Principles of
Communication
dB
OPERATIONS
and NOISE
Mark Jayson Briones,
ECE, ECT
dB Operations
dB CALCULATIONS
DECIBEL
A means of
expressing
change in
power level
Comparison of
two powers and
does not
express a fixed
value unless it
refers to dB
above or below
some specific
reference point
dB CALCULATIONS
Change in POWER
expressed in Decibels
PdB = 10 log (Pout/Pin)
Change in VOLTAGE
expressed in Decibels
VdB = 20 log Vout/Vin ;
Change in CURRENT
expressed in Decibels
IdB = 20 log Iout/Iin ;
(if R2=R1)
(if R2=R1)
POWER with RESPECT to SPECIFIC
REFERENCE POWER LEVEL
dBk = 10 log (Plevel/1x103 W)
dBw = 10 log (Plevel/1 W)
dBm = 10 log (Plevel/1x10-3 W)
dBu = 10 log (Plevel/1x10-6 W)
dBn = 10 log (Plevel/1x10-9 W)
dBp = 10 log (Plevel/1x10-12 W)
dBf = 10 log (Plevel/1x10-15 W)
SAMPLE QUESTIONS
1. How much in decibels is 125 W larger than 45 W?
a. 2. 78 dB
b. 4.44 dB
c. 27.8 dB
d. 44.4 dB
2. Determine the attenuation in decibels for the
attenuator having an input power of 100mW and an
output power of 3.5 mW.
a. 0.035 dB
b. -1.46 dB
c. 14.6 dB
d. -14.6 dB
SAMPLE QUESTIONS
3. Express 38 dB as a voltage ratio.
a. 6309
b. 79.43
c. 158.87
d. 63.09
4. Express 425 mW as dBm.
a. 2.63 dBm
b. 26.3 dBm
c. -2.63 dBm
d. -26.3 dBm
dB OPERATIONS
dB1 + dB2 = dBT
dB + dBm = dBm
dB + dBu = dBu
dB + dBk = dBk
dBm1 + dBm2
Invalid operation
dBw1 + dBw2
Note: convert power
levels into watts and
perform the operation
dBk + dBw
SAMPLE QUESTIONS
1. What is the power output of a 3-stage amplifier having gains equal
to 25 dB, -35 dB and 50 dB respectively, if the input power is 10 mW?
a. 100 W
b. 100 mW
c. 10 W
d. 10 mW
2. The input to a three-stage system is -12 dBm. The power gain of
the first stage is 40 dB, for the second stage a loss of 35 dB, and for
the third stage, a gain of 25 dB. What is the power output in dBm?
a. -18 dBm
b. 18 dBm
c. 36 dBm
d. -36 dBm
SAMPLE QUESTIONS
3. What is the difference between -18 and 22 dBm?
a. 40 dB
b. 40 dBm
c. 4 dB
d. 4 dBm
POWER EXPRESSED in
NEPERS
Power can also be defined in
terms of the natural
logarithm to the base
e=2.718281828.
PNeper = ½ ln (Pout/Pin)
When
defined as
the natural
logarithm,
the unit is
Neper.
1 Neper = 8.686 dB ; 1 dB = 0.1151 Neper
REVIEW QUESTIONS
1. The correct symbol for decibel?
a. DB
b. dB
c. db
d. Db
2. A voltage change that is equal to twice its original
value corresponds to a change of
a. 3 dB
b. 6 dB
c. 9 dB
d. 30 dB
REVIEW QUESTIONS
3. Which of the following is not an actual
amount of power?
a. dB
b. dBm
c. dBw
d. dBk
4. What does a power difference of – 3 dB mean?
a. a loss of one third of the power
b. a loss of one-half of the power
c. a loss of 3 watts of power
d. a noise figure value of 3
REVIEW QUESTIONS
5. A gain of 60 dB is the same as a gain of
a. 10 volts/volt
b. 100 volts/volt
c. 1000 v/v
d. 10000 v/v
6. A network has a loss of 20 dB. What power ratio
corresponds to this loss?
a. 0.01
b. 0.1
c. 10
d. 100
REVIEW QUESTIONS
7. The input current of a network is 190 uA and
the output is 1.3 uA. The loss in dB is
a. 20.2
b. 21.6
c. 28.6
d. 43.3
8. What is the gain in dB if the output to input
ratio is 1000.
a. 20
b. 30
c. 40
d. 10
REVIEW QUESTIONS
9. What is the equivalent output of a circuit in dBm if
it has an output of 10 watts?
a. 10 dBm
b. 30 dBm
c. 20 dBm
d. 40 dBm
10. If four networks connected in series have gains of 0.5 dB, -0.3 dB, -2 dB and 6.8 dB, the overall gain is
a. 2 dB
b. – 2 dB
c. 4 dB
d. – 4 dB
REVIEW QUESTIONS
11. Determine the attenuation in dB for the attenuator having
an input power of 100 mW and an output power of 3.5 mW.
a. 0.035 dB
b. – 1.46 dB
c. 14.56 dB
d. – 14.56 dB
12. If you have available number of power amplifiers with a
gain of 100 each, how many such amplifiers do you need to
cascade to give an overall gain of 60 dB?
a. 20
b. 3
c. 4
d. 5
REVIEW QUESTIONS
13. A network has an input of 75 dB and an
output of 35 dB. The loss of the network is
a. – 40 dB
b. 40 dB
c. 40 dBm
d. – 40 dBm
14. The input is 0.1 W and the network gain is 13
dB, the output is
a. 2 W
b. 2.5 W
c. 1.5 W
d. 1.8 W
REVIEW QUESTIONS
15. The input is 1 W and the network loss is 27 dB,
the output is
a. 1 nW
b. 3 mW
c. 2 mW
d. 4 mW
16. A combiner has two inputs of +30 dBm each, what
is the resultant output?
a. 36 dBm
b. 30 dBm
c. 60 dBm
d. 33 dBm
REVIEW QUESTIONS
17. The input power to a loss free cable is 5 W. If the
reflected power is 7 dB down on the incident power, the
output power to the load is
a. 4 W
b. 5 W
c. 6 W
d. 7 W
18. If an electronic network has an input of 40 W and
an output of 40 W, the gain is
a. 0 dB
b. 1 dB
c. 6 dB
d. 12 dB
REVIEW QUESTIONS
19. A filter has a voltage gain of -12 dB. If the input
voltage is 48 volts, the output voltage is
a. 8 volts
b. 10 volts
c. 12 volts
d. 24 volts
20. A theoretical antenna has a gain of 1 dB. It’s gain
in nepers is
a. 8.686
b. 0.1151
c. 6.868
d. 0.5111
Noise
NOISE
Any unwanted form of electrical energy,
usually random or aperiodic in character,
which tends to interfere with the proper and
easy reception and reproduction of
transmitted signals.
DISTORTION and
INTERFERENCE
DISTORTION
INTERFERENCE
Any waveform
perturbation or
deviation
caused by the
imperfect
response of the
system to the
desired signal
Contamination by
extraneous or
external signals from
human sources,
other transmitters,
power lines,
machinery,
switching circuits,
and related sources
RESULTS OF NOISE
hiss - loudspeakers
snow or confetti – TV CRT
bit error – digital transmission
GENERAL CATEGORIES
of NOISE
CORRELATED
NOISE
UNCORRELATED
NOISE
unwanted electrical
energy that is
present as a direct
result of a signal,
such as harmonic
and intermodulation
distortion.
noise present
regardless of
whether there is a
signal or none.
CORRELATED NOISE
INTERMODULATION
DISTORTION
HARMONIC
DISTORTION
unwanted crossproduct (sum and
difference)
frequencies created
when two or more
signals are
amplified in a nonlinear device.
unwanted multiples
of a single
frequency created
when amplified in
non-linear device
% THD = Vhigher/Vfundamental
UNCORRELATED
NOISE
EXTERNAL
noise created outside
the receiver and
allowed to enter the
circuit
INTERNAL
noise created within the
receiver or the device.
EXTERNAL
NOISE
ATMOSPHERIC NOISE (STATIC)
caused by lightning discharges
in thunderstorms and other
natural electric disturbances
occurring in the atmosphere.
less severe at frequencies
above 30 MHz
EXTERNAL NOISE
EXTRATERRESTRIAL NOISE (SPACE NOISE)
Severe at
10 MHz –
1.45 GHz
EXTRATERRESTRIAL
NOISE
SOLAR NOISE
Constant noise radiation from
the sun (6000°C). Due to
electrical disturbances due to
solar cycle activities that
repeat every 11 years and
supercycles every 99 years
EXTRATERRESTRIAL
NOISE
COSMIC NOISE (THERMAL,
BLACKBODY, GALACTIC)
Noise radiated from
distant stars and other
heavenly bodies
EXTERNAL
NOISE
INDUSTRIAL NOISE (MAN MADE
NOISE)
Noise coming from automobile and
aircraft ignition, electric motors
and switching equipment,
fluorescents, leakage from high
voltage lines and a multitude of
other heavy electric machines.
Severe at 15 – 160 MHz
INTERNAL NOISE
THERMAL AGITATION NOISE
White, Gaussian, Johnson Noise
Due to the rapid and random motion of the
molecules, atoms and electrons of which any
component (resistor) made of.
INTERNAL NOISE
R
Noise Power
Noise Voltage
Pn=kTB
Vn=√4kTBR
Vn
INTERNAL NOISE
In
R
Noise Current In=√4kTBG
INTERNAL NOISE
Noise Bandwidth and Noise
Voltage of RC Network
Bn = 1/4RC
fc=1/2piRC
R
Vn = √(kT/C)
Vn
C
SAMPLE QUESTIONS
1. An amplifier operating over a 4 MHz BW has a 100
ohm input resistance and is operating at 300 deg K.
Determine the noise power generated.
a. 1.656 x 10 exp -13 W
b. 16.56 fW
c. 1656 nW
d. 1.656 pW
SAMPLE QUESTIONS
2. An amplifier with an input resistance of 1000 Ω is
operating over a 4 MHz bandwidth. Calculate the rms
noise voltage if the amplifier is operating at 27 C.
a. 8.14 nV
b. 8.14 uV
c. 6.6 nV
d. 6.6. uV
SAMPLE QUESTIONS
3. Determine the equivalent noise bandwidth for a
single RC low pass filter if R = 20 kΩ and C =
0.1uF.
a. 125 Hz
b. 250 Hz
c. 125 MHz
d. 250 MHz
INTERNAL NOISE
SHOT NOISE
Random variations in the arrival
of electrons (or holes) at the
output electrode of an amplifying
device
When amplified, it is supposed to
sound as though a shower of lead
shot were falling on a metal sheet.
in=√(2eipB)
SAMPLE QUESTIONS
1. Determine the noise current for a diode with a
forward bias of 3.5 mA over a 100 kHz BW.
a.10.58 nA
b. 0.335 nA
c. 21.2 nA
d. 5.6 nA
2. Determine the open circuit noise current in 1 Hz of
bandwidth at 290 K for a diode biased at 1mA.
a. 1.78 pA
b. 17.88 pA
c. 1.78 nA
d. 17.88 nA
INTERNAL NOISE
TRANSIT TIME NOISE
High frequency noise resulting from delayed
travel of electrons from emitter-collector of a
vacuum tube or transistor
MISCELLANEOUS
NOISE
a. FLICKER NOISE (MODULATION
NOISE, PINK NOISE)
Low frequency noise from carrier
density fluctuations. It is
proportional to emitter current
and junction temperature,
inversely proportional to frequency
(negligible above 500 Hz)
MISCELLANEOUS
NOISE
b. RESISTANCE NOISE
Due to the base (greatest
contributor), emitter and
collector internal resistance
c. NOISE in MIXERS
Low transconductance of mixers
compared to amplifiers and
inadequate image frequency
rejection.
MISCELLANEOUS
NOISE
d. CROSSTALK
Interference signal from one
channel to another
e. AVALANCHE NOISE
Large noise spikes due to
collision that result in
avalanching action
MISCELLANEOUS
NOISE
f. BURST NOISE (POP-CORN)
Low frequency noise observed in BJT
which appears as a series of burst of
two or more levels.
g. PARTITION NOISE
Random fluctuations in the division
of current in two or more electrodes.
MISCELLANEOUS
NOISE
h. IMPULSE NOISE
Non-continuous, consisting of
irregular pulses or noise spikes of
short duration and of relatively high
amplitude. The spikes are called
HITS.
NOISE ANALYSIS and
CALCULATIONS
ADDITION of NOISE due to SEVERAL
SOURCES:
Vn total = √4kTBRt
RESISTORS IN SERIES:
Rt = R 1 + R 2 + R 3 + . . .
RESISTORS IN PARALLEL:
Rt = 1/(1/R1 + 1/R2 + 1/R3 + . . .)
NOISE ANALYSIS and
CALCULATIONS
A1
ADDITION of
NOISE due to
SEVERAL
AMPLIFIERS
in CASCADE:
R1
Vn1
A2
R2
Vn2
R3
Vn3
When a noise resistance is transferred
from the output of a stage to its input, it
must be divided by the square of the
voltage gain of that particular stage.
NOISE ANALYSIS and
CALCULATIONS
ADDITION of NOISE due to SEVERAL
AMPLIFIERS in CASCADE:
Vn total = √4kTBReq
Req = equivalent noise resistance as referred to
the input
Req = R1 + R2/A12 + R3/A12 A22
SAMPLE QUESTIONS
1. Two resistors, 20 KΩ and 50 kΩ at ambient temperature.
Calculate, for a bandwidth equal to 100 kHz, the thermal
noise voltage for the two resistors connected in parallel.
a. 0.4782 uV
b. 4278 uV
c. 4.78 uV
d. 47.8 uV
2. Calculate the noise voltage at the input of a TV RF
amplifier using a device that has a 200-Ω equivalent noise
resistance and a 300-Ω input resistor. The bandwidth of the
amplifier is 6 MHz and the temperature is 17 C.
a. 6.93 nA
b. 6.93 uA
c. 69.3 nA
d. 69.3 uA
SAMPLE QUESTIONS
3. The first stage of a 2-stage amplifier has a voltage
gain of 10, a 600-Ω input resistor, a 1600-Ω equivalent
noise resistance and a 27-KΩ output resistor. For the
second stage, the values are 25, 81 KΩ, 10 KΩ and 1
MΩ, respectively. Find the equivalent noise resistance
of this 2 stage amplifier.
a. 2518 Ω
b. 816 Ω
c. 620 Ω
d. 914 Ω
NOISE ANALYSIS and
CALCULATIONS
SIGNAL-to-NOISE RATIO (S/N)
Relative strength of the signal power to
the noise power at the same point
S/N = (signal power/noise power)
= (Ps/Pn) ; power ratio
S/N = (signal voltage/noise voltage)2
= (Vs/Vn)2 ; voltage ratio
NOISE ANALYSIS and
CALCULATIONS
NOISE FACTOR (F) and NOISE FIGURE (NF)
The figure of merit that indicates the
degradation in the signal-to-noise ratio
as the signal propagates through an
amplifier or a communications system.
NOISE ANALYSIS and
CALCULATIONS
a. NOISE FACTOR from SIGNAL to NOISE
RATIO
F = (input S/N) / (output S/N)
NFdB = 10 log F
NOISE ANALYSIS and
CALCULATIONS
b. NOISE FACTOR from
EQUIVALENT NOISE RESISTANCE
Generator (antenna)
Amplifier (receiver)
Av
Ra
Vi
Rt
Rt
Vn
F = 1 + (Req’/Ra)
Req’ = Req - Rt
Vo
NOISE ANALYSIS and
CALCULATIONS
c. Noise Factor in Terms of Equivalent
Noise Temperature
F = 1 + (Teq/T0)
Teq = T0 (F – 1)
SAMPLE QUESTIONS
1. The signal in a channel is measured to be 23 dB
while noise in the same channel is measured to be 9
dB. The signal to noise ratio is
a. 9/23
b. 23/9
c. 32 dB
d. 14 dB
2. What is the S/N at the output of an amplifier whose
F = 10 dB and the input S/N = 25 dB?
a. 35 dB
b. 15 dB
c. 25 dB
d. 10 dB
SAMPLE QUESTIONS
3. The equivalent noise temperature of the amplifier
is 25 K. What is the noise figure?
a. 10.86
b. 1.086
c. 0.1086
d. 1.86
4. A receiver is connected to an antenna whose
resistance is 60 Ω has an equivalent noise resistance
of 40 Ω. Calculate the receiver’s noise figure in
decibel and its equivalent noise temperature.
a. 1.67 and 194 K
b. 2.23 and 194 K
c. 1.67 and 174 K
d. 2.23 and 174 K
NOISE ANALYSIS and
CALCULATIONS
CARRIER to NOISE RATIO (C/N)
Is the ratio of the wideband carrier to
the wideband noise power
The signal-to-noise ratio measured
before the signal is demodulated, that
is, while it still has a carrier.
NOISE UNITS and
LEVELS
Test tone
A pure signal at a signal frequency and
power level
Usual reference 1000 Hz at 1 mW applied
at a point of 0 relative level
Weighting Curve
Curve showing the relative interfering
effects of sinusoidal tones compared to a
reference frequency
NOISE UNITS and
LEVELS
Weighting Network
Equalizer which attenuates frequencies in the
same manner as would be done by the
average ear with the specific listening
apparatus to which the weighting refers.
Relative Level
The difference between the power of a signal
at one point and its power a reference point
NOISE UNITS and
LEVELS
1. dBrn (144
Weighting Curve,
Western Electric 144
Handset)
2. dBa (F1A Weighting
Curve, F1A Handset)
dB above reference
noise
Reference level: -85
dBm at 1 kHz (pure
tone)
Reference level: -90
dBm at 1 kHz (pure
tone)
dB above reference
noise adjusted
-82 dBm at 3 kHz
(random, white noise)
NOISE UNITS and
LEVELS
3. dBrnC (C-Message
Weighting Curve, 500
type handset)
4. pWp (pW
psophometrically
weighted)
dB above reference
noise
Psophometrically
weighted picowatt
Reference level: -90
dBm at 1 kHz (pure
tone)
Reference level: -90
dBm at 800 Hz
-88 dBm at 3 kHz
(random, white noise)
NOISE UNITS and
LEVELS
5. dBaO
6. dBrnCO
dBa adjusted at 0 dBm
level point
dBrnC at 0 dBm level
point
SAMPLE QUESTIONS
1. At what power level does a 1 kHz tone cause
zero interference, 144 weighted?
a. – 90 dB
b. – 90 dBm
c. 90 dBm
d. 90 dB
2. What is the reference noise level?
a. 10 pW
b. 0 dBm
c. 1 mW
d. – 90 dBm
SAMPLE QUESTIONS
3. What is the reference frequency of CCITT
psophometric noise measurement?
a. 800 Hz
b. 1500 Hz
c. 3400 Hz
d. 1000 Hz
4. A decibel notation relative to a reference noise
level.
a. dBa
b. dBp
c. dBf
d. dBr n
SAMPLE QUESTIONS
5. 15 dBa F1A weighted
a. – 90 dBm
b. – 82 dBm
c. – 85 dBm
d. – 70 dBm
6. The extent of noise referred to a test tone of
zero dBm.
a. dBa
b. dBm
c. dBa0
d. dbr n C
SAMPLE QUESTIONS
7. How many dBmV units correspond to a 1 mV signal
level?
a. 0
b. 1
c. 3
d. 6
REVIEW QUESTIONS
1. Any unwanted form of energy that tends to
interfere with the wanted signal is called
a. noise
b. spectrum
c. radiation
d. Absorption
2. The following are characteristics of noise except
a. unwanted energy
b. present in the channel
c. predictable in character
d. due to any cause
REVIEW QUESTIONS
3. The noise created outside the receiver
a. internal
b. external
c. shot
d. industrial
4. Noise that is produced by the active components
within the receiver
a. thermal
b. external
c. internal
d. White
REVIEW QUESTIONS
5. Noise created by man.
a. solar
b. industrial
c. extra-terrestrial
d. Galactic
6. The most prevalent noise found in urban areas, and
is normally caused by arc discharge from automobile
or aircraft ignition systems, induction motors,
switching gears, high voltage lines and the like.
a. industrial
b. Johnson
c. Flicker
d. Mixer
REVIEW QUESTIONS
7. Noise from distant planets, stars, galaxies,
and other celestial objects
a. extra-terrestrial
b. atmospheric
c. transit time
d. thermal
8. Which of the following is not a source of space
noise?
a. sun
b. stars
c. lightning
d. black body
REVIEW QUESTIONS
9. Noise caused by random variations in the arrival of
electrons at the output electrode of an amplifying
device
a. shot
b. random
c. impulse
d. transit time
10. Noise that is due to the random and rapid motion
of the charge carriers inside a resistive component.
a. Johnson
b. Thermal
c. Mixer
d. Transit time
REVIEW QUESTIONS
11. Atmospheric noise is less severe at frequencies above
a. 10 GHz
b. 30 MHz
c. 1 GHz
d. Audio level
12. Indicate the noise whose source is in a category
different from that of the other three.
a. solar
b. cosmic
c. atmospheric
d. Galactic
REVIEW QUESTIONS
13. Which of the following types of noise becomes of
great importance at high frequencies?
a. shot
b. random
c. impulse
d. transit time
14. Indicate which of the following does not occur in
transistors
a. shot
b. partition
c. flicker
d. resistance
REVIEW QUESTIONS
15. The value of a resistor creating thermal agitation
noise is doubled. The noise power generated is
therefore
a. halved
b. quadrupled
c. doubled
d. unchanged
16. The noise power generated by a resistor is
proportional to
a. temperature
b. bandwidth
c. a and b
d. none of these
REVIEW QUESTIONS
17. If bandwidth is doubled, the signal power is
a. unchanged
b. quadrupled
c. tripled
d. Doubled
18. In noise analysis, the reference temperature is
a. 75 K
b. 250 K
c. 290 K
d. 300 K
REVIEW QUESTIONS
19. Noise figure for an amplifier with noise is
always
a. 0 dB
b. Infinity
c. Less than 1
d. Greater than 1
20. Which noise figure represents the lowest
noise?
a. 1.5 dB
b. 2 dBm
c. 3.7 dB
d. 4.1 dB
REVIEW QUESTIONS
21. The noise figure of a totally noiseless device.
a. unity
b. infinity
c. zero
d. 100
To God be the glory!