master degree in electronic engineering€¦ · 4.1 --frequency modulation (fm) 5 --the realization...

L.X.R: miniproject, baseband signal transmission

Master Degree in Electronic Engineering

Analog and Telecommunication Electronic Course

(ATLCE-01NWM)

Prof. Dante Del Corso

Miniproject :

“Baseband signal transmission: technique for modulation

and demodulation”

Name: LI.XINRUI

Student ID: s219989

E-mail:[email protected]


Catalogue

1 --Introduction of technique of baseband signal transmission

1.1 --Typical techniques are applied to transmitted signal.

1.2 --The definition of baseband signal.

1.3 --The role of modulation technology in signal transmission

theory.

2 -- Linear-modulation

2.2 --Linear -modulation -Amplitude Modulation(AM)

2.3 --Linear -modulation –DSB-SC modulation

2.4 --Linear -modulation –SSB-SC modulation

2.5 -- Linear -modulation –VSB-SC modulation

3 --Demodulation

3.1 --AM demodulation

3.2 --DSB-SC demodulation

3.3 --SSB-SC demodulation

3.4 --VSB-SC demodulation

4 --Non-linear modulation

4.1 --Frequency modulation (FM)

5 --The realization of FM circuit

5.1 --Direct FM modulation circuits

5.2 --Indirect FM modulation circuit

6 --Demodulation for FM signals

7 --PM modulation analysis

8 --The applications of modulation techniques

9 -- Advanced technique-Software Defined Radio (SDR)

10 --Reference materials


1. Introduction of technique of baseband signal transmission.

Recently, there are dramatically evolution 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.

What is well known to us is how the audio and the TV and cellphone

works. However, someone is not familiar with the radiation

technique, this thesis is based on the theory analysis and introduction

of how the baseband signal to be transmitted and received without

change or lose the original information.

The following graph illustrates the basic architecture of baseband

transmission between receiver and transmitter.


A very simple model to demonstrate the transmission principle for

baseband signal. Seeing graph1.1

antenna

antenna

graph1.1

Actually, the graph just gives us a general idea of the structure of

transmission concept. It should contains a lot of elements such as,

filters, amplifiers and so on, which will be shown later.

1.1 Typical techniques are applied to transmitted signal.

Before going to see the contents of modulation and demodulation,

let us have a look at the several techniques for transmitting signals:

Baseband transmission.

Baseband transmission which sends the wanted signal directly

to the destination without the M-DeM techniques is wildly

applied to the computer inner parallel buses, most Local Area

Network (LAN) such as, Ethernet and Token Ring.

transmitter Baseband

signal

receiver Original

signal


Benefits: convenient for short transmission, low attenuation,

high speed transmission and low error probability.

According to the features of baseband transmission, this technique is

much more popular in digital transmission field and the graph1.2

gives us the basic model.

Graph1.2

Passband transmission.

This technique means the long-distance transmission. Since the

majority of original signals to be transmitted are baseband

signals which have many AC components, hence it is not

suitable for L-D transmission.

Passband transmission technique is related to transmitting

signal directly inner channels and based on the modulation and

demodulation technologies. It is likely to have very high error

probability and low speed but is a good choice for the L-D

transmission.

Enco

der

Scramb

ler

Sou

rce

Mo

du

lator

Deco

der

Descram

bler

sink

Dem

od

ulato

r

channel

noise


Broadband transmission

Broadband transmissions are divided into multiple bands or

channels by multiplexers using a multiplexing scheme such as

frequency-division multiplexing (FDM). Each channel has a carrier

frequency that is modulated to carry the signal from a given source.

At the receiving station, multiplexers separate the various signals.

Guard bands are used to prevent interference among channels.

Broadband transmission is typically used for environments in

which video, audio, and data need to be transmitted simultaneously.

Cable television systems are based on broadband transmission

technologies. Other examples of broadband services include

T-carrier services, Asynchronous Transfer Mode (ATM), and

variants of Digital Subscriber Line (DSL). Its bandwidth typical is

higher than 128Kbps.

Graph1.3

1.2 The definition of 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. It can

be seen from graph 2.1. In telecommunications and signal

processing, baseband signals are transmitted without modulation,

that is, without any shift in the range of frequencies of the signal,

and are low frequency - contained within the band of frequencies

from close to 0 hertz up to a higher cut-off frequency or maximum

bandwidth. Baseband can be synonymous with lowpass or

non-modulated, and is differentiated from passband, bandpass,

carrier-modulated, intermediate frequency, or radio frequency (RF).

Compared with RF signal, the baseband signal has a rather lower

frequency, which can be regard as original signal before transmitted.

Voice, image and audio signals could be stored in the baseband

signal.

Graph 1.4

1.3. The role of modulation technology in signal transmission

theory.

In order to find the influence of modulation, first we need analyze


the reason why we cannot use the baseband transmission technology

to transmit the baseband signal. To radiate the baseband signal which

has very lower frequency, we need use the radiation system and

electromagnetic theory. Due to the very lower frequency of baseband

signal and the drawback of baseband transmission technique for L-D

transmission, the original signal will be transmitted as the

electromagnetic wave into the space and be recovered by the

demodulation technique.

Now, I give the direct reason why we prefer to use the modulation

technique! According to the antenna theory which is that in order to

radiate the electromagnetic field very far, the length of antenna

should be corresponding with the wavelength. Recalling that

λ = c/f, if f is very low, we get inconvenient to design one antenna

that has a large size. If so, we need shift the spectrum to a radio

frequency (a few MHz-1GHz).

Now let’s give a description of modulation technology.

2. Linear-modulation

The purpose of using the modulation is adding the information

inner baseband signal to the carrier. Carrier is a kind of signal

with higher frequency, which can be derived using frequency

synthesizer. This technique includes the linear modulation and

non-linear modulation.


Grap

h2.1

dem

onstr

ates

the

gene

ral model of RF circuit

Graph2.1

2.1 Linear -modulation -Amplitude Modulation(AM)-

Amplitude modulation (AM) is a modulation technique in

which the amplitude of a high frequency sine wave (usually at

a radio frequency) is varied in direct proportion to that of a

modulating signal. The modulating signal carries the required

information and often consists of original data, as in the case

of AM radio broadcasts or two-way radio communications.

The high frequency sine wave (the carrier) is modulated by

adding the modulating signal to it in a mixer. A simplified AM

radio transmitter system is shown as graph2.3.


Graph2.2

The mathematic model is shown as graph2.3

Graph 2.3

Notation: m (t) is called modulation signal, and A0 is the addition

DC component with zero-main which could be random or

determined signal. cos(ωct)=Xc(t) is called carrier and sAM(t) is

modulated signal. The formulas of AM signal in time and frequency

domains are shown as following

Assuming the upper fre-boundary of m(t) is ωH and its band width is

Bm=fH. We therefore can get the waveform and spectrum of AM

signal and the BW of AM signal is twice than modulation signal m(t)

as shown in graph2.4.


Graph2.4

Totally speaking, from the spectrum graph2.4 (b) we can

conclude that both upper sideband and lower sideband contain the

total origin information. In addition, we can see 2 phases at ω=±ωC

and AM signal is double sideband signal with carrier and its

BW=2Bm=2fH.

The other significant parameters are power of vAM(t),

modulation efficiency ήAM and modulation index m.

power of vAM(t) and modulation efficiency(ME) ήAM

According to the definition of the efficient power with the

AC signal, we have the following formulas:

Recalling that no DC component for m(t),namely

<m(t)>t=0 and <cos2(ωct)>t=<

1

2∗ cos(2ωct) +

1

2 >t=1/2

PAM=A02/2+<m

2(t)>t/2=PC+PS. notation PC= A0

2/2 called


carrier power and PS=<m2(t)>t/2 called sideband power, so we

get the definition of ή ≜ PS/PAM=<m2(t)>t/(A0

2+<m

2(t)>t) < 1

always!

modulation index m

Modulation index m is defined as the maximum changes of

modulated signal marked as |ΔVAM(t)|max and the magnitude of

carrier. However m can be expressed as (VM-Vm)/(VM+Vm).

m=1 is called full-modulated; m>1 is called over-modulated.

General case is 0<m<1. Now let me give one typical example.

Voice signal modulation:

A simple form of amplitude modulation was originally used to

modulate audio voice signals onto a low-voltage direct current (dc)

carrier on a telephone circuit. A microphone in the telephone handset

acts as a transducer, and uses the sound waves produced by the

human voice to vary the current passing through the circuit. At the

other end of the telephone line, a second transducer (in the form of a

small loudspeaker mounted in the remote handset) uses the varying

voltage to produce sound waves that are close enough to the original

speech patterns to be recognizable as the voice of the caller.

Although the human voice is composed of frequencies ranging from

300 to approximately 20,000 hertz, the public switched telephone

system limits the frequencies used to between 300 and 3,400 hertz,


giving a total bandwidth of 3,100 hertz. This bandwidth is perfectly

adequate for purely voice transmission, since the higher frequencies

in the human voice (i.e. those above 3,100 hertz) are not really

needed for recognizable speech reproduction. The use of a limited

bandwidth also makes the telephone system much simpler from an

engineering perspective.

Whereas telephone signals can be transmitted at audio

frequencies, the same is not really a practical proposition for radio

transmissions. The main reason for this is that the optimum length of

a radio antenna is a half or a quarter of a wavelength. Since a typical

audio frequency of 3,000 hertz has a wavelength of approximately

100 kilometers, the antenna would need to have a length of 25

kilometers to be effective - not a realistic proposition. By

comparison, a radio frequency of 100 megahertz would have a

wavelength of approximately 3 meters, and could use an antenna 80

centimeters long. It becomes necessary, therefore, to use a radio

frequency carrier signal in order to transmit audio signals, which are

used to modulate the carrier waveform.


Graph2.5

2.2 Linear -modulation –DSB-SC modulation

Introduction: DSB-SC is also one technique for amplitude

modulation. Since from the expression of ME (modulation

efficiency ), only PS is related to the information or we can say

baseband signal or modulating signal and the denominator of ME is

the sum of PS and PC, while PC is only related to the additional

component A0,therefore we can see the ME is not high for

modulating signal. What’s more, We can conclude that only the

sideband signal has much significant for transmitting information or

massage. In order to economize the resources of channel, we prefer


to take measures to avoid the carrier component to be sent to the

space, and then we derive the definition of DSB-SC, which can

compensate this drawback.

Definition: Double-sideband suppressed-carrier transmission

(DSB-SC) is transmission in which frequencies produced by

amplitude modulation (AM) are symmetrically spaced above and

below the carrier frequency and the carrier level is reduced to the

lowest practical level, ideally being completely suppressed.

In the DSB-SC modulation, unlike in AM, the wave carrier is not

transmitted; thus, much of the power is distributed between the

sideband, which implies an increase of the cover in DSB-SC,

compared to AM, for the same power used. Above all, DSB-SC

transmission is a special case of double-sideband reduced carrier

transmission. It is used for radio data systems.

The basic mathematics model is shown as figure2.6


Graph2.6

The basic real circuit is shown as following circuit

The expression can be described as following one:

Notation: m(t)=Vmcos(ωmt) is called modulating signal and

Vccos(ωct)=xc(t) is called carrier.

Spectrum analysis of DSB-SC

DSB-SC is basically an amplitude modulation wave without the

carrier, therefore reducing power waste, giving it 100% efficiency.

This is an increase compared to normal AM transmission (DSB),

which has a maximum efficiency of 33.333%, since 2/3 of the power

is in the carrier which carries no intelligence, and each sideband

carries has same information. The spectrum can be seen from

graph2.7


Graph2.7

From the above figure, we can summary that the DSB-SC signal

suppresses the carrier component, and the modulation efficiency

(ME)=100%. The graph2.8 shows us very important information that

when m(t)=0, the SDSB(t) has a phase shift 180o, which is the big

difference from general AM signal.

graph2.8

Numerical analysis of the DSB-SC ME and power:

PDSB=<m2(t)>t/2 is the same as PAM, however its ME ήDSB=2*ήAM

means it improve the ME.

Since from figure 2.7, we can also find the disadvantage that is

wasting the channel capacity to transmit information by DSB-SC.

Actually, we can use some method to get the single upper or lower

sideband instead of DSB to avoid sending the information repeatedly

Phase shift 180o


with 2 sideband signals, thus, we can economize half resources

compared to DSB-SC.

2.3 Linear -modulation –SSB-SC modulation

Single-Side Band Suppressed-Carrier (SSB-SC) is a refinement of

amplitude modulation which uses transmitter power and bandwidth

more efficiently. Amplitude modulation produces an output signal

that has twice the bandwidth of the original baseband signal.

Single-sideband modulation avoids this bandwidth doubling, and the

power wasted on a carrier, at the cost of increased device complexity

and more difficult tuning at the receiver.

The general ideas to create the SSB signal is based on two

techniques, namely filtering method and phase shift by 90o, which

will be introduced later.

HPF or LPF filtering to create SSB

One method of producing an SSB signal is to remove one of the

sidebands via filtering, leaving only either the upper sideband

(USB), the sideband with the higher frequency, or less

commonly the lower sideband (LSB), and the sideband with the

lower frequency. Most often, the carrier is reduced or removed

entirely (suppressed), being referred to in full as single sideband

suppressed carrier (SSBSC). Assuming both sidebands are

symmetric, which is the case for a normal AM signal, no


information is lost in the process. Since the final RF

amplification is now concentrated in a single sideband, the

effective power output is greater than in normal AM (the carrier

and redundant sideband account for well over half of the power

output of an AM transmitter).

The mathematics model is shown as graph2.9

Graph2.9

Where HSSB(ω) is the transfer function of sideband filter, whose

function is transferring the DSB-SC signal to SSB-SC signal

without change the shape which corresponding the wanted

information.

Corresponding to get the upper or lower sideband signal, there

are two kinds of filters, namely HPF and LPF, which can be

seen from graph2.10.

Lower sideband


Graph2.10

Phase-shift method for SSB-SC

Using filtering method to create the SSB-SC is a kind rather

easy way to create SSB in theory, however the single-side band

filter is very difficult to realize. The mean reason why we

cannot is that ideal such kind of filters is not feasible. The true

filter usually has a transition band from pass band to stop band,

however, a majority of modulating signals have many low

frequency components, which made the modulated DSB signal

has very narrow band between upper and lower sidebands,

hence we need a dramatic cut-off characteristic at fcut-off. Such

kinds of filters are rather difficult to make, which tends us to

drive another way to realize SSB-SC signals.

In time domain, the SSB-SC signal can be expressed as:

Notation: m (t) is modulating signal, m(t)-hat is the Hilbert

upper sideband


transform of m(t), ωc is carrier frequency,”-” represents lower

sideband signal vs “+” represents upper sideband signal.

The model for creating SSB-SC is shown as graph 2.11

Graph2.11

Notation: Hh(ω) is the transfer function of Hilbert transform

filter, which translate all the components coming from m(t) with

phase shift -90o. It is also the big problem for obtaining such

kinds filters with accuracy -90o phase shift.

Parameter analysis

Bandwidth: the BW is half of DSB signal and similar

with that of modulating signal

BWSSB=1

2BWDSB=Bm=fH , where fH is the highest

frequency component of m(t)

Power

ME(modulation efficiency) ήSSB


Since its spectrum has no carrier frequency component,

ME is 100%, namely ήSSB=100%.

2.4 Linear -modulation –VSB-SC modulation

VSB-SC is called Vestigial Sideband suppression carrier. The idea is

design a kind of filter called VSB filter to filter one kind sideband

and keeps a little part of the other one. The model is shown as graph

2.12

Graph2.12

Notation that in order to demodulate the baseband signal from sVSB(t),

the condition is HVSB(ω+ωC)+HVSB(ω-ωC)=constant for |ω|<=ωH.

In other word, the transfer function HVSB(ω) is reciprocal around

carrier frequency ωc, which can be seen clearly from graph 2.13

Graph2.13

According to the theory, we can drive its spectrum SVSB(ω)


The whole principle for VSB technique can be seen from graph 2.14

(a)

(b)

Graph2.13

3. Demodulation

Demodulation is the inverse processor corresponding to

modulation techniques, which is translated modulated signal,

such as SAM(t), SDSB(t), SSSB(t) and SVSB(t) to modulating signal


m(t). there are devise demodulation methods for different kinds of

modulation signals, which are displayed as following cases.

3.1 AM demodulation

Two techniques will be introduced for AM signal

demodulation, namely envelope detection and coherent

demodulation process.

envelope detection

From grap2.4 (a), we can conclude that the envelope of

sAM(t) is corresponding to the shape of m(t). We therefore

can use detector to detect the envelope of sAM(t). The

detector can be built by half-wave rectifier or full-wave

rectifier and a LPF.

The basic structure of detector is shown as graph3.1.

sAM(t) m(t)

graph3.1

Rectifier can be realized by simple structure consisting of only diode,

R and C. When 1/ωc <<RC<<1/ωH is satisfied, the output envelop

from detector is quite similar with that of m(t),, which is roughly

expressed by mo(t)=A0+m(t), in case of distortion of demodulated

signal, A0>=|m(t)max|.

Using LPF is to filter the wave fluctuation coming from carrier; the

Rectifier LPF


whole structure can be seen from following circuit.

The equivelent detector circuit is shown as below picture

The physical meaning of detection process is that LPF consists of

resistor and capacitor and the recharge time-constant is based on

RDC (RD is the conductance of diode VD), which is very small and

the discharge time-constant is based on RC which is very large. The

envelope therefore can be kept finally, moreover DC components

can be filtered by the capacitor C. the output is just the envelope of

modulating signal.

High demodulation efficiency and feasibility are the significant

features for detector. Compared with coherent method, there is no

need of same carrier signal in the receiver.

Graph3.2 shows us the general process of envelop detection

Low-frequenc

y amplifier


Graph3.2

The bridge built with diodes also can be applied to detect envelope.

The structure is using four diodes in each branch instead of single

diode without any changes of resistor and capacitor.

Bridge-detector component

coherent demodulation

From mathematics analysis, we can conclude that applying

the mixer with inputs sAM(t) and xc(t) can recover the

modulating signal m(t).

Only one LPF is needed to separate the modulating signal,


which can be written as mo(t) = 1/2[Ao+m(t)].

Graph 3.3 and graph3.4 give us the basic idea realization for

coherent demodulation process

Graph3.3

Graph3.4

The reference signal xc(t) can be generated by PLL

technique.

Phase-lock loop (PLL) with one input of original carrier

and output signal from PLL, this technique shown as

graph3.5 is good for creating stable local frequency

component ωc. this method corresponds to graph3.5

Graph3.5 Graph3.6

Graph3.6 illustrates the whole circuit for coherent demodulation.


3.2 DSB-SC demodulation

As what we can see from graph2.8, the waveform is not

corresponding to the envelope of modulating signal m(t). We

therefore need use the coherent demodulation process to

recover m(t), which is quite similar with AM coherent

demodulation. According to the graph3.3, the output from the

mixer for DSB demodulation is following expression.

From the LPF, we can get mo(t)=1

2𝑚(𝑡) and the process can

be seen as graph3.7

Graph3.7

Here, the reference signal xω(t) can be create in another way.

When using DSB modulation, we let the demodulated signal

contain a part carrier component, which can be used to recover

the carrier (reference signal) at the receiver point.

3.3 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 as graph3.3.

The mathematics expressions are displayed as following


The output from multiplier is

Via PLF, we get mo(t) = 1

4𝑚(𝑡), finally we get the modulating

signal without any distortion. The whole process is shown as

graph3.8

Graph3.8

3.4 VSB-SC demodulation

Coherent technique is also applied to VSB demodulation

shown as graph3.9

Graph 3.9

Output from the multiplier is in time

domain.

In frequency domain, we have


After the LPF, we obtain that

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|<=ω

H

The whole process is seen as following picture.

Up to now, we have analyzed some kinds of the linear

modulation and demodulation techniques. Now the following

table1 provides us comparison among them.

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

Large band is

occupied


efficiency

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

Table1


4. Non-linear modulation

Apart from modulating the amplitude of carrier, we can also

modulate the frequency or phase in carrier with m(t). The frequency

or phase vary along with the variations of m(t). According to this

purpose, we have FM and PM two different techniques.

In communication system, if we use baseband signal to

modulate the angle or phase for carrier or we do the reverse

procedure, we have to apply the non-linear spectrum transformation.

Therefore, we call such kinds of techniques are non-linear

modulations.

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).

4.1. Frequency modulation (FM)

In telecommunications and signal processing, frequency

modulation (FM) is the encoding of information in a carrier

wave by varying the instantaneous frequency of the wave.

Compare with amplitude modulation, in which the

instantaneous frequency of the carrier wave varies, while the

amplitude remains constant.

In analog signal applications, the difference between the


instantaneous and the base frequency of the carrier is directly

proportional to the instantaneous value of the input-signal

amplitude.

Frequency modulation is used in radio, telemetry, radar,

especially for broadcasting music and speech, two-way radio

systems, magnetic tape-recording systems and some

video-transmission systems. In radio systems, frequency

modulation with sufficient bandwidth provides an advantage in

cancelling naturally-occurring noise.

Concept for instantaneous phase and frequency

For an oscillator, the local output signal

(t)Aa(t) m cos with Am is the amplitude; (t) is the

total phase

According to the relationship between phase and frequency,

we have

dt

(t)dt ω

tdtωtt

00

)(t called instantaneous radius and 0 called initial phase.

If )(t is related to the time variation, instantaneous phase

can be written as 00

tdtωtt

and the general form is

00

cos tdtωAtat

m .

Basic analysis of FM


If the information to be transmitted (i.e. the baseband signal)

is xm(t) and the sinusoidal carrier is xc(t) = Accos (2πfct),

where fc is the carrier's base frequency, and Ac is the carrier's

amplitude, the modulator combines the carrier with the

baseband data signal to get the transmitted signal:

Notation: f(τ) is the instantaneous frequency of local os-

cillator 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).

Generally speaking, a baseband modulated signal may be

approximated by a sinusoidal continuous wave signal with a

frequency fm. the integral of such a kind signal is

In this case the, the expression for y(t) simplifies to:

where the amplitude Am of the modulating sinusoid is

represented by the peak deviation fΔ.


Modulation index

As in other modulation systems, the value of the modulation

index indicates by how much the modulated variable varies

around its unmodulated level. It relates to variations in the

carrier frequency:

Where fm is the highest frequency component present in

modulating signal xm(t), Δf is the maximum deviation of the

instantaneous frequency from the carrier frequency, general

when h<<1, the modulation is called narrowband FM

otherwise wideband FM.

According to the signal theory, since we use the sinusoidal

signal which has harmonics components. We have to

consider and analyze them but are not shown here.

Graph 4.1 gives us the general concept of FM technique.

Graph4.1


We can see from the blue carve that the frequency of FM signal is

related to the amplitude variation of m(t).

5. The realization of FM circuit

There are two different ways to realize FM circuit, namely direct

and indirect FM modulation circuits according to different fields.

5.1. Direct FM modulation circuit

According to the expression and definition of FM, the general

idea for building FM circuit is trying to control the output

frequency for resonance circuit with modulating signal m

(t).This technique is called direct FM modulation. Its junction

capacitance varies with the inverse voltage. The basic

characteristic of resonance circuit is shown as graph5.1

Graph5.1

fr=1

2π√𝐿𝐶

FM circuit with var-diode

The var-diode can be inserted in LC resonance circuit.

Moreover, m(t) controls its capacitance directly and also

)1(

0

D

jj

Vu

CC


it controls resonance frequency ωr or fr indirectly.

Barrier capacitance

After adding bias and modulating voltage, the total

voltage

Finally we can get

Therefore, we replace the capacitor C with Cj showing as

graph5.2

graph5.2

According to the resonance circuit theory, the

instantaneous frequency is

Crystal oscillator FM circuit

Due to have the high stability at center frequency, we

prefer to choose crystal oscillator circuit. Using the

series structure of Cj and crystal oscillator, which is

Ω tUV(T)UVu Ω mQΩQ cos

Ω t)m(

Cjo

)VV

Ω tU()

V

V(

C

)V

Ω tUV(

C

Vu(

CC

c

γ

QD

Ω mγ

D

Q

j

γ

D

Ω mQ

j

D

jj

cos1

cos11

cos1

)1

0

00

22 )1())(

1(1)(

xVV

tuLCx c

QD

cj


shown as graph5.3

Graph5.3

The principle for this structure is quite similar with the

above one. Using m(t) controls resonance frequency.

Reactance tube FM circuit

Reactance tube which is the same as var-diode is also

voltage control device. Controlled source can be electron

tube, transistor and FET. The circuit is graph5.4

Graph5.4

5.2. Indirect FM modulation circuit

Do the integration for modulating signal m(t) and after that do

the phase modulation. As for m(t), the process is frequency

M(t)


modulation. Therefore, the key point for this technique is

phase modulation circuit. At this point, we can also use

var-diode inserted in oscillator circuit for high frequency

amplifier. Due to the effect of var-diode, the resonance

frequency will change. Finally, the carrier will has phase shift

when coming to this resonance circuit. Graph 5.5 is single

stage PM circuit.

Graph5.5

Var-diode Cj and inductor L create a resonance circuit as the

phase shift network. R1,R2 and R3 are called isolation resistors.

Three capacitors whose values are 0.001uF are short circuit for

high frequency and open circuit for modulating signal m(t).

6. Demodulation for FM signals

FM and PM demodulation processes are called frequency

output M(t)

Carrier in


detector and phase detector respectively. However, they have

some differences.

First, we have to change FM signal into AM signal or pulses

with different duty circle, then use AM demodulation techniques

or rectifier for pulse to demodulate m(t) from FM signal. This

process is shown as graph5.6

Graph5.6

The relationship between output voltage uΩ(t) and input FM

instantaneous frequency offset Δf is called discriminator curve,

which is shown as graph5.7

Graph5.7

From the above curve we can obtain two important parameters for


FM demodulation process, namely discriminator transconductance

gd and bandwidth B

and B=2Δf in general

Several different circuits for FM demodulation process:

Slope discriminator

Slope discriminator depends on LC parallel resonance

circuit to change FM to AM and uses the

amplitude-frequency curve of resonance circuit. Most

popular circuit is double loop detuning frequency

discriminator which is graph 5.8

graph5.8(a) circuit graph5.8(b) discriminator curve

For this circuit, resonance frequency f1is lower than center

frequency fc in the first loop and resonance frequency f2 is

larger than fc. when fc-f1=f2-fc, we can get the

discriminator curve graph5.8(b).

Phase discriminator

Phase discriminator is based on the phase-frequency

cff

ddf

dug


curve for resonance circuit to change the FM signal to

AM signal.

The principle is shown as graph5.9(a) and the equivalent

circuit is graph 5.9(b)

Graph5.9 (a)

Graph5.9 (b)

We can see from graph5.9(a), there is a inductor coupler

(transformer). Two branches with resistor R and

capacitor C3 and C4 are symmetric.

Ratio discriminator

Ratio discriminator has both discrimination function and

clipping function, which is shown as graph5.10


Graph5.10

PLL-discriminator

A short description of PLL. PLL consists of phase

detector, loop filter, voltage control oscillator

components.

Graph5.11 provides us the basic structure of it.

Graph5.11

Notation: VI is called input signal, VO is called output

signal from VCO, VD is called error voltage from PD,

VC is control voltage for VCO.

From its characteristic curve, we can conclude it works

as a bandpass filter, whose equivalent bandwidth is


The characteristics can be seen from graph5.12

Graph5.12

More details of PLL can be seen from the

material ”learning notes for pll from analog and

telecommunication electronic in polito”. The link is

http://areeweb.polito.it/didattica/corsiddc/01NVD/

which is authored by prof. Dante Del Corso.

FM PLL demodulation is based on the PLL feature. We

can write Fp=Fpo+KM(t).From graph5.12, we can see that

there is full Fp spectrum in the lock range marked by L

and VC(t) is proportional to M(t). Due to the idea, we

can use this structure by letting PLL working at this

state to demodulate FM.

7. PM modulation analysis

As we all know, the phase Φ is the integration of angle frequency

ω therefore, the PM modulation technique is quite similar with FM

http://areeweb.polito.it/didattica/corsiddc/01NVD/


modulation to some degree. We give the definition as following.

PM process is that using modulating signal m(t) control the phase of

carrier, thus the carrier phase variation ΔΦ is proportional to m(t).

According to the definition, PM expression is shown as

and instantaneous phase can be

with kp called proportion coefficient, (t)Δ p called instantaneous

phase shift.

Then, we can drive instantaneous frequency

from above equation,

we can immediately get

like AM and FM, it also has modulation index called mp which is

defined as maximum phase shift ,namely m =

The waveform is show as graph5.13

graph5.13

(t)uktωUu ΩpccmPM cos

(t)Δtω(t)uktω(t) pcΩpc

)()()(

)( tdt

tduk

dt

tdt pcpc

dt

tdukt pp

)()(

maxtΔ p maxtΔ p


Fortunately, we can use FM modulation to realize PM signal, which

means when we do the FM modulation meanwhile we also have

modulated carrier phase. Therefore, we also have some kinds of

circuits to do PM modulation. seeing FM modulation circuit part to

get more details.

In particular for a single large sinusoidal signal, PM is similar to FM,

and its bandwidth is approximately 2(mp + 1)fM, where fM =ωm/2π

and mp is modulation index.

8. The applications of modulation techniques

AM, FM and PM are widely used for radio and radar fields, even

if they are almost replaced by digital field. They are also still

necessary for some certain fields.

Graph 8.1 is the basic structure of BeiDou Navigation

Satellite System，BDS ,designed by China. We can see there

are AM modulation and I-Q demodulation processes in the

transmitter and receiver respectively.


Graph8.1

For considering AM decoder, we can analysis NE567 IC,

whose internal circuit is shown as graph8.2.

Graph8.2

Actually, for this chip, we can use it in the lab, but just get the

amplitude of input signal demodulated at the output. The part of

its datasheet can be seen as following figure


Figure1

The typical characteristics are shown as following serial figures.

stereo FM transmitter

Graph8.3 is a small stereo FM transmitter. Output can be tuned

from 88to 108Mhz and the transmitter can be battery powered.


Graph8.3

This circuit is based on the Rohm BA1404 datasheet. The

maximum voltage should not exceed 3V. The IC can be driven

from a 7805 Regulator with a couple of 1N4001 diodes to reduce

the supply voltage to about 2.8 Volts. The IC can dissipate

500mW (this is not RF power output but power dissipated by the

IC). RF output power is typically 500mW but range depends

upon antenna coupling and efficiency, environment and size of

antenna. A small telescopic whip has an expected range of at

least 100 meters or more.

LM1596/LM1496 Balanced Modulator-Demodulator

The LM1596/LM1496 are doubled balanced

modulator-demodulators, which produce an output voltage


proportional to the product of an input (signal) voltage and a

switching (carrier) signal. Typical applications include

suppressed carrier modulation, amplitude modulation,

synchronous detection, FM or PM detection, broadband

frequency doubling and chopping.

The basic connection can be seen from graph8.4

The typical characteristics are the followings

9. Advanced technique-Software Defined Radio (SDR).


Introduction: Software-defined radio (SDR) is a radio

communication system where components that have been typically

implemented in hardware (e.g. mixers, filters, amplifiers,

modulators/demodulators, detectors, etc.) are instead implemented

by means of software on a personal computer or embedded system.

The basic structure can be seen from graph9.1

Graph9.1

One of a basic component is called software defined antennas,

which adaptively "lock onto" a directional signal, so that receivers

can better reject interference from other directions, allowing it to

detect fainter transmissions. Wireless mesh network where every

added radio increases total capacity and reduces the power required

at any one node. Each node only transmits loudly enough for the

message to hop to the nearest node in that direction, reducing

near-far problem and reducing interference to others.

Since SDR is the advanced knowledge, here there is not enough


descriptions and more details and applications can be seen in the

future.

Nowadays, digital transmission is much more popular than analog

transmission due to the good properties for digital signal, such as

high transmission speed, low error probability, etc. Digital devices

translate and reassemble data and in the process are more prone to

loss of quality as compared to analog devices. Computer

advancement has enabled use of error detection and error correction

techniques to remove disturbances artificially from digital signals

and improve quality. Recalling that cellphones in 40 years ago are

based on analog signal transmission with a lot of noises. That’s the

reason why it is replaced with 2nd generation technique (GSM),

which is based on modulation and demodulation technologies. After

that there exits 3rd generation (3G), which improves the speed of

information transmission dramatically.

10. Reference materials

1. <<Fundamentals of Telecommunications>>, published by

John Wiley & Sons.1999 ISBNs: 0-471-29699-6 (Hardback);

0-471-22416-2 (Electronic).

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

version-2004.

3. Datasheet of LM1596/LM1496 Balanced


Modulator-Demodulator.

4. Datasheet of IC SI4730 based on broadcast AM/FM/SW/LW

radio receiver.

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

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

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

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

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

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

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