1B Transmitter and Receiver PDF

Transmitter Why Transmitter/Modulation is needed? The Problem Spectrum of Natural Speech Speech, music and data signal contain low frequency components. Transmitting low frequency signals using radio waves requires very very long antenna. ...

Transmitter Why Transmitter/Modulation is needed? The Problem Spectrum of Natural Speech Speech, music and data signal contain low frequency components. Transmitting low frequency signals using radio waves requires very very long antenna.  How to reduce antenna length? Ans: Shift the spectrum to a higher frequency band. Signal with low Short antenna freq components High frequency Modulator e.g. speech, music, data The process of shifting the frequency components from a low band to a higher frequency band is known as modulation.  In modulation, a high frequency sinewave, called the carrier is used in the frequency shifting. Inside the modulator, the low frequency signal modifies certain characteristics (amplitude, frequency or phase) of the carrier. The modified carrier is a high frequency signal, called the Modulated carrier or Modulated signal. Shifting low to high frequency using Amplitude Modulation (AM) Spectrum of the AM signal V(f) kHz fc - fH fc + fH kHz 995 1005 fH 5 Modulated signal Modulator Modulating signal Modulated carrier (low frequency) (high frequency) All the frequency components in the AM signal are at high Carrier frequency. Vp fc >> fH kHz fc 1000  AM Modulation and Demodulation V(f) V(f) kHz kHz kHz fH fc - fH fc + fH fH 5 5 995 1005 Modulator Demodulator A B Transmitter Receiver At the Transmitter, the modulator shifts the low frequency to a higher frequency band. At the Receiver, the demodulator shifts the high frequency back to low. The spectrum at point B is the same as the spectrum at point A  Radio Frequency (RF) Bands The carriers used in modulation are high frequency sinewaves. They range from 30 kHz to 300 GHz. This range of frequency is called the Radio Frequency (RF) band. Because the RF band is very wide, the frequencies are sub- divided into smaller bands. Each sub-band is named based on their frequency values.  Radio Frequency (RF) Bands Low Frequency (LF) 30kHz – 300kHz Medium Frequency (MF) 300 kHz – 3 MHz High Frequency (HF) 3 MHz – 30 MHz RF band Very High Frequency (VHF) 30 MHz – 300 MHz 300 MHz – 3 GHz Ultra High Frequency (UHF) 3 GHz – 30 GHz Super High Frequency (SHF) 30 GHz – 300 GHz Extremely High Frequency (EHF) In some cases, the sub-band is also named based on their wavelength. Low Frequency (LF) Long Wave (LW) 30kHz – 300kHz Medium Frequency (MF) Medium Wave (MW) 300 kHz – 3 MHz High Frequency (HF) 3 MHz – 30 MHz Short Wave (SW) RF band Very High Frequency (VHF) 30 MHz – 300 MHz 300 MHz – 3 GHz Ultra High Frequency (UHF) 3 GHz – 30 GHz Super High Frequency (SHF) 30 GHz – 300 GHz Extremely High Frequency (EHF)  Wavelength, λ Let v(prop) = velocity of propagation v(t) t Wavelength, λ = Distance travelled by the radio wave in T seconds (one cycle of the signal). Distance = velocity x time v(prop) Hence, λ = v(prop) x T metres λ= f Max v(prop) = c =speed of light = 3 x 108 m/s occurs in free space. v(prop) on earth is slower.  Wavelength, λ Example: v(prop) = velocity of f = 20MHz propagation v(t) t Assume v(prop) = c = speed of light, for convenience v(prop) 3x108 λ= = 15m λ= f 20x10 6  Power Amplifier The low output power from the modulator is increased by the Power Amplifier. Class C Amplifiers are normally used as Power Amplifier because its efficiency is higher than Class A, B and Class AB amplifiers.  Power Amplifier efficiency, η A class C power amplifier draws 2.5A from a 24V power supply. The RF output power is 48W. a) How efficient is the amplifier? b) How much power is dissipated as heat in the transistor? 2.5A 24V 48W  Power Amplifier efficiency, η a) How efficient is the amplifier? PDC = VDC IDC = 24V x 2.5A = 60 W η = ( Po / PDC) x 100 % η = ( 48 /60 ) x 100 % η = 80% b) How much power is dissipated as heat in the transistor? Heat dissipated in transistor = PDC – Po = 60W – 48W = 12W  Power Amplifier The transistors in this Power Amplifier are mounted on heat sink to dissipate the heat. The heat sink has fins to increase the surface area. Filter  FILTERS A filter is a frequency-selective circuit. The behaviour of a filter is described by its frequency response. To measure the frequency response, a sinewave is pumped into the input of the filter. V(t) V(t) Filter t Circuit t vi vo The Voltage gain, Av = Vo/Vi is computed for each input frequency. V(t) V(t) Filter t Circuit t vi vo Voltage gain, Av = Vo/Vi The frequency response of the filter is a plot of voltage gain at various input frequency. AV frequency of input sinusoidal signal Frequency Response of Ideal Low Pass Filter (LPF) Passes input frequencies between 0 and fc Voltage Av=Vo/Vi=0  Vo=AvVi =0 gain (AV) Nothing comes out Passband Stopband frequency of 0 fC input sinusoidal signal cut-off frequency  Frequency Response of Ideal High Pass Filter (HPF) Voltage gain AV Stopband Passband Frequency of input sinusoidal signal 0 fC cut-off frequency  Frequency Response of Ideal Band Pass Filter (BPF) Voltage gain AV Stopband Passband Passband Stopband 0 fL fU Frequency of input sinusoidal signal Lower cut-off Upper cut-off frequency frequency  Example A 1kHz square wave with zero dc level is passed through the following filters. Sketch the spectrum of the output signal. a) Ideal LPF, cut-off frequency, fc = 4 kHz. Input waveform Input spectrum vi(t) Vp 0 t 0 1 3 5 7 …. f (kHz)  Low pass filter, cut-off freq = 4kHz Input Spectrum Output Spectrum (a) Vp Freq response Vp f (kHz) 0 1 3 4 5 7 …. f (kHz) 0 1 3 5 7  Example A 1kHz square wave with zero dc level is passed through the following filters. Sketch the spectrum of the output signal. b) Band pass filter, cut off freqs = 2kHz & 6kHz (c) Vp Freq response Vp 0 1 23 5 6 7 …. f (kHz) 0 1 3 5 7 f (kHz)  Practical Filters AV Ideal Filters Sharp transition from passband to stopband. Nice to have but unfortunately cannot get. Passband Stopband f fc AV Practical Filters Gradual transition from passband to stopband f  Bandwidth of BPF = width of passband = fu - fL Lower cut-off freq Upper cut-off freq The centre frequency of a BPF is the frequency at which the voltage gain is maximum. AV f fcentre If the gain in the passband is constant, the centre frequency of a BPF is in the middle of the passband. fcentre Receiver  Function of Radio Receivers 1. Pick up the radio wave. This job is performed by the receiving antenna. Electromagnetic Induction 30  Function of Radio Receivers 2. Tuning This is done by a BPF. Frequency response of BPF BPF 1000 kHz 970 1000 1030 kHz Wanted Station The BPF in radio receivers is usually implemented using parallel LC circuits known as tuned circuits. 31  Function of Radio Receivers 3. Demodulation BPF Demodulator Extracting the original information-bearing signal from a carrier wave 32  Function of Radio Receivers 4. Amplify the demodulated signal BPF Demodulator Amplifier 33  Tuned Radio Frequency (TRF) Receivers BPF Demodulator Amplifier TRF stands for Tuned Radio Frequency. Besides tuning, the BPF also amplifies the signal. Hence the BPF is often called a TUNED AMPLIFIER. 34  TRF Receivers The TRF receiver is simple but it has 2 problems. 1st problem: – The bandwidth of BPF is not constant. It increases with centre frequency. 35  BPF Bandwidth of BPF in receiver must be constant. However, in all BPF, bandwidth increases with centre frequency. Sample Experiment results. Centre frequency of BPF Bandwidth of BPF 600 kHz 5 kHz 1000 kHz 20 kHz 1500 kHz 40 kHz 36  TRF Receivers 2nd problem: The demodulator has to operate at high frequency. 30 MHz BPF Demodulator 30 MHz Wanted Station The demodulator needs to operate at the same frequency as the wanted station. If the input frequency to the demodulator can be lowered, the demodulator circuit will be easier to design. 37  Superhet Receivers The SUPERHET receiver solves the 2 problems of TRF receivers What are we trying to achieve? Reduce input frequency to demodulator. Maintain bandwidth of receiver constant. Reduce input frequency to demodulator - The Superhet Receiver MIXER is added to reduce the input frequency to demodulator. TRF Receiver BPF Demodulator Amplifier  Multiplication Principle 1: The multiplication of 2 cosine signals produces sum and difference frequency components. f1 Vp vp f2 f vp 10MHz 1MHz 19MHz f f 9MHz It is possible to produce a lower frequency using multiplication. 40 How does the MIXER work? Wanted Station 41 The MIXER shifts the high frequency at point A to a lower frequency at point D. Hence, the demodulator need not operate at high frequency. 42 The MIXER shifts the high frequency at point A to a lower frequency at point D. This lower frequency is called the Intermediate Frequency (IF). 43  The Superhet Receiver What are we trying to achieve? Reduce input frequency to demodulator. Solved using MIXER Maintain bandwidth of receiver constant. Next to solve 44  (recall) BPF In all BPF, bandwidth increases with centre frequency. Sample Experiment results. Centre frequency of BPF Bandwidth of BPF 600 kHz 5 kHz 1000 kHz 20 kHz 1500 kHz 40 kHz 45  The Superhet Receiver constant IF  centre frequency of IF BPF constant  bandwidth of IF BPF constant  only one station comes out  problem solved! 46  How does the Superhet Receiver maintain IF constant? By changing fLO when we tune the RF BPF to another station such that IF = fLO – fc is always constant. Changing fLO when we tune the RF BPF to another station is called GANGED TUNING. 47  Example – Ganged Tuning IF = 455 kHz (chosen by designer) fc fLO IF 660 kHz 600 + 455 = 1115kHz 455 kHz 775 kHz 775 + 455 = 1230kHz 455 kHz 1050 kHz 1050 + 455 = 1505kHz 455 kHz fLO - fc = IF = 455kHz, constant fLO = fc + IF = fc + 455kHz 48 Amplitude Modulation And Frequency Modulation Amplitude Modulation and Frequency Modulation Two common types of modulation : ‒ Amplitude Modulation (AM) ‒ Frequency Modulation (FM) Amplitude Modulation (AM) In AM, the amplitude or height of the carrier wave is changed Occurs when a voice signal's varying voltage is applied to a carrier frequency The carrier's frequency does not change. Frequency Modulation (FM) In FM, the frequency of the carrier is changed The transmitter’s sine wave frequency changes based on the information signal