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PFIEV, Systems, pdf
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October 2013Lectured by
Assoc Prof. Thuong Le-Tien
Slides with references from HUT Finland, Mc. Graw Hill Co., and A.B. Carlson’s Communication Systems book
ANALOG MODULATION SYSTEMS
Analog Carrier Wave Systems
Carrier wave techniques form a bases for telecommunication systems
Topics today in CW-applications:
– Single conversion radio receiver
• FM radio (analog) stereo multiplexing
– Measurement equipment
• Spectrum analyzer
– Multiplexing techniques
• Frequency Division Multiplexing (FDM)
• Quadrature-carrier multiplexing
– Phase-locked loop (PLL)
• FM-demodulator
• frequency synthesis
• Costas loop
Heterodyne Single-conversion receiver
Assume reception of a bandpass signal
Multiplication at the receiver with the local oscillator signal having frequency of fLO yields signals at two CW-bands
Therefore, IF can be selected asor LO can be selected as
( ) ( )cos ( )c c
x t A t t t
( ) ( ) ( )
( )cos( )cos ( )
( )cos ( ) / 2 ( )cos ( ) / 2
IF LO c
LO c
LO c LO c
x t x t x t
A t t t t
A t t t A t t t
IF LO cf f f
LO c IFf f f
Mirror frequency
Select for IF for instance
For the reason that cosine is even function there are two frequency bands that convert to intermediate frequency namely
This means that both bandpass signals at the received frequencies are converted to the intermediate frequency.
Example: Assume we set
therefore receiver picks signals at the bands of
However, this is usually not wanted, and the other band must be filtered away by the first bandpass filter at the receiver
( )cos ( ) / 2m LO c
A t t t
110MHz, 10MHzLO IFf f
'
' ,
IF LO c LO c c LO
c LO IF c LO IF
LO IF
110MHz 10MHz
= 120MHz 100MHz
c LO IFf f f
Filtering mirror frequencies (image rejection filtering)
(selected)
' 2 (see the figure)
IF LO C
C LO IF
C C IF
f f f
f f f
f f f
*Should pass the messagebut not the mirror image
*
2 IFf
2T RF IFB B f
Single conversion - basic characteristics
SC can be used with all CW methods
The RF stage provides image rejection
The IF stage provides gain and interference rejection
– note that the fractional BW= BT/fIF is selected by adjusting fIF Remember from the second lecture that system design is easier
if the fractional bandwidth is kept relatively small: For analog FM broadcasting:
when it was required
Tuning of the receiver to a desired band is easy by adjusting the local oscillator. (Often BRF is selected to be so wide and fLO so high that the first bandpass filter (amplifier) center frequency requires no tuning, as usually in FM radios)
0 01 0 10
. / . B f
/ 200kHz /10.6MHz 0.02IF IF
B f
Example of Frequency ConversionScanning spectrum analyzer
VCO, BRF and BIF filters form together a scanning bandpass filter (SBF)
Ramp generator takes care of sweeping SBF
After the IF filter the envelope detector yields signal whose power is comparable to the power that has passed the SBF
Sweep rate and BIF determine system resolution. High resolution->small BIF and sweep rate as discussed soon
When larger sensitivity is desired sweep rate must be decreased
Spectrum analyzer includes often integrator (or averaging function) to improve SNR via inclusion of multiple sweep data
BRFBIF
Frequency-division multiplexing (FDM)
FDM receiverFirst the FDM wave is demodulated. Then each subcarrieris detected by using separate bandpass filters anddetectors.
AT & T FDM hierarchy in PSTN
voice channel
Advanced FDM: xDSL with OFDM Conventional FDM:
– Each channel occupies accurately certain frequency band
– Bandwidth efficiency increased by using SSB modulation
– Usage of guard bands wastes resources
– A lot of filtering functions (complex circuitry)
Modern FDM: OFDM (orthogonal frequency division multiplexing) and DMT (discrete multitone modulation) yield increased spectral adaptation. Applied in xDSL (digital subscriber line techniques).
DMT with cable attenuation only
DMT with cable attenuation, interferenceand cross-talk
rejected sub-band
FM stereo multiplexing (MPX-system) The MPX encoder utilizes various linear modulation methods
L+R and L-R signals are transmitted on different channels
SCA (Subsidiary Communication Authorization) is used to transmit background music for selected subscribers
FM stereo decoder System is based on detecting both L+R and L-R signals from
which the R and L can be calculated
Compatibility to mono-phonic transmission is granted by using the unmodulated L+R and DSB modulated L-R signal at 23-53 kHz that is automatically filtered out in mono-phonic reception
Quadrature-carrier multiplexing
Two signals x1 and x2 are transmitted via same channel
Signals can be analog or digital CW or baseband signals (QPSK, DSB, SSB ...)
Task: show that the signals x1 and x2 can be detected independently at the receiver!
x t A x t t x t tC C C C( ) ( )cos( ) ( )sin( )
1 2
( )Cx t
Quadrature-carrier reception
In order to detect the x1 component multiply by the cos-wave:
In order to detect the x2 component multiply by sin-wave:
Note
– Second-order frequency must be filtered away
– The local oscillator must be precisely in-phase to the received signal, otherwise cross-talk will follows
cos( ) ( )cos( ) ( )sin( )
( ) cos( ) / ( )sin( ) /
C C C
C C
t x t t x t t
x t t x t t
1 2
1 21 2 2 2 2
sin( ) ( )cos( ) ( )sin( )
( ) cos( ) / ( )sin( ) /
C C C
C C
t x t t x t t
x t t x t t
1 2
2 11 2 2 2 2
Phase-locked loops (PLLs)
Phase-locked loop is a feedback arrangement capable to synchronize itself to a noisy external reference
The output signals of the loop can be used to produce for instance multitude of locked frequencies
PLL application areas include...
– modulators
– demodulators
– frequency synthesis
– multiplexers
– signal processors
The PLL principle
The PLL circuit consists of
– phase comparator (in the figure below the multiplier)
– lowpass filter
– feedback amplifier
– VCO (voltage controlled oscillator), whose output frequency is linearly proportional to input amplitude
Principle: phase difference of Xc(t) and v(t) adjusts VCO
Phase comparator output iscomparable to phase difference of input signals
PLL phase comparator realizations
Circuits: (a) analog and (b) digital phase comparator circuit
Note that for (a) output is proportional to
– input signal phase difference
– input signal amplitudes (unintended AM thus harmful)
In (b) AM effects are compensated and response is more linear
XOR-circuit
1 1sin( cos( ) sin( ) sin( )2 2
a
ideal
pulse ratio: 50/50
FM detection by PLL
( ) 2 ( )tv v
t K y t dt
time domain
phase domain
frequency domain
( )( )
( ) ( )
v
t
v
d tt
dt
t d
1 1( ) ( ) (0) ( )22
t
v d V f V fj f
sin ( ) ( ) ( ) ( ) v
t t t t
PLL FM-demodulator: the feedback analysis
1
1 2
( )( ) ( )
1 ( ) ( )
H fY f X f
H f H f
( )( ) ( )
1 ( ) /
1 ( )( )
( )
( )
a
a v
v
a v
K H fY f f
K H f K jf
jfKH ff
K jf KH f
K K K
Solve transfer function with feedback:
This is applied to the linearized PLL yielding relationship between the input phase and output voltage:
2 1( ) ( ) ( ) ( ) ( )Y f X f H f Y f H f
1 2 1( ) ( ) ( ) ( ) ( ) ( )Y f H f H f Y f X f H f
( )Y f
Applying the FM signal to the linearized PLL model
Remember the FM wave:
where the modulating signal is denoted by x(t). The input FM phase to the system is thus
This is in frequency domain:
assuming no DC component or V(0) = 0, or
( ) / 2 ( )d t dt f x t
( ) 2 ( )t
t f x d
( ) 2 ( ) /( )f f X f j f
0
1 1( ) ( ) (0) ( )22t
v d V f V fj f
Applying FM signal to the detector... (cont.)
Thus the input is and the output is
where the loop equivalent transfer function is
Assume that the first order LP function is used or
( ) ( ) /( )f f X f jf
1 ( ) ( )( ) ( ) ( )
( )L
v v
jfKH f f X fY f f H f
K jf KH f K
( )( )
( ) ( / )L
H fH f
H f j f K
1( )
1 ( / )L
H fj f K
( )( ) ( ), 1
1 ( / )v v
f fX f WY f X f
K j f K K K
( ) ( )v
fy t x t
K
Y(f)
a vK K K
PLL based frequency synthesizer
VCOVCO
Filt.Filt.Phasedetector
Phasedetector
Divide by10
Divide by10
10out inf f
inf
By adjusting thedivider differentfrequencies can be producedwhose phase is locked into fin
Reference signal finis locked for instanceto the fundamental frequency of a crystal oscillator
Detecting DSB using PLL-principle
An important application for PLLs is in synchronization of receiver local oscillator in synchronous detection
In the Costas PLL (below) two phase discriminators are used to:
– cancel out DSB modulation x(t) in the driving signal
– synchronize the output frequency to the center frequency of the DSB spectra (the suppressed carrier)
– to detect the DSB signal
Cos
tas
PL
L d
etec
tor
for
DS
B
PD: phase detector (=multiply+LPF)
Loop drives phase error to zero
LPF yields constant (zero) output when loop is locked to carrier
1sin cos sin 2 sin 02ss ss ss ss
Frequency synthesizer with fixed and adjustable outputs
Linearized PLL models (a) time domain (b) phase (c) frequency domain
Monochrome
TV transmitter
Monochrome TV receiver
The aspect ratio
Color subcarrier modulation system
The chrominance signal
The luminance signal
Color demodulation system
Color vector
Scene capabilities of conventional NTSC system and HDTV
Viewing angles as a function of distance (a) conventional NTSC (b) HDTV
HDTV transmitter
HDTV receiver