1. 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
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2. 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

3. 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Due to these limitations this receiver is not used and have been replaced by Superhetrodyne receivers.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

4. 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
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5. Superhetrodyne Receivers (Superhet)
Suppose f1 = 1000KHz (From antenna/selector)
LO frequency f2 = 1455KHz
Output of Mixer are:
f2 – f1 = = 455KHz
f2 + f1 = = 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

6. 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

7. 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
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8. 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.
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9. 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

10. 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.)
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11. 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
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12. 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) + -
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13. 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

14. 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

15. 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

16. 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)
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17. Coherent Detection of DSBSC Modulated Waves (2/2)
Unwanted
This unwanted signal is removed by LPF
The overall output vo(t) is :-
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18. 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
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19. 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

20. 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

21. 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
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22. 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
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23. 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

24. 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 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

25. 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

26. 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

27. Demodulation of SSB Waves
Product Modulator
LPF
SSB wave
S(t)
cos(2πfct)
V(t)
V0 (t)
Scaled message signal
Unwanted component
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28. 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
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29. VSB Transceiver Transmitter Receiver
Hi()
m(t)
LPF
Ho()
Transmitter
Receiver
e(t)
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30. Department of Electronics and Communication Engineering, KUET
Thanks for Your Kind
Attention
Department of Electronics and Communication Engineering, KUET