AM Radio Receivers, Demodulation, DSBSC and SSB 18년 5월 25일 21시 36분 24초 AM Radio Receivers, Demodulation, DSBSC and SSB Dr. Monir Hossen ECE, KUET Department of Electronics and Communication Engineering, KUET © 2007 Microsoft Corporation. All rights reserved. Microsoft, Windows, Windows Vista and other product names are or may be registered trademarks and/or trademarks in the U.S. and/or other countries. The information herein is for informational purposes only and represents the current view of Microsoft Corporation as of the date of this presentation. Because Microsoft must respond to changing market conditions, it should not be interpreted to be a commitment on the part of Microsoft, and Microsoft cannot guarantee the accuracy of any information provided after the date of this presentation. MICROSOFT MAKES NO WARRANTIES, EXPRESS, IMPLIED OR STATUTORY, AS TO THE INFORMATION IN THIS PRESENTATION.

Department of Electronics and Communication Engineering, KUET 2 AM Radio Receivers The power of the received signal received by the radio receiver (RR) is usually in picowatts. Important terms of RR: Sensitivity => It is the ability of a RR to amplify weak signals. Selectivity => It is the ability of a RR to reject unwanted signals. Types of RR: Tuned radio frequency receivers Superhetrodyne receivers Department of Electronics and Communication Engineering, KUET 2

Tuned Radio Frequency Receivers Antenna RF Amp. 1st Stage 2nd Stage RF Amp. Detector Audio Amp. Power Amp. Limitations: Not suitable for above 1650KHz Instable Insufficient selectivity Bandwidth variations Due to these limitations this receiver is not used and have been replaced by Superhetrodyne receivers. Department of Electronics and Communication Engineering, KUET 3

Superhetrodyne Receivers (Superhet) All modern radio receivers (RRs) are superhetrodyne It utilize mixing (hetrodying) of two frequencies It is the most superior RR circuit Selector CKT f1 f2 – f1 Mixer Output f2 + f1 Local Osc. f2 Department of Electronics and Communication Engineering, KUET 4

Superhetrodyne Receivers (Superhet) Suppose f1 = 1000KHz (From antenna/selector) LO frequency f2 = 1455KHz Output of Mixer are: f2 – f1 =1455-1000 = 455KHz f2 + f1 = 1455+1000 = 2455KHz Here, output 455KHz is called intermediate frequency (IF). IF of AM is 455KHz and it is nationally or internationally accepted. Department of Electronics and Communication Engineering, KUET 5

Block Diagram of a Superhetrodyne Receiver Antenna RF Amp. f2 – f1 = 455 KHz f1 µv Mixer mv IF Amp. Detector Audio Amp. Power Amp. Shaft of tuner LO 20 – 20,000 Hz AGC f2 AGC is an electronic device. It changes the gain of a RR automatically with the changing of strength of the signal so that the output remains constant. Department of Electronics and Communication Engineering, KUET 6

Demodulation or Detection The process of separating original signal from the modulated wave is called demodulation or detection. Functions of Detector: It rectify the radio wave, i.e., eliminates the negative ½ cycle of the wave. The positive ½ cycle is passed through the LP filter ckt which suppresses the HF carrier. Classification of Detection Methods: Square law diode detectors Linear diode detectors Department of Electronics and Communication Engineering, KUET 7

Square Law Diode Detector Significant (Enlarge part) va Compressed part It utilizes the non-linear portion of the dynamic v-i characteristics of a diode. Department of Electronics and Communication Engineering, KUET 8

Square Law Diode Detector The dynamic current characteristic obeys approximately the square-law relation: Where, ia is ac anode current and anode voltage Frequency contains in 1st term, The 2nd term contains, The high frequency (RF) terms are bypassed through the shunt capacitor. Department of Electronics and Communication Engineering, KUET 9

Output voltage without filter Linear Diode Detector Amplitude t t Amplitude Amplitude t va ia Amplitude t Output voltage without filter This detector is extremely popular in commercial receivers. It utilizes the linear region of the dynamic v-i characteristic of the diode (rectification char.) Department of Electronics and Communication Engineering, KUET 10

Double Side Band Suppressed Carrier (DSBSC) Modulation A DSBSC modulated wave consists simply the product of the message signal & carrier signal: A device that used to achieve this requirement is called product modulator The balanced modulator The ring modulator DSBSC modulated wave Carrier signal Message signal Department of Electronics and Communication Engineering, KUET 11

The Balanced Modulator (1/2) It consists of two standard amplitude modulators arranged in balanced configuration to suppress carrier. Two modulators are identical except for the sign reversal of message signal. Amplitude Modulator Oscillator ∑ S1(t) S2(t) S(t) m(t) - m(t) Accos(2πfct) + - Department of Electronics and Communication Engineering, KUET 12

The Balanced Modulator (2/2) Output and From (1)-(2) we have: So the balanced modulator output is equal to the product of the message signal and the carrier Department of Electronics and Communication Engineering, KUET 13

The Ring Modulator (1/2) It is a popular and widely used modulator. It is also called lattice or double balanced modulator. The lattice modulator consists of an input transformer, an output transformer and four diodes connected in a bridge circuit. The carrier signal is applied to the center taps of the input and output transformers. The modulating signal is applied to the input transformer. The output appears across the output transformer. The carrier sine wave is considerably higher in frequency and amplitude than the modulating signal. Department of Electronics and Communication Engineering, KUET 14

The Ring Modulator (2/2) The carrier sine wave is used as a source of forward and reverse bias for the diodes The carrier turns the diodes off and on at a high rate of speed. The diodes act like switches that connect the modulating signal at the secondary of T1 to the primary of T2. When the carrier is positive the outer diodes are on. When the carrier is negative the inner diodes are on. Department of Electronics and Communication Engineering, KUET 15

Coherent Detection of DSBSC Modulated Waves (1/2) The modulating signal m(t) is received from DSBSC wave s(t) is = s(t)*local oscillator (LO) wave + LPF. Assume that LO output is exactly coherent or synchronized in both frequency and phase. But, in general demodulation process using LO signal of the same frequency with arbitrary phase difference φ. Amplitude Modulator Local Oscillator LPF v(t) vo(t) s(t) cos(2πfct+φ) Department of Electronics and Communication Engineering, KUET 16

Coherent Detection of DSBSC Modulated Waves (2/2) Unwanted This unwanted signal is removed by LPF The overall output vo(t) is :- Department of Electronics and Communication Engineering, KUET 17

Quadrature Carrier Multiplexing (1/2) A quadrature carrier multiplexing or Quadrature amplitude Modulation (QAM) scheme enables two DSBSC modulated waves to occupy the same transmission bandwidth. Product Modulator Amplitude Modulator -900 phase shifter ∑ Multiplexed signal s(t) m1(t) m2(t) Accos(2πfct) + QAM Transmitter Department of Electronics and Communication Engineering, KUET 18

Quadrature Carrier Multiplexing (2/2) Here, two product modulator supplied with two carrier waves of the same frequency but differing in phase by -900 Sum of this two product modulator output is:- Product Modulator -900 phase shifter Multiplexed signal s(t) cos(2πfct) QAM Receiver LPF Department of Electronics and Communication Engineering, KUET 19

Single Side Band (SSB) Modulation We know transmission of information is concerned in only one sideband because both upper and lower sidebands are identical by virtue of their symmetry about the carrier frequency So, if both the carrier & a sideband are suppressed at the transmitter no information is lost When only one sideband is transmitted, the modulation is called SSB modulation Department of Electronics and Communication Engineering, KUET 20

Frequency Domain Description of SSB Modulation (1/2) Let us consider a message signal m(t) whose spectrum is M(f) of bandwidth -fc-W -fc -fc+W M(f) fc-W fc+W fc f DSBSC M(f) -W W M(0) Message spectrum DSBSC spectrum Department of Electronics and Communication Engineering, KUET 21

Frequency Domain Description of SSB Modulation (2/2) When only the upper sideband (USB) is transmitted: -fc-W -fc fc+W fc f SSB Spectrum When only the lower sideband (LSB) is transmitted: -fc+W -fc fc-W fc f SSB spectrum Department of Electronics and Communication Engineering, KUET 22

Advantages and Disadvantages of SSB Modulation Reduced bandwidth requirement Elimination of the high-power carrier wave Disadvantages: The cost is high Complexity of its implementation Department of Electronics and Communication Engineering, KUET 23

Frequency Discrimination Method for Generating an SSB Modulated Wave For this method, the message signal should satisfy two conditions: The message signal m(t) has little or no low frequency content. Audio signal is an example of this type of signal. Example: In telephony, the useful frequency content of a speech signal is restricted to 0.3 - 3.4 KHz. Thereby creating an energy gap from 0 to 300 Hz. 2. The highest frequency component of the message signal m(t) is much less than the carrier frequency fc . Department of Electronics and Communication Engineering, KUET 24

Block Diagram of Frequency Discrimination Method Product Modulator BPF SSB modulated wave m(t) Ac cos(2πfct) Product Modulator BPF SSB modulated wave m(t) A1 cos(2πfct) A2 cos(2πfct) Department of Electronics and Communication Engineering, KUET 25

Phase Discrimination Method for Generating an SSB Modulated Wave In-Phase path Q Phase Ac cos(2πfct) Wide band -900 phase shifter m(t) Hillbert transform Oscillator -900 phase shifter Acsin(2πfct) SSB wave ∑ A message signal m(t) and a quadrature component equal to is called the Hillbert transform of m(t) This modulator is also known as Hillbert modulator Department of Electronics and Communication Engineering, KUET 26

Demodulation of SSB Waves Product Modulator LPF SSB wave S(t) cos(2πfct) V(t) V0 (t) Scaled message signal Unwanted component Department of Electronics and Communication Engineering, KUET 27

Vestigial Sideband (VSB) Modulation VSB inherits the advantages of SSB and DSB but avoid their disadvantages at small cost It is easy to generate but BW is just 25% larger (merit / demerit?) than SSB but less than DSB Instead of rejecting one sideband (e.g., SSB) it gradually cutoff one sideband Vestigial shaping filter is used to produces VSB from DSB SSB (Upper sideband) VSB Spectrum DSB Department of Electronics and Communication Engineering, KUET 28

VSB Transceiver Transmitter Receiver Hi() m(t) LPF Ho() Transmitter Receiver e(t) Department of Electronics and Communication Engineering, KUET 29

Department of Electronics and Communication Engineering, KUET Thanks for Your Kind Attention Department of Electronics and Communication Engineering, KUET