NEW YORK CITY COLLEGE of TECHNOLOGY THE CITY UNIVERSITY OF NEW YORK DEPARTMENT OF ELECTRICAL ENGINEERING AND TELECOMMUNICATIONS TECHNOLOGIES Course : EET 24 Communications Electronics Module : AM Tx and Rx Prepared by: Dr. Djafar K. Mynbaev Spring 28 D. Mynbaev, EET 24 Module, Spring 28
Topic : AM transmission and reception. Introduction Quiz # 5 (AM) next week. AM review AM transmission: AM transmission system AM modulation Block diagram of an AM transmitter AM demodulation: AM demodulation Operation AM detector Block diagram of an AM receiver Superheterodyne receiver Key words AM transmission: AM transmission system AM modulation Block diagram of an AM transmitter AM demodulation: AM detector Block diagram of an AM receiver Superheterodyne receiver D. Mynbaev, EET 24 Module, Spring 28 2
AM transmission system: The operation of AM transmission can be presented in the following form: (Remember, we need amplitude modulation to better transmit an information signal.). 2.5 AM transmitter 5.. -5. Input: information signal -. Amplitude modulation transmitter end. 5.. -5. -. Transmission of AM signal AM receiver Amplitude demodulation receiver end 2.5 Output: information signal D. Mynbaev, EET 24 Module, Spring 28 3
Modulating (information) signal AM modulation: We want to generate a high-frequency signal that will deliver low-frequency information signal. In AM we do so by alternating the amplitude of a carrier signal: v(t) = (A C + A M cos M t) cos C t. variable part of amplitude carrier signal * Carrier signal Amplitude-modulated signal D. Mynbaev, EET 24 Module, Spring 28 4
Amplitude modulation Block diagram of an AM transmitter To implement the idea, an AM transmitter has to perform the operations described by the formula of an AM signal: v(t) = A C cos C t + A M cos M t cos Ct. (Observe the new format of this formula.) Thus, an AM transmitter should include the following modules (See []and [2]): Voice Transducer Modulator driver Modulating (information) signal Functional block diagram of an AM transmitter Modulator Amp Carrier oscillator Buffer amp Carrier signal Antenna A transducer (microphone) converts voice into electrical signal. This information signal after processing by modulator driver is presented to a modulator. On the carrier side, an oscillator generates a carrier signal that (after passing through a buffer amplifier) is also presented to the modulator. This module produces an AM signal that (after additional amplification) is broadcasted by an antenna. D. Mynbaev, EET 24 Module, Spring 28 5
AM demodulation - operation To recover the information from a received (AM!) signal, we need to demodulate this signal.. 5.?. -5. 2.5 -. from that (received AM) signal? How can we obtain this (information) signal D. Mynbaev, EET 24 Module, Spring 28 6
AM demodulation -operation. 2.5 5.. -5. -. Compare these two signals: Information signal is a positive envelope of an AM signal. Therefore, the first step in demodulation should be removing the negative half of an AM wave. What device do we need to perform this operation? D. Mynbaev, EET 24 Module, Spring 28 7
AM demodulation - operation This is a signal we need 2.5 A B G T This is a signal we ve obtained N94 D 2 AM AM Signal Input kohm R This is a signal we received 3V k Hz 2k Hz Remind the diode operation. After the diode we have obtained a signal that contains high-frequency oscillations and low-frequency information signal. What operation do we need to perform to obtain a desired signal? D. Mynbaev, EET 24 Module, Spring 28 8
AM demodulation - detector This is a signal we need 2.5 A B G T This is a signal we ve obtained N94 D 2 AM AM Signal Input kohm R 6nF C This is a signal we received 3V k Hz 2k Hz By inserting a capacitor (6 nf), we make a low-pass filter that filters out the highfrequency components. How important a capacitance value? Discuss operation of a R-C low-pass filter in terms of critical frequency: X C = R f C = /2π RC. D. Mynbaev, EET 24 Module, Spring 28 9
AM demodulation - detector 2.5 This is a signal we need This is a signal we ve obtained A B G T N94 D 2 AM AM Signal Input 3V k Hz 2k Hz kohm R 6nF C This is a signal we received A capacitor is too small (6 nf) many high-frequency components retain. D. Mynbaev, EET 24 Module, Spring 28
AM demodulation - detector This is a signal we need 2.5 G This is a signal we ve obtained A B T N94 D 2 AM AM Signal Input 3V khz 2kHz kohm R 6nF C This is a signal we received A capacitor is too large (6 nf) distortion of an information signal called diagonal clipping loss of information and poor quality (fidelity) of a sound in audio system []. A good hint for a filter designer is that f M < f CR << f C, which means that we make a filter suppress a carrier signal as much as possible and, at the same time, doesn t suppress modulating (information) signal that much. To evaluate the level of suppression, we need to use well known formula Av = / ( + (f /f C ) 2 ). D. Mynbaev, EET 24 Module, Spring 28
AM demodulation: AM receiver A B G T Output - Detected envelope AM Signal Input AM 2 N94 D 3V k Hz 2kHz kohm R A 6nF C nf Cc Out k Ohm RL Input AM signal [5], Fig. 7-3. B An AM receiver (demodulator) includes a diode (detector) that strips off the negative half wave. A resistance of a diode R and a capacitor C form a low-pass filter that removes high-frequency carrier signal. The output follows the peak variations (envelope) of an input (AM) signal. This circuit is called envelope detector. Coupling capacitor, C C and load resistor R L work as a high-pass filter that suppresses the frequencies higher than modulating frequency. D. Mynbaev, EET 24 Module, Spring 28 2
AM demodulation - detector N94 D 2 AM Signal Input AM kohm R 6nF C 3V k Hz 2k Hz AM receiver peak amplitude detector This circuit is called peak amplitude detector, or simply detector. A detector is a heart of any type of an AM receiver; however, a receiver has to include some other modules. For example, a detector requires several milliwatts of input signal power; therefore, the input signal must be amplified on the way from an antenna to a detector. See [], pages 48-5, Figures from 5- through 5-7. D. Mynbaev, EET 24 Module, Spring 28 3
AM demodulation - receivers Antenna Speaker RF amplification and tuning AM detector Audio amplifier Block diagram of a tuned radio frequency (TRF) receiver []. A TRF receiver involves several stages of radio frequency (RF) tuning and amplification. This operation is presented by the block RF amplification and tuning. An AM detector has been discussed in the previous slides. An audio amplifier provides amplification in the range of audio frequencies (< 2 khz). This type of receiver is no longer in use because () amplification in RF range is difficult to implement and (2) there are several stages of amplifiers and tuning at one station (frequency) must be done at every stage independently. (See []. Also see [2], Figure 5-8, Page 52.) These problems can be resolved if we will make tuning a one-step operation and shift signal to be amplified at the lower frequency range. D. Mynbaev, EET 24 Module, Spring 28 4
AM demodulation - receivers Amplitude (V) A C m A C /2 m A C /2 A M f M f M - f C f C f M + f C Frequency (Hz) So far we have considered AM demodulation from a waveform (time-domain) perspective. Let s take a look at this operation from a frequency-domain standpoint: We need to extract one signal (f M ) from the received combination of three signals (f C, f M f C, and f M + f C ). This can be achieved by using an AM detector. However, the real problem of AM demodulation is tuning and amplification. We will resolve these problems by frequency conversion of a received AM signal so that information is carried by the signal whose frequency has been translated to an intermediate frequency (IF) that is between carrier and modulating frequencies. D. Mynbaev, EET 24 Module, Spring 28 5
AM demodulation superheterodyne receiver Antenna Local Oscillator (LO) f LO Audio amplifier Speaker RF stage f C Mixer f IF = f C - f LO IF amplifier Detector Automatic gain control (AGC) Ganged tuning Block diagram of a superheterodyne AM receiver []. An intermediate frequency (IF) carries an information signal, that is recovered by an AM detector. The main advantage of a superheterodyne receiver is that amplification always occur at the same fixed IF frequency. This result is achieved by ganged tuning of a local oscillator and a circuitry (RF stage) accepting an incoming AM signal. Automatic gain control (AGC) provides the constant gain for incoming signals with various power. Further discuss the receiver operation. D. Mynbaev, EET 24 Module, Spring 28 6
Assignments:. Reading: a. Textbook: Pages 9-95 and 8-29. b. Paul Young, Electronic Communication Techniques, 5 th Edition, Prentice Hall, 24: Section Amplitude Modulation. 2. Homework problems: Chapter 3: ## 4-48. 3. Carefully review the examples given in this lecture. References:. Jeffrey S. Beasley and Garry M. Miller, Modern Electronic Communication, 8 th ed., Prentice Hall, 25. 2. Paul H. Young, Electronic Communication Techniques, 5 th ed., Prentice Hall, 24. 3. Robert L. Boylestad, Introductory Circuit Analysis, th ed., Prentice Hall, 24. 4. Thomas L. Floyd, Electronic Devices, 7 th ed., Prentice Hall, 25. 5. Richard H. Berube, Learning Electronics Communications Through Experimentation Using Electronics Workbench Multisim, Prentice Hall, 22. D. Mynbaev, EET 24 Module, Spring 28 7