variations of am
DESCRIPTION
Variations of AM. Bandwidth Efficiency Single Side Band Vestigial Side Band Power Efficiency Reduced Carrier Suppressed Carrier Complication Circuits: generation, tuning, detection Filter, tone variation, carrier recovery. Double Side Band Suppressed Carrier (DSBSC). - PowerPoint PPT PresentationTRANSCRIPT
![Page 1: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/1.jpg)
![Page 2: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/2.jpg)
Bandwidth EfficiencySingle Side BandVestigial Side Band
Power EfficiencyReduced CarrierSuppressed Carrier
ComplicationCircuits: generation, tuning, detectionFilter, tone variation, carrier recovery
![Page 3: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/3.jpg)
Double Side Band Suppressed Carrier (DSBSC)
It is a technique where the sidebands are It is a technique where the sidebands are transmitted without the carrier (carrier is being transmitted without the carrier (carrier is being suppressed/cut)suppressed/cut)
Characteristics:Characteristics: Power content lessPower content less Same bandwidthSame bandwidth Disadvantages - receiver is complex and Disadvantages - receiver is complex and
expensive.expensive.
![Page 4: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/4.jpg)
Frequency spectrum of DSB-SSC
![Page 5: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/5.jpg)
Phasor diagram of DSB-SC
![Page 6: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/6.jpg)
We know that, the total power transmitted in AM is
![Page 7: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/7.jpg)
If the carrier is suppressed, then the total power transmitted in DSB-SC-AM is P t = PLSB+PUSB
![Page 8: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/8.jpg)
![Page 9: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/9.jpg)
![Page 10: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/10.jpg)
Single Side Band (SSB)
Improved DSBSC and standard AM, waste power and occupy Improved DSBSC and standard AM, waste power and occupy large bandwidth.large bandwidth.
SSB is a process of transmitting one of the sidebands of the SSB is a process of transmitting one of the sidebands of the standard AM by suppressing the carrier and one of the sidebandsstandard AM by suppressing the carrier and one of the sidebands
Advantages:Advantages: Saving powerSaving power Reduce BW by 50%Reduce BW by 50% Increase efficiency, increase SNRIncrease efficiency, increase SNR
DisadvantagesDisadvantages Complex circuits for frequency stabilityComplex circuits for frequency stability
![Page 11: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/11.jpg)
Frequency spectrum of SSB
![Page 12: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/12.jpg)
Vestigial Side Band (VSB) VSB is mainly used in TV broadcasting for their VSB is mainly used in TV broadcasting for their
video transmissions.video transmissions. TV signal consists ofTV signal consists of
Audio signal – transmitted by FMAudio signal – transmitted by FM Video signal – transmitted by VSBVideo signal – transmitted by VSB
A video signal consists a range of frequency and A video signal consists a range of frequency and fmax = 4.5 MHz.fmax = 4.5 MHz.
If it transmitted using conventional AM, the If it transmitted using conventional AM, the required BW is 9 MHz (BW=2fm). But according to required BW is 9 MHz (BW=2fm). But according to the standard, TV signal is limited to 7 MHz onlythe standard, TV signal is limited to 7 MHz only
So, to reduce the BW, a part of the LSB of picture So, to reduce the BW, a part of the LSB of picture signal is not fully transmitted.signal is not fully transmitted.
![Page 13: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/13.jpg)
The frequency spectrum for the TV The frequency spectrum for the TV signal / VSB:signal / VSB:
![Page 14: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/14.jpg)
![Page 15: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/15.jpg)
Generation of Amplitude ModulationIt is broadly divided into two types namely (i) Linear modulator or large signal
modulator.(ii) Non Linear modulator or small signal modulator
Linear modulator or large signal modulator: In this type of modulators the devices are operated
in the linear region of its transfer characteristics thus the relation between the amplitude of the modulating signal and the resulting depth of modulation is linear. It can be divided into two types
(i) Transistor modulator(ii) Switching modulator
![Page 16: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/16.jpg)
In this linear modulator the pair of non-linear elements having (as near as possible) the same characteristics in a balanced circuit cancel out the unwanted components. `
Non-linear Modulator or Small signal modulator
These modulators makes use of non-linear V-I characteristics of the devices and are, in general suited for use at low voltages.
The important type is: Square law modulator
In simple words it can be defined as the devices used in these modulator are operated in non linear region of its characteristics.
![Page 17: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/17.jpg)
Square law modulatorA square law modulator requires to add up
the carrier and modulating signal to obtain AM with carrier.
Thus a square law modulator has three features.
1. Summer-To add carrier and modulating signal
2. A non-linear (active) element.3. Bandpass filter for extracting desired
modulating products.
![Page 18: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/18.jpg)
![Page 19: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/19.jpg)
![Page 20: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/20.jpg)
A. Modulating Signal
B. Carrier
C. Sum of carrier and
modulating signal
D. Diode current
E. AM output across
tuned circuit
![Page 21: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/21.jpg)
Square law modr. using FET
![Page 22: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/22.jpg)
The FET is biased in a non-linear region of its transfer characteristics, to obtain the desired output.
The output tank circuit RLC is tuned to the carrier frequency to select the desired modulating components.
Mathematical AnalysisThe current of a non-linear element
suitably biased in its non-linear region is given by the square law equation
i.e., io = a1V1+a2V12 +...........
![Page 23: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/23.jpg)
Where V1 = input voltage applied to the FET
V1 = Am sin ωmt +Ac sin ωct
Substituting above equation in prev. eqn.
![Page 24: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/24.jpg)
Neglecting the second order terms,
![Page 25: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/25.jpg)
When the bandpass filter is tuned to the carrier frequency it allows only wc , (ωc − ωm) and (ωc + ωm) terms and it eliminates all other terms. Hence we obtain,
![Page 26: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/26.jpg)
Product Modulators Used for generation of DSB-SC signals.Here, a DSB-SC signal is obtained by simply
multiplying the modulating signal Vm(t) with carrier signal cos ωct.
![Page 27: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/27.jpg)
Let the modulating signal be Vm (t) = Am sin ωmt
and the carrier signal C(t) = Ac sin ωct
When multiplying both the carrier and message signal, the resulting signal is the DSB-SC-AM signal.
S(t)DSBSC =Vm (t)C(t)
Therefore S(t)DSBSC = Am sin ωmt Ac sin ωct
![Page 28: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/28.jpg)
![Page 29: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/29.jpg)
Balanced Modulators (practical) for DSB-SC generation:There are two ways of generating DSB-SC (i) Balanced modulator.
(ii) Ring modulator.Balanced modulator: a pair of symetric non-linear elements are
used.They cancel out the unwanted
components
![Page 30: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/30.jpg)
Diode balanced Modulator:
![Page 31: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/31.jpg)
The carrier voltage is applied in parallel to diode D1 and D2
the modulating voltage in push-pull to the diodes.
![Page 32: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/32.jpg)
![Page 33: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/33.jpg)
![Page 34: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/34.jpg)
A bandpass filter is that type of filter which allows to passband of frequencies.
Since the bandpass filter is centred around ωc it will pass a narrow band of frequencies
BPF is centred at ωc with a small bandwidth of 2ωm to preserve the sidebands.
Therefore, the output of BPF centered around ωc is given by
Which is the expression for a DSB-SC signal.
![Page 35: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/35.jpg)
Switching or Ring or Chopper modulator.
![Page 36: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/36.jpg)
A ring modulator uses four diodes in ring form.
diodes are controlled by a square wave carrier signal applied through a center-tapped transformer.
If the carrier is large enough to cause the diodes to switch states, then the circuit acts like a diode switching modulator
The modulating signal is inverted at the carrier rate. This is essentially multiplication by ±1.
![Page 37: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/37.jpg)
When the carrier is +, outer diodes will conduct.
When the carrier is - , inner diodes will conduct.
Thus carrier switches the diodes on / off.
Bandpass filters are used to extract the frequency of interest.
![Page 38: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/38.jpg)
Diode conduction for + and – carrier:
![Page 39: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/39.jpg)
![Page 40: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/40.jpg)
Generation of SSB-SC AM
SSB-SC waves can be generated in two ways:(i) Frequency discrimination or filter
method(ii) Phase discrimination method
Phase discrimination method itself can be divided into two types.
(a) Phase shift method(b) Modified phase shift-weavers
method.
![Page 41: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/41.jpg)
Filter method of SSB generation:
![Page 42: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/42.jpg)
first a DSB-SC signal is generated simply by using an ordinary product modulator or a balanced modulator.
After this from the DSB-SC signal one of the two sidebands is filtered out by a suitable Band pass filter (BPF).
The design of band pass filter is quite critical and puts some limitations on the modulating or baseband and carrier frequencies.
![Page 43: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/43.jpg)
Limitations of frequency discrimination method.
is useful only if the baseband signal is restricted at its lower edge due to which the upper and lower sidebands are non-overlapping.
the base band signal must be appropriately related to the carrier frequency (design of the BPF becomes difficult if ωc >> ωm.
![Page 44: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/44.jpg)
Phase shift method of SSB generation :
![Page 45: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/45.jpg)
The phase shift method avoids filter.
makes use of two balanced modulators and two phase shifting networks.
One modulator receives the carrier with the phase shift of 90◦ and the modulating signals directly.
The other modulator receives modulating signal with the phase shift of 90◦ and the carrier signal directly.
![Page 46: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/46.jpg)
For BM 1
Vm (t) = Am sin (ωmt)Vc (t) = Ac sin (ωct +90◦)
For BM 2
Vm (t) = Am sin (ωmt +90◦)Vc (t) = Ac sin ωct
![Page 47: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/47.jpg)
Output of balanced modulator M1 will contain sum
and difference frequencies
![Page 48: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/48.jpg)
Output of balanced modulator M1 will contain sum and difference frequencies
![Page 49: Variations of AM](https://reader035.vdocuments.us/reader035/viewer/2022062423/5681458c550346895db277cc/html5/thumbnails/49.jpg)