IF for selectivity. • Example: • fIF1=10.7 MHz, fIF2=455
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- 24. Superheterodyne receiver Advantage Explanation Highselectivity Fixed IF allows precise filtering High sensitivity Large stable gain at IF Stable operation Only one tuning adjustment Constant bandwidth Independent of received frequency Ease of demodulation Low fixed IF simplifies design Advantages
- 25. Superheterodyne receiver • Disadvantages ImageFrequency Interference — requires good RF selectivity. Complexity — more stages and alignment. Frequency Drift — local oscillator must remain stable. Higher cost than TRF.
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- 29. Superheterodyne receiver The image(22.8 MHz) is far from the desired signal (1.4 MHz), so a simple RF filter can easily reject it.
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- 31. Superheterodyne receiver • Example— Find the resonant frequency, Q and bandwidth for LC tuned circuit: Given: =100 H , =100 pF, Series resistance (coil 𝐿 𝜇 𝐶 loss) =2 𝑅 Ω . Quality factor Q for a series RLC:
- 32. Superheterodyne receiver The samecoil in the last example has effective resistance that increases with frequency. Suppose measured effective resistance is:
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