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Principles of Communications School of Engineering
Divino Fiel A. de Bien, ECE 1
1 danivy
Principles of Communication
Signal-to-Noise Ratio (SNR)
Engr. Divino Fiel A. de Bien Lecturer
2 danivy
OBJECTIVES 1. State the signal-to-noise ratio
2. Describe noise figure
3. Identify equivalent noise temperature
4. Calculate the S/N, the noise figure and the
equivalent noise temperature of certain
amplifiers
5. Describe techniques for making noise
measurements
Principles of Communications School of Engineering
Divino Fiel A. de Bien, ECE 2
3 danivy
SIGNAL-TO-NOISE RATIO • abbrev. as S/N or SNR, usually expressed in dB
• Ratio of the relative strength of the desired signal power to the noise power at the same point.
• Expresses in decibels the amount by which a signal level exceeds its corresponding noise
• 10dB – for barely intelligible speech
• 90dB or more – for compact-disc audio system
• Minimum requirement S/N ratio in corresponding end instruments:
a. Voice = 30dB
b. Video = 45dB
c. Data = 15 dB
4 danivy
SIGNAL-TO-NOISE POWER RATIO 1. Ideal Case
(S/N)out = (S/N)in
(S/N)In (S/N)out
Principles of Communications School of Engineering
Divino Fiel A. de Bien, ECE 3
5 danivy
SIGNAL-TO-NOISE POWER RATIO 2. Practical Case
(S/N)In (S/N)out
(S/N)out = (Apsin )/(ApN in+Ninternal)
6 danivy
S (dB) = 10 log Ps N PN
S (dB) = 20 log Vs N VN
( S + N ) N
( S + N + D ) N
Principles of Communications School of Engineering
Divino Fiel A. de Bien, ECE 4
7 danivy
NOISE FIGURE - abbrev. as NF or just F
- Is a figure of merit, indicating how much a
component, stage, or series of stages degrades
the signal-to-noise ratio of a system
- Is a measure of the noise produced by a practical
network compared to an ideal network (i.e one that
is noiseless)
8 danivy
Noise Factor - Is equal to the S/N of an ideal system divided by
the S/N at the output of the receiver or amplifier under test, both working at the same temperature over the same bandwidth and fed from the same source.
- Degradation of the signal-to-noise ratio by the network
- Occasionally called Noise Factor
- Is a measure of the noisiness of the input stage. It determines the effect that the internal noise, produced by that stage, has on the received signal.
Principles of Communications School of Engineering
Divino Fiel A. de Bien, ECE 5
9 danivy
Noise Figure - Sometimes the term Noise Ratio, reserving the
term “Noise figure” for decibel form
(S/N)i
(S/N)o NF = 10 log NF NF(dB)=
NF(dB) = (S/N)i dB – (S/N)o dB
10 danivy
A. For a noiseless network
(S/N)i = (S/N)o ;
NF = 1
NF(dB) = 0
B. For a network that contributes some noise
(S/N)i > (S/N)o ;
NF > 1
Principles of Communications School of Engineering
Divino Fiel A. de Bien, ECE 6
11 danivy
EXAMPLES 1. A radio receiver produces a noise power of
200mW with no signal. The output level increases
to 5W when a signal is applied. Calculate (S +
N)/N as a power ratio and in decibels.
2. A transistor amplifier has a measured S/N power
of 10 at its input and 5 at its output. Calculate the
transistors’ noise figure in dB.
12 danivy
EXAMPLES 3. The signal power at the input to an amplifier is
100µW and the noise power is 1µW. At the output,
the signal power is 1W and the noise power is
30mW. What is the amplifier noise figure, as a
ratio?
4. The signal at the input of an amplifier has an S/N
of 42dB. If the amplifier has a noise figure of 6dB,
what is the S/N at the output(in dB)?
Principles of Communications School of Engineering
Divino Fiel A. de Bien, ECE 7
13 danivy
EXAMPLES 5. Consider a receiver with an NF of 10dB. Its output
S/N ratio is 50dB. What is its input equivalent S/N
ratio?
14 danivy
EQUIVALENT NOISE TEMPERATURE - The thermal noise that the device adds to a
system
- It is the absolute temperature of a resistor that,
connected to the input of a noiseless amplifier of
the same gain, would produce the same noise at
the output as the device under discussion.
Principles of Communications School of Engineering
Divino Fiel A. de Bien, ECE 8
15 danivy
EQUIVALENT NOISE RESISTANCE
►
►
S
N i
S
N o
a. Real Noisy Amplifier
S
N i
S
N o Noiseless
Amplifier
Power Gain A
NF = 0dB
Power Gain A
b. Equivalent Noise Resistance and
Ideal Amplifier
16 danivy
ADVANTAGES OF NOISE
TEMPERATURE
1. It is additive, like noise power
2. For low noise levels is that it shows a greater
variation for any given noise-level change than
does the noise figure; changes are easier to grasp
in their true perspective.
Principles of Communications School of Engineering
Divino Fiel A. de Bien, ECE 9
17 danivy
Teq = 290(NF-1)
NF = Teq + 1
290
Where:
Teq – equivalent temperature
NF – Noise figure
18 danivy
NOISE FIGURE – NOISE TEMPERATURE
CONVERSION NFdB Teq(K)approx NFdB Teq(K) approx
15 8950 6 865
14 7000 5 627
13 5500 4 439
12 4300 3 289
11 3350 2.5 226
10 2610 2.0 170
9 2015 1.5 120
8 1540 1.0 75
7 1165 0.5 35.4
Principles of Communications School of Engineering
Divino Fiel A. de Bien, ECE 10
19 danivy
EXAMPLES 1. An amplifier has a noise figure of 2dB. What is its
equivalent noise temperature?
2. Consider a receiver with an equivalent noise
temperature of 290K. What is its NF?
3. A satellite receiving system includes a dish
antenna (Teq=35K) connected via coupling
network (Teq=40K) to a microwave receiver
(Teq=52K).
20 danivy
EXAMPLES What is the noise power to the receivers
input over a 1-MHz frequency range. Determine
the receivers NF.
4. A receiver has an NF of 10dB. What is its
equivalent noise temperature in Kelvin's?
Principles of Communications School of Engineering
Divino Fiel A. de Bien, ECE 11
21 danivy
CASCADED AMPLIFIERS
► ► Stage 1 Stage 2
Gain A1
NF1
Gain A2
NF2
NF1 + (NF2-1) + (NF3 –1) + (NF4-1) + ….
A1 A1A2 A1A2A3 NFT =
FRIIS’ FORMULA:
22 danivy
EXAMPLES 1. A three-stage amplifier has stages with the following
specifications:
Calculate the power gain, noise figure, and noise
temperature for the entire amplifier, assuming matched conditions.
Stage Power
Gain Noise
Figure
1 10 2
2 25 4
3 30 5
Principles of Communications School of Engineering
Divino Fiel A. de Bien, ECE 12
23 danivy
2. Given the parameters for three power amplifiers as indicated below, determine the following:
a. The order in which the amplifiers must be connected.
b. The overall noise figure for the cascaded system.
GA= 12dB
NFA = 2 GB= 6dB
NFB = 1.7 GC= 20dB
NFC = 4
24 danivy
Assignment 1. For an amplifier with an output signal voltage of
4V, an output noise voltage of 0.005V, and an
input and output resistance of 50 ohms, determine
the signal-to-noise power ratio.
2. For a non ideal amplifier and the following
parameters, determine
a. Input S/N ratio (dB)
b. Output S/N ratio (dB)
c. Noise factor and noise figure
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