Miniproject: Baseband signal transmission techniques Name: LI. XINRUI E-mail: [email protected]Master Degree in Electronic Engineering Analog and telecommunication electronic course (ATLCE-01NWM) 1 L.X.R miniproject, baseband signal transmission Monday, June 08, 2015
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Master Degree in Electronic Engineering · Master Degree in Electronic Engineering Analog and telecommunication electronic course (ATLCE-01NWM) L.X.R miniproject, baseband signal
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Miniproject: Baseband signal transmission techniques
Analog and telecommunication electronic course (ATLCE-01NWM)
1 L.X.R miniproject, baseband signal transmission Monday, June 08, 2015
2 Monday, June 08, 2015 L.X.R miniproject, baseband signal transmission
Introduction of technique of baseband signal transmission - the definition of baseband signal - typical techniques are applied to transmitted signal - The role of modulation technology in signal transmission theory. Linear- modulation -amplitude modulation (AM) - DSB-SC modulation - SSB-SC modulation - VSB-SC modulation Demodulation - AM demodulation envelope detection coherent demodulation - DSB-SC demodulation - SSB-SC demodulation - VSB-SC demodulation
Syllabus
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Non –linear modulation - frequency modulation (FM) direct FM indirect FM - phase modulation (PM) Demodulation - FM demodulation slope discriminator phase discriminator ratio discriminator PLL discriminator - PM demodulation the applications of modulation techniques - BeiDou Navigation Satellite System - AM decoder (NE567) - stereo FM transmitter - LM1596 Balanced Modulator-Demodulator - advanced technology – Software Defined Ratio
Syllabus
4
Theme : baseband signal transmission
Monday, June 08, 2015 L.X.R miniproject, baseband signal transmission
Introduction of technique of baseband signal transmission
Recently, there are dramatically evolutions of information techniques, such as cellphone, computer and network etc, along with the development of technology. Particularly, in the electronic and wireless transmission fields, for instance, the speed of network is much higher than before. Even though, the most popular way to transmit baseband signal is based on the digital technique, such as, improving the channel transmission or develop the digital elements, I would like to study the analog signal transmission in the free space according to the analog techniques. The following graph illustrates the basic architecture of baseband transmission between receiver and transmitter.
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The modulation and demodulation techniques are applied to baseband signal transmission
What are not shown in the picture Filter amplifier
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Definition for baseband signal
A signal is baseband if it has a very narrow frequency range, i.e. a spectral magnitude that is nonzero only for frequencies in the vicinity of the origin (termed f = 0) and negligible elsewhere.
Compared with RF signal, it has rather low frequency. Baseband signals always represent basic information voice image audio.etc
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Typical techniques are applied to transmitted signal
baseband transmission applied to computer inner parallel buses, most Local Area Network such as, Ethernet and Token Ring Benefits: convenient for short transmission; low attenuation ; high speed transmission low error probability no need of M-DeM cells Drawback: not suitable for long transmission since it has many AC components
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Typical techniques are applied to transmitted signal
Pass-band transmission Passband transmission technique is related to transmitting signal directly inner channels and based on the modulation and demodulation technologies. Benefits: high radiation capability capability for long-distance transmission Drawback: low speed, high error probability
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Typical techniques are applied to transmitted signal
broadband transmission Broadband transmissions are divided into multiple bands or channels by multiplexers using a multiplexing scheme such as frequency-division multiplexing (FDM). Benefits: high bandwidth (>128Kbps typical) high speed Drawback: strict requirements complex structure
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Linear modulation
Amplitude Modulation (AM)
Definition: the amplitude of a high frequency sine wave (usually at a radio frequency) is varied in direct proportion to that of a modulating signal
AM radio transmitter system
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Linear modulation
Mathematic model
m(t) modulating signal A0 is additional DC component Xc(t) = cos(ωct) called carrier sAM(t) called AM signal or modulated signal
Mathematic expression:
From the spectrum : Center frequency at ω=±ωC
AM bandwidth: 2Bm(2fH)
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Linear modulation
AM parameters:
Power pAM
No DC component for m(t) <m(t)>t=0
<cos2(ωct)>t=<1
2∗ cos 2ωct +
1
2 >t=1/2
finally, PAM=A02/2+ <m2(t)>t/2 =PC+ PS
with carrier power PC and sideband power PS Modulation efficiency ήAM
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Linear modulation
Modulation index m :
Modulation index m is defined as the maximum changes of modulated signal marked as|ΔVAM(t)|max also could be expressed with (VM-Vm)/(VM+Vm) m= (VM-Vm)/(VM+Vm) or |ΔVAM(t)|max Interpolation of m: m=1 is called full-modulated; m>1 is called over-modulated. general case : 0<m<1
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Linear modulation
Example of voice signal modulation:
Human voice : Frequency range from 300 to 20,000 hertz
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Linear modulation
DSB-SC modulation:
Mathematic expression:
Notation: m(t)=Vmcos(ωmt) called modulating signal Vccos(ct)=Xc(t) called carrier
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Spectrum analysis:
Linear modulation
DSB-SC modulation:
no carrier component amplitude attenuation 1/2
What about DSB-SC in time domain ?
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Linear modulation
DSB-SC modulation:
m(t)=0
Parameters analysis: power PDSB=<m2(t)>t/2 is the same as AM ME ήDSB=100% improve the modulation
efficiency
In time domain
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Linear modulation
SSB-SC modulation:
Basic idea: since both sidebands carry the same information, we can only keep one sideband (upper or lower)component to save resource
expression in time domain: How can we create the SSB-SC signal ? 1. using a sideband filter, which can be HPF or LPF HPF or LPF can be applied to remove one of the sideband, leaving only either the upper sideband (USB) or the lower sideband (LSB) according to which sideband we want to keep. Assuming both sidebands are symmetric, no information is lost in the process and the effective power output is greater than in normal AM.
method model
Where HSSB(ω) is the transfer function of sideband filter
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Linear modulation
SSB-SC modulation:
Lower sideband
upper sideband
Requirements: need a dramatic cut-off characteristic at fcut-off
LPF
HPF
Benefit: rather easy way to create SSB Drawbacks: difficult to make such kinds of filters (very steep filter)
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Linear modulation
SSB-SC modulation:
How to create SSB-SC signal 2. Phase-shift method Notation: Hh(ω) is the transfer
function of Hilbert transform filter, which translates all the components coming from m(t) with phase shift -90o
Drawbacks: big problem for obtaining such kinds filters with a strict -90o phase shift.
Parameter analysis: bandwidth
BWSSB=1
2BWDSB=Bm=fH
Power
modulation efficiency ήSSB=100%
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Linear modulation
VSB-SC modulation:
VSB-SC is called Vestigial Sideband suppression carrier. The idea is designing a kind of filter called VSB filter to filter one sideband and keeps a little part of the other one
Requirement: HVSB(ω+ωC)+HVSB(ω-ωC)=constant for |ω|<=ωH. This is derived from VSB demodulation techniques.
HVSB(ω) is reciprocal around carrier frequency ωc
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Linear modulation
VSB-SC modulation:
Expression:
2 cases according to SSB-SC principle
Benefits: we could make VSB filter without the strict requirement like SSB However we still have some requirement.
Now, let’s have a look at the demodulation techniques
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Output: mo(t)=A0+m(t) No distortion when A0>=|m(t)max|
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demodulation
1. envelope detection
process of envelop detection
Bridge circuit with diodes rectifying circuit
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demodulation
2. coherent demodulation
applying the mixer with inputs sAM(t) and xc(t) can recover the modulating signal m(t).
Output: mo(t) = 1/2[Ao+m(t)].
PLL applied stable frequency component ω called
reference signal.
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DSB-SC demodulation
demodulation
Only coherent demodulation method can be used, since the DSB envelope is not corresponding with the shape of m(t)
The structure is the same as AM one.
From the LPF, we can get mo(t)=1
2𝑚(𝑡)
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demodulation
SSB-SC demodulation
With the same factor as DSB-SC, the coherent demodulation is also applied for SSB-SC demodulation process with the same principle circuit
The output from mixer is
The output from LPF is mo(t) =1
4𝑚(𝑡)
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VSB-SC demodulation
demodulation
Coherent technique is applied:
Output from the multiplier
Spectrum domain:
From LPF we get:
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demodulation
VSB-SC demodulation
From above equation, we can conclude that in order to get the modulating signal m (t) without distortion from VSB-SC signal, we have HVSB(ω+ωC)+HVSB(ω-ωC)=constant for |ω|<=ωH
Up to now, we have analyzed some kinds of the linear modulation and demodulation techniques Comparison among these AM, DSB-SC, SSB-SC, VSB-SC
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Modulation type Demodulation
methods
benefits drawbacks
AM Envelope detector
and coherent
demodulation
Simple structure
for demodulation
process; envelope
detection can be
applied;
Low modulation
efficiency;
Large band is
occupied
DSB-SC Coherent
demodulation
No carrier power;
High modulation
efficiency
Large band is
occupied
SSB-SC Coherent
demodulation
No carrier power;
High modulation
efficiency;
Few band
occupied;
Difficult to
realize steep sideband
filter;
VSB-SC Coherent
demodulation
No carrier power;
High modulation
efficiency;
Few band
occupied;
Strict condition
limited
Comparison chart
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Non-linear modulation
Non-linear modulation
Frequency modulation (FM): frequency variation of carrier Δωc is proportional to modulating signal m(t).
Phase modulation (PM): phase variation of carrier ΔΦ is proportional to modulating signal m(t).
Frequency modulation (FM): encoding information in a carrier wave by varying the instantaneous frequency of the wave and keep the amplitude constant.
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Non-linear modulation
FM modulation
Instantaneous phase and frequency introduction:
(t)Aa(t) m cos is called the total phase (t)
Relationship between phase and frequency
dt
(t)dt ω
tdtωtt
00
ωc called instantaneous radius and φ0 called initial phase.
instantaneous phase: 00
tdtωtt
xm(t) is called transmitted signal. sinusoidal carrier: xc(t) = Accos (2πfct)
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f(τ) is the instantaneous frequency of local oscillator and fΔ is the frequency deviation, which represents the maximum shift away from fc in one direction, assuming xm(t) is limited to the range ±1 namely, xm(t)=sin(2πfmt).
Non-linear modulation
FM modulation
expression for y(t) simplifies to
Modulation index:
Δf is the maximum deviation of the instantaneous frequency fm : highest frequency component
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Non-linear modulation
FM modulation
When h<<1, narrowband FM otherwise wideband FM. The realization of FM circuit 1. Direct FM modulation circuit FM circuit with var-diode Crystal oscillator FM circuit Reactance tube FM circuit 2. Indirect FM modulation circuit
Basic idea: control the output frequency for resonance circuit with modulating signal m (t)
Direct FM modulation circuit
1
2π 𝐿𝐶 fr=
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Non-linear modulation
Direct modulation
Reactance tube FM circuit
Reactance tube which is the same as var-diode is also voltage control device
M(t)
Modulating signal
Indirect FM modulation circuit
First, do the integration for modulating signal m(t) and after that do the phase modulation.
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Carrier in
M(t)
output
Non-linear modulation
indirect modulation
Integrator
isolation resistors: R1,R2 and R3 Three capacitors: short circuit for high frequency and open circuit for modulating signal m(t)
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Non-linear modulation
PM modulation
Analysis: phase Φ is the integration of angle frequency ω PM modulation technique is quite similar with FM modulation using modulating signal m(t) controls the phase of carrier, thus the carrier phase variation ΔΦ is proportional to m(t). The expression is : instantaneous phase:
(t)uktωUu ΩpccmPM cos
(t)Δtω(t)uktω(t) pcΩpc
kp :proportion coefficient, called instantaneous phase shift. (t)Δ p
instantaneous frequency: )(
)()()( t
dt
tduk
dt
tdt pcpc
dt
tdukt pp
)()( with
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PM modulation
Non-linear modulation
Modulation index :mp maximum phase shift maxtΔ p
bandwidth: 2(mp + 1)fM with fM =ωm/2π
PM circuits correspond to the FM circuits. when doing FM , we get PM as well.
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demodulation
FM demodulation
FM demodulation process is called frequency detector or discriminator 1. Modifying FM to AM or pulses with different duty cycle 2. Applying AM demodulation methods
Fig. basic principle for FM demodulation process
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demodulation
FM demodulation
1. Slope discriminator
Circuit : double loop detuning frequency discriminator
When fc-f1=f2-fc
f1<fc
f2>fc
Discriminator transconductance : CONSTANT
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demodulation FM demodulation
2. Phase discriminator
Phase discriminator is based on the phase-frequency curve for resonance circuit to change the FM signal to AM signal. Another idea is using steep filter to transfer FM to AM
equivalent circuit basic circuit
Output voltage : Kd called voltage transmission coefficient
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demodulation FM demodulation
3. Ratio discriminator
Ratio discriminator has both discrimination function and clipping function
Clipping to avoid the parasitic signal
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demodulation
4. PLL-discriminator
FM demodulation
Recalling the behavior of PLL, we can use certain region
Fp=Fpo+KM(t)
there is full Fp spectrum in the lock range marked with “L” and VC(t) is proportional to M(t)
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The applications of modulation techniques
BeiDou Navigation Satellite System
I-Q demodulation
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The applications of modulation techniques
AM decoder: NE567 IC
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The applications of modulation techniques
AM decoder: NE567 IC
Configuration
PLL cell
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The applications of modulation techniques
stereo FM transmitter: Rohm BA1404
• maximum voltage <3V • battery can be used
7805 Regulator with a couple of 1N4001 diodes can drive this IC
reducing supply voltage to about 2.8 V
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The applications of modulation techniques
LM1596/LM1496 Balanced Modulator-Demodulator
Applications: suppressed- carrier modulation , AM, syn-detection, FM or PM detection.
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The applications of modulation techniques
LM1596/LM1496 Balanced Modulator-Demodulator
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The applications of modulation techniques
Advanced technique-Software Defined Radio (SDR)
Software structure instead of hardware components (e.g. mixer filter amplifier modulator,etc.) Based on computers or embedded systems
Software defined antenna
Benefits: • Increasing capacity • Reducing
interference to others
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summary Modulation techniques: Linear-modulation: • AM modulation: