005 Snr Analysis

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



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



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



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.



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



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)?



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.



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.



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



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?



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



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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005 Snr Analysis