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CHAPTER 2

CHAPTER 2. Amplitude Modulation 2.2 AM RECEIVERS. OBJECTIVES. To define AM demodulation To define and describe the receiver parameters To describe the operation of a tuned radio frequency (TRF) receiver To describe the operation of a superheterodyne receiver. LECTURE OVERVIEW.

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CHAPTER 2

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  1. CHAPTER 2 Amplitude Modulation 2.2 AM RECEIVERS

  2. OBJECTIVES • To define AM demodulation • To define and describe the receiver parameters • To describe the operation of a tuned radio frequency (TRF) receiver • To describe the operation of a superheterodyne receiver

  3. LECTURE OVERVIEW • Demodulation • Receiver parameters • Tuned radio frequency (TRF) receiver • Superheterodyne receiver

  4. Introduction • AM demodulation – reverse process of AM modulation. • Demodulator: converts a received modulated-wave back to the original source information. • Basic understanding of the terminology commonly used to describe radio receivers & their characteristics is needed to understand demodulation process

  5. Simplified block diagram of an AM receiver

  6. Receiver Parameters • Selectivity • Bandwidth improvement • Sensitivity • Dynamic range • Fidelity • Insertion Loss • Noise temperature & Equivalent noise temperature

  7. Selectivity • Used to measure the ability of the receiver to accept a given band of frequencies and reject all others. • Way to describe selectivity is to simply give the bandwidth of the receiver at the -3dB points. • Not necessarily a good means of determining how well the receiver will reject unwanted frequencies.

  8. Cont’d… • Give the receiver bandwidth at two levels of attenuation. Eg: -3dB, -60dB • The ratio of two BW ~ Shape factor SF = B(-60 dB) / B(- 3dB) Where SF – Shape factor B(-60dB) – BW 60dB below max signal level B(-3dB) – BW 3dB below max signal level

  9. Cont’d… • If both BW equal, the shape factor would be 1. • Impossible to achieve in practical circuit • Example application for SF nearly 1 • Satellite • Microwave • Two way radio Rx

  10. Bandwidth Improvement • Thermal noise directly proportional to bandwidth. • Reduce BW ~ reduce noise, improving system performance. • Reducing BW = improving the noise figure of the RX

  11. Cont’d… Bandwidth Improvement, BI BI = BRF /BIF Where BRF = RF Bandwidth (Hz) BIF = IF Bandwidth (Hz) Noise figure improvement, NF = 10 log BI

  12. Sensitivity • The minimum RF signal level that can be detected at the input to the Rx and still produce a usable demodulated information signal. • Usually stated in micro volts of received signal. • Rx sensitivity also called Rx threshold.

  13. Cont’d… • Depends on: • The noise power present at the input to the Rx. • Rx noise figure. • AM detector sensitivity. • BI factor of the Rx • To improve ~ reduce the noise level • Reducing the temperature or Rx BW or RX noise figure

  14. Dynamic range • The difference (in dB) between the minimum input level necessary to discern a signal and the input level that will overdrive the Rx and produce distortion. • Input power range over which the Rx is useful.

  15. Cont’d… • A dynamic range of 100dB is considered about the highest possible. • A low dynamic range can cause a desensitizing of the RF amplifiers and result in severe intermodulation distortion of the weaker input signal.

  16. Fidelity • A measure of the ability of a communication system to produce (at the output of the Rx) an exact replica of the original source information.

  17. Cont’d… • Forms of distortion that can deteriorate the fidelity of a communication system:- • Amplitude • Frequency • Phase

  18. Noise Temperature & Equivalent noise Temperature • Thermal noise directly proportional to temperature ~ can be expressed in degrees, watts or volts. • Environmental temperature, T (kelvin) T = N/KB Where N = noise power (watts) K = Boltzman’s Constant (1.38 X 10-23 J/K) B = Bandwidth (Hz)

  19. Cont’d… • Equivalent noise temperature, (Te) Te = T(F-1) Where T = environmental temperature (kelvin) F = Noise factor • Te often used in low noise, sophisticated radio receivers rather than noise figure.

  20. Insertion loss • IL is a parameter associated with the frequencies that fall within the passband of a filter. • The ratio of the power transferred to a load with a filter in the circuitto the power transferred to a load without the filter. IL (dB) = 10 log (Pout /Pin)

  21. AM RECEIVERS • Two basic types of radio receivers. • Coherent • Synchronous receivers • The frequencies generated in the Rx & used for demodulation are synchronized to oscillator frequencies generated in Tx. • Non-coherent • Asynchronous receivers • Either no frequencies are generated in the Rx or the frequencies used for demodulation completely independent from the Tx’s carrier frequency. • Non-coherent detection = envelope detection.

  22. Coherent demodulation SSB X LPF cos wct COHERENT • EXAMPLE OF COHERENT DEMODULATION: SSB • The received signal is heterodyned /mixed with a local carrier signal which is synchronous (coherent) with the carrier used at the transmitting end.

  23. Non-Coherent Rx • Tuned Radio Frequency Rx • Superheterodyne Rx

  24. Non-coherent tuned radio frequency receiver (TRF Rx)

  25. Cont’D • RF amplifier - to filter and amplify the received signal to a level sufficient to drive the detector • Audio detector - converts RF signals directly to information • Audio stage – amplifies the information signals to a usable level • Advantages – simple and have relatively high sensitivity

  26. DisAdvantages of trf • Bandwidth is inconsistent and varies with center frequency when tuned over a wide range of input frequencies • This is caused by a phenomenon called the skin effect • Skin effect phenomenon: B = f/Q Where Q is quality factor.

  27. Cont’d… • Instability due to large number of RF amplifiers all tuned to the same center frequency. • Can be reduced by tuning each amplifier to a slightly different frequency, slightly above or below the desired center frequency. • Their gains are not uniform over a very wide frequency range because of the non-uniform L/C ratios of the transformer-coupled tank circuits in the RF amplifiers

  28. AM superheterodynereceiver

  29. Cont’d… • RF section: • Preselector is use to provide enough initial bandlimiting to prevent a specific unwanted radio frequency called the image frequency from entering the receiver • Preselector also reduces the noise bandwidth of the receiver • RF amplifier determines the sensitivity of the receiver

  30. CONt’d… • Mixer/ converter section: • Is a nonlinear device and its purpose is to convert radio frequencies to intermediate frequencies (RF-to-IF translation) • IF section: • Most of the receiver gain and selectivity is achieved in the IF section • IF is always lower in frequency than the RF because it is easier and less expensive to construct high-gain, stable amplifiers for low-frequency signals.

  31. Cont’d… • Detector section: • To convert the IF signals back to the original source information • Audio amplifier section: • Comprises several cascaded audio amplifiers and one or more speakers

  32. Frequency conversion • High side injection, flo = fRF + fIF • Low side injection flo = fRF - fIf

  33. INTERMEDIATE FREQUENCY (IF) • Mixers generate signals that are the sum and difference of the incoming signal frequency (fS) and the frequency of the local oscillator (fLO). • The difference frequency is more commonly chosen as the IF. • Some receivers use the sum frequency for the IF.

  34. IMAGES • An image (fIM) is an undesired signal that is separated from the desired signal frequency (frf) by two times the IF (fIF). • fI = frf + 2fIF or frf - 2fIF • Images interfere with the desired signal. • Images can be eliminated or minimized by: • Proper selectionof the IF in design. • Use of highly selective filters before the mixer. • Use of a dual conversion receiver.

  35. con’t’d… • Image frequency fim = fRF + 2fIF • Image Frequency rejection ratio IFRR = √ (1 + Q²ρ²) Where ρ = (fim/fRF) –(fRF/fim)

  36. Example For a citizens band receiver using high-side injection with an RF carrier of 27 MHz and an IF center frequency of 455 kHz, determine a. Local oscillator frequency b. Image frequency c. IFRR for a preselector Q of 100

  37. AM APPLICATION • AM Radio broadcasting • Commercial AM radio broadcasting utilizes the frequency band 535 – 1605 kHz for transmission voice and music. • Carrier frequency allocation range, 540-1600 kHz with 10 kHz spacing.

  38. Cont’d… • Radio stations employ conventional AM for signal transmission – to reduce the cost of implementing the Rx. • Used superheterodyne Rx. • Every AM radio signal is converted to a common IF frequency of fIF = 455 kHz.

  39. AT THE END OF THIS CHAPTER, YOU SHOULD BE ABLE • To define AM demodulation • To define and describe the receiver parameters • To describe the operation of a tuned radio frequency (TRF) receiver • To describe the operation of a superheterodyne receiver

  40. END OF CHAPTER 2.2 : AM RECEIVER

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