analog noise part 3.pdf

Analog

Communication 6/13/2015Analog Communication - NOISE

1

Noise in CW

modulation Systems

NOISE IN AM RECEIVERS USING ENVELOPE DETECTION

NOISE IN FM RECEIVERS

6/13/2015 Analog Communication - NOISE 2

Noise in AM DSB-FC Receivers

6/13/2015Analog Communication - NOISE

3

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)

-

CQCIaCC

0

2

a

2

CAMC,

00

2

a

2

C

CaC

Filtered

W

W

signalAM



4

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

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.


5


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


6

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

7

Threshold effect

Threshold Effect : loss of message in an envelope

detector that operates at a low CNR. 6/13/2015Analog Communication - NOISE

8

Noise in AM DSB-SC Receivers


9



10

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



11

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



12

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

Noise performance of AM

receivers


13

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!

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


14

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


15


FM signal:

Filtered noise n(t):


16

)]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


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.


17


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.


18


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


19



20

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)



21



22



23



24


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

25


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

26

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


27

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.


28

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

29


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.


30


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:


31

this slide is made

and compiled by abhishek kumar SAY THANKS TO HIM

32

Use of pre-emphasis and de-

emphasis in an FM system


33

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

34

Documents

analog noise part 3.pdf