Master Degree in Electronic Engineering · Master Degree in Electronic Engineering Analog and telecommunication electronic course (ATLCE-01NWM) L.X.R miniproject, baseband signal

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

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


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.


The modulation and demodulation techniques are applied to baseband signal transmission

What are not shown in the picture Filter amplifier


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


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



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



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


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


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)


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

definition: ήAM=Ps/PAM=<m2(t)>t/(A02+<m2(t)>t) < 1

always!


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


Linear modulation

Example of voice signal modulation:

Human voice : Frequency range from 300 to 20,000 hertz


Linear modulation

DSB-SC modulation:

Mathematic expression:

Notation: m(t)=Vmcos(ωmt) called modulating signal Vccos(ct)=Xc(t) called carrier


Spectrum analysis:

Linear modulation

DSB-SC modulation:

no carrier component amplitude attenuation 1/2

What about DSB-SC in time domain ?


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


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


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)


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%


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


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


demodulation

AM demodulation envelope detection coherent demodulation DSB-SC demodulation SSB-SC demodulation VSB-SC demodulation

1. envelope detection

Detector: rectifier and a LPF

Diode rectifier

condition :1/ωc <<RC<<1/ωH

Output: mo(t)=A0+m(t) No distortion when A0>=|m(t)max|


demodulation

1. envelope detection

process of envelop detection

Bridge circuit with diodes rectifying circuit


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.


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)


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)


VSB-SC demodulation

demodulation

Coherent technique is applied:

Output from the multiplier

Spectrum domain:

From LPF we get:


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


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


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.



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)


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).


FM modulation

expression for y(t) simplifies to

Modulation index:

Δf is the maximum deviation of the instantaneous frequency fm : highest frequency component



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=



Direct modulation

FM circuit with var-diode

22 )1())(

1(1)(

xVV

tuLCx c

QD

cj

Finally, we get the resonance frequency

Crystal oscillator FM circuit

Using m(t) controls resonance frequency

Basic steps: 1. Integrating m(t) 2. phase 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.


Carrier in

M(t)

output


indirect modulation

Integrator

isolation resistors: R1,R2 and R3 Three capacitors: short circuit for high frequency and open circuit for modulating signal m(t)



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


PM 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.


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


demodulation

FM demodulation

1. Slope discriminator

Circuit : double loop detuning frequency discriminator

When fc-f1=f2-fc

f1<fc

f2>fc

Discriminator transconductance : CONSTANT


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


demodulation FM demodulation

3. Ratio discriminator

Ratio discriminator has both discrimination function and clipping function

Clipping to avoid the parasitic signal


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)


The applications of modulation techniques

BeiDou Navigation Satellite System

I-Q demodulation



AM decoder: NE567 IC



AM decoder: NE567 IC

Configuration

PLL cell



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



LM1596/LM1496 Balanced Modulator-Demodulator

Applications: suppressed- carrier modulation , AM, syn-detection, FM or PM detection.

Monday, June 08, 2015 L.X.R miniproject, baseband signal transmission 50


LM1596/LM1496 Balanced Modulator-Demodulator



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

Monday, June 08, 2015 L.X.R miniproject, baseband signal transmission 52

summary Modulation techniques: Linear-modulation: • AM modulation:

• DSB-SC modulation • SSB-SC modulation • VSB-SC modulation Non-linear modulation:

• FM modulation: • PM modulation

Demodulation techniques: • Envelope detection • Coherent demodulation • FM/PM demodulation circuits

Case study:

• BDS, GPS, NE567,LM1596/LM1496,SDR

Reference materials 1. <<Fundamentals of Telecommunications>>, published by John

Wiley & Sons, 1999.

2. <<High frequency electrons>> published by Chen He.

3. Datasheet of LM1596/LM1496 Balanced Modulator-Demodulator. 4. Datasheet of IC SI4730 based on broadcast AM/FM/SW/LW radio

receiver. 5. 0http://www.allaboutcircuits.com/vol_3/chpt_9/6.html

6. http://www.radio-electronics.com/info/rf-technology-design/am-

amplitude-modulation/what-is-am-tutorial.php

7. http://en.wikipedia.org/wiki/Frequency_modulation

53 L.X.R miniproject, baseband signal transmission Monday, June 08, 2015

http://www.allaboutcircuits.com/vol_3/chpt_9/6.html



http://www.radio-electronics.com/info/rf-technology-design/am-amplitude-modulation/what-is-am-tutorial.php




















http://en.wikipedia.org/wiki/Frequency_modulation




Documents

Master Degree in Electronic Engineering · Master Degree in Electronic Engineering Analog and telecommunication electronic course (ATLCE-01NWM) L.X.R miniproject, baseband signal