Transcript Chapter 5(cont)_NOISE

Chapter 6
Noise
Noise is a term generally used to refer
to any undesired disturbances that
mask the received signal in a
communication system.
• Thermal noise
• Shot noise
6.1 Thermal Noise
Also known as Johnson Noise or Nyquist noise
The thermal noise current i in a resistor R may
t
be expressed by its mean square value and is
given by:
2
t
i
4 KTB

R
where K is Boltzmann's constant, T is the absolute
temperature and B is the post-detection bandwidth.
• Electrons within any resistor never remain
stationary and this constitutes a randomly
varying current known as thermal current.
• Motion due to their thermal energy.
SHOT NOISE
• Discrete nature of electrons causes a signal
disturbance called shot noise.
• Deviation of the actual number of electrons
from the average number is known as shot
noise.
• Present for BOTH current: Signal and dark
current.
6.2 Shot Noise due to Dark Current
 When there is no optical power incident on the photodetector a

small reverse leakage current still flows from the device terminals
and this contributes to the total system noise
The shot noise due to the dark current, id is given by:
i  2eBI d
2
d
where e is the charge of an electron and Id is the dark current.
6.3 Shot Noise on the Photocurrent
The shot noise, is on the photocurrent Ip is given by:
i  2eBI p
2
s
6.4 Overall Receiver Noise



Figure 6.1 shows a block schematic of the front end of
an optical receiver and the various noise sources
associated with it.
The majority of the noise sources shown apply to both
main types of optical detector (p-i-n and avalanche
photodiode).
The avalanche photodiode receiver is the most complex
case as it includes noise resulting from the random
nature of the internal gain mechanism (dotted in Fig.
6.1).
Figure 6.1
6.4.1 p-n and p-i-n Photodiode Receiver
 The total shot noise i
2
TS
i
TS
is given by:
 2eB I p  I d 

The thermal noise due to the load resistance RL is
given by:
2
TH
i
4 KTB

RL
 The signal to noise ratio (SNR) for the p-n or p-i-n
photodiode receiver may be obtained by summing all
the noise contributions.
 It is given by:
I p2
S

N 2eB I  I   4 KTB  i 2
p
d
amp
RL
where iamp=total noise from
amplifier circuit
The noise associated with the amplifier, iamp can be combined with
the thermal noise from the load resistor it using the noise figure, Fn
for the amplifier to give:
i i
2
t
2
amp
4 KTBFn

RL
The expression for the SNR can now be written in the form:
2
p
I
S

N 2eB I  I   4 KTBFn
p
d
RL
6.4.2 Receiver Capacitance
 The total capacitance to the front end of an optical receiver is
given by:
CT = Cd + Ca
where Cd is the detector capacitance and Ca is the amplifier
input capacitance.
 Need
to minimize in order to preserve the post detection
bandwidth B. To increase B it is necessary to reduce RL
1
B
2RL CT
Figure 6.4
Equalizer compensates
for distortions



However, a thermal noise penalty is introduced when B is
increased by decreasing RL
A trade-off therefore exists between the maximum
bandwidth and the level of thermal noise which may be
tolerated.
This is especially important in receivers which are
dominated by thermal noise.
6.4.3 Avalanche Photodiode (APD) Receiver
 The internal gain mechanism in an APD increases the signal
current into the amplifier and so improves the SNR.
 However, the dark current and quantum noise are increased by
the multiplication process and may become a limiting factor.
 This is because the random gain mechanism introduces excess
noise into the receiver in terms of increased shot noise above the
level that would result from amplifying only the primary shot
noise.

Thus if the photocurrent is increased by a factor M, then the
shot noise is also increased by an excess noise factor Mx, such
that the total shot noise is is given by:
i  2eB I p  I d M
2
S
2 x
where x is between 0.3 and 0.5 for silicon and between 0.7 and
1.0 for germanium or III-V alloy.

The total SNR for the avalanche photodiode may be obtained as
M 2 I p2
S

N 2eB I  I M 2 x  4 KTBFn
p
d
RL

This can be rewritten:
I p2
S

N 2eB I  I M x  4 KTBFn M  2
p
d
RL



It may be seen that the first term in the denominator increases
with increasing M whereas the second term decreases.
For low M the combined thermal and amplifier noise term
dominates and the total noise power is virtually unaffected
when the signal level is increased, giving an improved SNR.
However, when M is large, the thermal and amplifier noise
term becomes insignificant and the SNR decreases with
increasing M at the rate of Mx.

An optimum value of the multiplication factor Mop therefore exists
which maximizes the SNR.

It is given by:
M
2 x
op
4 KTFn

xeRL I p  I d 

The variation in M, for both silicon and germanium APD is
illustrated in Fig. 6.5.

This shows a plot SNR versus M with Fn equal to unity and
neglecting the dark current.
Figure 6.5
6.5 Receiver Structures
 There are 3 basic configurations for optical receivers:
a) Low Impedance Front End
b) High Impedance Front End
c) Transimpedance Front End
6.5.1 Low Impedance Front End
 Simplest and most common
 Low impedance front end allows thermal noise to
dominate within the receiver
 Impractical for long-haul, wideband optical fiber
communication systems.
Low Impedance Front End
Rb
Ra
6.5.2 High Impedance Front End
 High input impedance amplifier with large detector bias resistor to
reduce thermal noise.
 Degraded frequency response
 Needs equalizer
 Improvement in sensitivity over the low impedance front end
design, but creates a heavy demand for equalization and has
problems of limited dynamic range.
High Impedance Front End
6.5.3 Transimpedance Front End





Overcomes the drawbacks of the high impedance front end by
utilizing a low noise, high input impedance amplifier with negative
feedback.
Operates as a current mode amplifier where the high
input
impedance is reduced by negative feedback (vout = IpRL)
Provides a far greater bandwidth without equalization
than the
high impedance front end.
Has a greater dynamic range.
Preferred for use in wideband optical fiber communication receivers
Transimpedance Front End
Exercise 1:
The bandwidth was 10 MHz. The detected signal power was
2x10-12 W, and the thermal-noise power was 1.66x10-13 W at 300
K. Suppose the the photodetector is followed by an amplifier
giving the power gain 10 dB and having the noise temperature
454 K. Compute the SNR.
Exercise 2:
A 1-Mbps NRZ link uses a 100Ω load at 300 K. The wavelength is
0.82 µm, and the desired error rate is 10-4. The PIN detector
quantum efficiency is unity. Compute the optic power incident on
the photodetector.
Given that;