analog noise part 3.pdf
TRANSCRIPT
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Analog
Communication 6/13/2015Analog Communication - NOISE
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Noise in CW
modulation Systems
NOISE IN AM RECEIVERS USING ENVELOPE DETECTION
NOISE IN FM RECEIVERS
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Noise in AM DSB-FC Receivers
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t)f(t)sin(2n-t)f(t)]cos(2nm(t)kA[A
n(t) s(t) x(t)
signal -
N2
P)k(1A (SNR)
)2
N(2W N power noise Average -
2P)k(1A power signal Average -
t)fm(t)]cos(2k[1A s(t)
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CQCIaCC
0
2
a
2
CAMC,
00
2
a
2
C
CaC
Filtered
W
W
signalAM
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Noise in AM DSB-FC Receivers
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1Pk1
Pk
)(
)(merit of Figure
1 k
power noise Avg power carrier Avg
N2
PkA (SNR) -
(t)nm(t)kAAy(t)
(t)n(t),n m(t)]k1[A
(t)n(t)]nm(t)kA[A
x(t)of envelop )(
2
a
2
a
a
0
2
a
2
CAMO,
IaCC
QIaC
2
12
Q
2
IaCC
AMC
O
SNR
SNR
W
Assume
ty
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Threshold effect
The threshold is a value of carrier-to-noise ratio
below which the noise performance of ademodulator deteriorates much more rapidly
than proportionately to the carrier-to-noise ratio.
Every noncoherent detector exhibits a threshold
effect, below the threshold the restored
message signal becomes practically useless.
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Noise in AM DSB-FC Receivers
Figure of merit for DSB modulation:
where P denotes the average power of messagesignal m(t) and ka is the amplitude sensitivity of AMmodulator.
The best figure of merit is achieved if the modulationfactor is = kaAm = 1
DSB system using envelope detection must transmitthree times as much average power as asuppressed-carrier system
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Threshold effect
Physical explanation:
If the carrier-to-noise ratio is high enough then
the signal dominates and the noise causes only
a small unwanted AM and PM.
However, if the carrier-to-noise ratio is small
then the noise dominates which results in a
complete loss of information.
As a result, the demodulator output does not
contain the message signal at all.6/13/2015Analog Communication - NOISE
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Threshold effect
Threshold Effect : loss of message in an envelope
detector that operates at a low CNR. 6/13/2015Analog Communication - NOISE
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Noise in AM DSB-SC Receivers
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Noise in AM DSB-SC Receivers
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NW2
PAC)SNR( -
(baseband)
NW2
N2W power noise Average-
2
PAC s(t) of power Average-
df (f)S P
power signal Average-
bandwidth message : W
(f)S : m(t) ofdensity spectral Power
factor scaling :C re whe
)t(m)tf2cos(CA)t(s -
0
2C
2
DSB,C
00
2C
2
WW- M
M
CC
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Noise in AM DSB-SC Receivers
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Finding (SNR)O
)t(n2
1)t(mCA
2
1 y(t)
)tf4sin()t(nA2
1)tf4cos()t(n)t(mCA
2
1)t(n
2
1)t(mCA
2
1
)tf2cos()t(x)t(v -
)tf2sin()t(n)tf2cos()t(n)t(m)tf2cos(CA
)t(n)t(s)t(x -
IC
CQCCICIC
C
CQCICC
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Noise in AM DSB-SC Receivers
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Finding (SNR)O
1)SNR(
)SNR(
merit of Figure
NW2
PAC
2NW
4PAC)SNR( -
NW2n(t) noise filtered pass band of Power(t))Power(n
(passband) NW2
1(2W)N
4
1 power noise Average-
4
PAC power signal Average-
SCDSBC
O
0
2C
2
0
2C
2
O
0I
00
2C
2
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Noise performance of AM
receivers
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Note: For high value of (SNR)C, the noise performance of
coherent and noncoherent DSB are identical. But
noncoherent DSB has a threshold effect. Coherent AM
detectors have no threshold effect!
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Comparison of noise performance of
AM modulation schemes
Remarks
Curve I: DSB modulation and envelope detector with modulation factor = 1
Curve II: DSBSC and SSB with coherent demodulator
Note the threshold effect that appears at about 10 dB
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Noise in FM Receivers
w(t): zero mean white Gaussian noise with PSD = No/2
s(t): carrier = fc, BW = BT
BPF: [fC - BT/2 - fC + BT/2]
Amplitude limiter: remove amplitude variation.
Discriminator
Slope network : varies linearly with frequencyEnvelope detector
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Noise in FM Receivers
FM signal:
Filtered noise n(t):
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)]t(tf2cos[A)t(s
dt)t(mk2)t(
]dt)t(mk2tf2cos[A)t(s
CC
t0f
t0fCC
)t(n
)t(ntan)t(
))t(n())t(n(r(t)
where
)]t(tf2r(t)cos[
)tf2sin()t(n)tf2cos()t(n)t(n
I
Q1
2Q
2I
C
CQCI
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Noise in FM Receivers
Phasor diagram interpretation of noisy demodulatorinput:
Due to the PM generated by the noise, noiseappears at the demodulator output.
But the FM demodulator is sensitive to theinstantaneous frequency of the input signal.
The instantaneous frequency is the first derivative ofthe phase of input signal.
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Noise in FM Receivers
The instantaneous frequency is the first derivative ofthe phase of input signal.
Derivation in the time domain corresponds tomultiplication by (j2f) in the frequency domain.
Multiplication by (j2f) means that the frequencyresponse of derivation is:
Recall, power spectral density of the output processequals to the PSD of the input process multiplied bythe squared magnitude of the frequency responseH(f) of the LTI two-port.
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Noise in FM Receivers
Recall, power spectral density of the output processequals to the PSD of the input process multiplied by thesquared magnitude of the frequency response H(f) of theLTI two-port.
Therefore, the PSD SN0(f) of noise at an FM receiveroutput has a square-law dependence on the operatingfrequency.
The high-frequency noise is dominant at the output of anFM receiver
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Noise in FM Receivers
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Discriminator output
dt
)t(dn
A2
1)t(n
)]}t(sin[)t(r{dt
d
A2
1
)]}t()t(sin[)t(r{dt
d
A2
1)t(n
where
)t(n)t(mkdt
d
2
1)t(v
Q
Cd
C
Cd
df
(t)
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Noise in FM Receivers
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Noise in FM Receivers
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Noise in FM Receivers
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Noise in FM Receivers
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Noise in FM Receivers
Figure of merit for frequency modulation
If the (SNR)C is high enough and exceeds the
threshold level then:
where P denotes the average power of message
signal m(t), kf is the frequency sensitivity of FM
modulator and W denotes the bandwidth of
message signal.
Since:
and for wide-band FM we may write:6/13/2015Analog Communication - NOISE
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Noise in FM Receivers
Figure of merit for frequency modulation
we obtain by substituting:
where is the Modulation Index.
Note: An increase in the transmission bandwidth BTprovides a corresponding quadratic increase in theoutput signal-to-noise ratio (or in the figure of merit) ofthe FM system. 6/13/2015Analog Communication - NOISE
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FM threshold effect
The figure of merit discussed above is valid only ifthe carrier-to-noise ratio (SNR)C is high comparedwith unity.
It has been found experimentally that as (SNR)C isdecreased below a threshold, each FMdemodulator, either coherent or noncoherent,breaks:
At first isolated clicks are heard and if the (SNR)C is decreased further, the clicks rapidly merge into a crackling. sound
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FM threshold effect
A qualitative explanation
If (SNR)C is small then the noise becomes dominantand the phasor representation and thedecomposition of noise into a PM and AM are notvalid any more.
The phase of noise is a random variable and it maytake any value.
Recall, the FM demodulator is sensitive to thederivate of phase.
When the phase of demodulator input variessuddenly by 2 due to the noise then an impulse,i.e., click appears at the receiver output.
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Pre-emphasis and de-emphasis in
FM systems Recall: The power spectral density SN0(f) of noise at an FM
receiver output has a square law dependence on theoperating frequency.
The high-frequency noise is dominant at the output of an FMreceiver.
The power spectral densityof message signals usuallyfalls off at higherfrequencies.
Generally, the most part ofa message signal is in thelow-frequency region.
These facts may beexploited to improve thenoise performance of FMsystems 6/13/2015Analog Communication - NOISE
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Pre-emphasis and de-emphasis in
FM systems Basic idea
Apply a filter at the demodulator output whichreduces the high frequency content of the outputspectrum.
To compensate this attenuation, a pre-emphasis mustbe applied to the high-frequency signals at thetransmitter
Pre-emphasis at the transmitter:
A filter that artificially emphasize the high-frequencycomponents of the message signal prior to themodulation.
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Pre-emphasis and de-emphasis in
FM systems De-emphasis at the receiver:
An inverse operation performed by a filter placed
after the demodulation.
The de-emphasis filter restores the original signal by
de-emphasizing the high-frequency components.
Effects of pre-emphasis and de-emphasis filters cancel
each other:
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this slide is made
and compiled by abhishek kumar SAY THANKS TO HIM
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Use of pre-emphasis and de-
emphasis in an FM system
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Comparison of noise performance of
CW systems
Note: Threshold problem is more serious in FM modulation
than in AM. The higher the , the better the FM noise performance. But the price to be paid is the wider
transmission bandwidth6/13/2015Analog Communication - NOISE
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