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1.
CDMA
Code
division
multiple
access
(CDMA)
is
a
form
of
multiplexing (not a modulation scheme) and a method of multiple
access that does not divide up the channel by time (as in
TDMA), or frequency (as in FDMA), but instead encodes data
with a certain code associated with a channel and uses the
constructive interference properties of the signal medium to
perform the multiplexing. CDMA also refers to digital cellular
telephony systems that make use of this multiple access
scheme, such as those pioneered by Qualcomm, or W-CDMA.
1.1
Usage in mobile telephony
A number of different terms are used to refer to CDMA
implementations.
The
original
standard
spearheaded
by
QUALCOMM was known as IS-95, the IS referring to an Interim
Standard of the Telecommunications Industry Association (TIA).
IS-95 is often referred to as 2G or second generation cellular.
The QUALCOMM brand name cdma One may also be used to
refer to the 2G CDMA standard.
After a couple of revisions, IS-95 was superseded by the IS2000 standard. This standard was introduced to meet some of
the criteria laid out in the IMT-2000 specification for 3G, or third
generation, cellular. It is also referred to as 1xRTT which simply
means "1 times Radio Transmission Technology" and indicates
that IS-2000 uses the same 1.25-MHz shared channel as the
1
original IS-95 standard. A related scheme called 3xRTT uses
three 1.25-MHz carriers for a 3.75-MHz bandwidth that would
allow higher data burst rates for an individual user, but the
3xRTT scheme has not been commercially deployed.
More recently, QUALCOMM has led the creation of a new
CDMA-based technology called 1xEV-DO, or IS-856, which
provides the higher packet data transmission rates required by
IMT-2000 and desired by wireless network operators.
The QUALCOMM CDMA system includes highly accurate time
signals (usually referenced to a GPS receiver in the cell base
station), so cell phone CDMA-based clocks are an increasingly
popular type of radio clock for use in computer networks. The
main advantage of using CDMA cell phone signals for reference
clock purposes is that they work better inside buildings, thus
often eliminating the need to mount a GPS antenna on the
outside of a building.
Also frequently confused with CDMA is W-CDMA. The CDMA
technique is used as the principle of the W-CDMA air interface,
and the W-CDMA air interface is used in the global 3G standard
UMTS and the Japanese 3G standard FOMA, by NTT DoCoMo
and
Vodafone;
however,
the
CDMA
family
of
standards
(including cdmaOne and CDMA2000) are not compatible with
the W-CDMA family of standards.
Another important application of CDMA—predating and entirely
distinct from CDMA cellular—is the Global Positioning System,
GPS.
2
1.2
Coverage
As CDMA is newer than GSM, it may not be available in some
parts of the world. However, as the signal can be transmitted
over greater distances, it may give reception in more remote or
rural areas where a GSM phone does not pick up a signal.
1.3
CDMA
Mathematical foundation
exploits
at
its
core
mathematical
properties
of
orthogonality. Suppose we represent data signals as vectors.
For example, the binary string "1011" would be represented by
the vector (1, 0, 1, 1). We may wish to give a vector a name, we
may do so by using boldface letters, eg a. We also use an
operation on vectors, known as the dot product, to "multiply"
vectors, by summing the product of the components. For
example, the dot product of (1, 0, 1, 1) and (1, -1, -1, 0) would
be (1)(1)+(0)(-1)+(1)(-1)+(1)(0)=1+-1=0. Where the dot product
of vectors a and b is 0, we say that the two vectors are
orthogonal.
The dot product has a number of properties, and one will aid us
in understanding why CDMA works. For vectors a, b, c:
3
The square root of a.a is a real number, and is important. We
write
Suppose vectors a and b are orthogonal. Then:
4
1.4
Implementation
An example of 4 orthogonal digital signals.
Suppose now we have a set of vectors that are mutually
orthogonal to each other. Usually these vectors are specially
constructed for ease of decoding -- they are columns or rows
from Walsh matrices that are constructed from Walsh functions - but strictly mathematically the only restriction on these vectors
is that they are orthogonal. An example of orthogonal functions
5
is shown in the picture on the right. Now, associate with one
sender a vector from this set, say v, which is called the chip
code. Associate a zero digit with the vector -v, and a one digit
with the vector v. For example, if v=(1,-1), then the binary vector
(1, 0, 1, 1) would correspond to (1,-1,-1,1,1,-1,1,-1). For the
purposes of this article, we call this constructed vector the
transmitted vector.
Each sender has a different, unique vector chosen from that set,
but the construction of the transmitted vector is identical.
Now, the physical properties of interference say that if two
signals at a point are in phase, they will "add up" to give twice
the amplitude of each signal, but if they are out of phase, they
will "subtract" and give a signal that is the difference of the
amplitudes. Digitally, this behaviour can be modelled simply by
the addition of the transmission vectors, componentwise. So, if
we have two senders, both sending simultaneously, one with the
chip code (1, -1) and data vector (1, 0, 1, 1), and another with
the chip code (1, 1), and data vector (0,0,1,1), the raw signal
received would be the sum of the transmission vectors (1,-1,1,1,1,-1,1,-1)+(-1,-1,-1,-1,1,1,1,1)=(0,-2,-2,0,2,0,2,0).
Suppose a receiver gets such a signal, and wants to detect what
the transmitter with chip code (1, -1) is sending. The receiver
will make use of the property described in the above foundation
section, and take the dot product to the received vector in parts.
Take the first two components of the received vector, that is, (0,
-2). Now, (0, -2).(1, -1) = (0)(1)+(-2)(-1) = 2. Since this is
6
positive, we can deduce that a one digit was sent. Taking the
next two components, (-2, 0), (-2, 0).(1,-1)=(-2)(1)+(0)(-1)=-2.
Since this is negative, we can deduce that a zero digit was sent.
Continuing in this fashion, we can successfully decode what the
transmitter with chip code (1, -1) was sending: (1, 0, 1,
1).Likewise, applying the same process with chip code (1, 1): (1,
1).(0,-2) = -2 gives digit 0, (1, 1).(-2,0)=(1)(-2)+(1)(0)=-2 gives
digit 0, and so on, to give us the data vector sent by the
transmitter with chip code (1, 1): (0, 0, 1, 1).
Now, there are certain issues where this mathematical process
can be disrupted. Suppose that one sender transmits at a higher
signal strength than another. Then the critical orthogonality
property can be disrupted, and thus the system can fail. Thus
controlling power strength is an important issue with CDMA
transmitters.
A
TDMA
or
FDMA
receiver
can
in
theory
completely reject arbitrarily strong signals on other time slots or
frequency channels. This is not true for CDMA; rejection of
unwanted signals is only partial. If any or all of the unwanted
signals are much stronger than the desired signal, they will
overwhelm it. This leads to a general requirement in any CDMA
system to approximately match the various signal power levels
as seen at the receiver. In CDMA cellular, the base station uses
a fast closed-loop power control scheme to tightly control each
mobile's transmit power.
Suppose that noise present in a channel takes a zero bit to
some other value. Then this will also disrupt the orthogonality
7
property, and thus adding an extra level of forward error
correction (FEC) coding is also vital.
So far, we have assumed that CDMA timing is absolutely exact,
that is, transmitters exactly transmit at points in multiples of the
length of the chip sequence. Of course, in reality, this is
impractical to achieve, so all forms of CDMA use spread
spectrum process gain to allow receivers to partially discriminate
against unwanted signals. Signals with the desired chip code
and timing are received, while signals with different chip codes
(or the same spreading code but a different timing offset) appear
as wideband noise reduced by the process gain.
CDMA's main advantage over TDMA and FDMA is that the
number of available CDMA codes is essentially infinite. This
makes CDMA ideally suited to large numbers of transmitters
each generating a relatively small amount of traffic at irregular
intervals, as it avoids the overhead of continually allocating and
deallocating a limited number of orthogonal time slots or
frequency
channels
to
individual
transmitters.
CDMA
transmitters simply send when they have something to say, and
go off the air when they don't.
8
1.5

CDMA features
Narrowband
message
signal
multiplied
by
wideband
spreading signal or pseudo-noise code

Each user has his own pseudo-noise (PN) code

Soft capacity limit: system performance degrades for all
users as number of users increases

Cell frequency reuse: no frequency planning needed

Soft handoff increases capacity

Near-far problem

Interference limited: power control is required

Wide bandwidth induces diversity: rake receiver is used

It would take all the computers ever made as much time as
humans have been on earth to crack or decode a single
second of CDMA conversation
9
10
11
12
2.
Capacitors
The capacitor's function is to store electricity, or electrical
energy. The capacitor also functions as a filter, passing
alternating current (AC), and blocking direct current (DC). This
symbol
is used to indicate a capacitor in a circuit diagram.
The capacitor is constructed with two electrode plates facing
each other, but separated by an insulator. When DC voltage is
applied to the capacitor, an electric charge is stored on each
electrode. While the capacitor is charging up, current flows. The
current will stop flowing when the capacitor has fully charged.
The capacitance of a capacitor is generally very small, s o units
-6
-9
such as the microfarad ( 10 F ), nanofarad ( 10 F ), and
picofarad (10
-12
F) are used.
The capacitor has an insulator ( the dielectric ) between 2
sheets of electrodes. Different kinds of capacitors use different
materials for the dielectric.
2.1 Electrolytic Capacitors (Electrochemical
capacitors)
Aluminum is used for the electrodes by using a thin oxidization
membrane. Large values of capacitance can be obtained in
comparison with the size of the capacitor, because the dielectric
used is
very thin.
The most
important
characteristic
of
13
electrolytic capacitors is that they have polarity. They have a
positive and a negative electrode.[Polarised] This means that it
is very important which way round they are connected. If the
capacitor is subjected to voltage exceeding its working voltage,
or if it is connected with incorrect polarity, it may burst. It is
extremely dangerous, because it can quite literally explode.
Make absolutely no mistakes. Generally, in the circuit diagram,
the positive side is indicated by a "+" (plus) symbol. Electrolytic
capacitors range in value from about 1µF to thousands of µF.
Mainly this type of capacitor is used as a ripple filter in a power
supply circuit, or as a filter to bypass low frequency signals, etc.
Because this type of capacitor is comparatively similar to the
nature of a coil in construction, it isn't possible to use for high frequency circuits. (It is said that the frequency characteristic is
bad.) The photograph on the left is an example of the different
values of electrolytic capacitors in which the capacitance and
voltage differ. From the left to right: 1µF (50V) [diameter 5 mm,
high 12 mm] 47µF (16V) [diameter 6 mm, high 5 mm] 100µF
(25V) [diameter 5 mm, high 11 mm] 220µF (25V) [diameter 8
mm, high 12 mm] 1000µF (50V) [diameter 18 mm, high 40 mm]
The size of the capacitor sometimes
14
depends on the manufacturer. So the sizes shown here on this
page are just examples. In the photograph to the right, the mark
indicating the negative lead of the component can be seen. You
need to pay attention to the polarity indication so as not to make
a mistake when you assemble the circuit.
2.2
Ceramic Capacitors
Ceramic capacitors are constructed with materials such as
titanium acid barium used as the dielectric. Internally, these
15
capacitors are not constructed as a coil, so they can be used in
high frequency
applications. Typically, they are used in circuits which bypass
high frequency signals to ground. These capacitors have the
shape of a disk. Their capacitance is comparatively small. The
capacitor on the left is a 100pF capacitor with a diameter of
about 3 mm.The capacitor on the right side is printed with 103,
3
so 10 x 10 pF becomes 0.01 µF. The diameter of the disk is
about 6 mm. Ceramic capacitors have no polarity. Ceramic
capacitors should not be used for analog circuits, because they
can distort the signal.
Multilayer Ceramic Capacitors
The
multilayer ceramic capacitor has a many-layered dielectric.
These capacitors are small in size, and have good temperature
and frequency characteristics. Square wave signals
used in
digital circuits can have a comparatively high frequency
component included. This capacitor is used to bypass the high
frequency to ground. In the photograph, the capacitance of the
16
component on the left is displayed as 104. So, the capacitance
4
is 10 x 10 pF = 0.1 µF. The thickness is 2 mm, the height is 3
mm, the width is 4 mm.The capacitor to the right has a
3
capacitance of 103 (10 x 10 pF = 0.01 µF). The height is 4 mm,
the diameter of the round part is 2 mm. These capacitors are not
polarized. That is, they have no polarity.
2.3
Polystyrene Film Capacitors
In these devices, polystyrene film is used as the dielectric. This
type of capacitor is not for use in high frequency circuits,
because they are constructed like a coil inside. They are used
well in filter circuits or timing circuits which run at several
hundred KHz or less. The component shown on the left has a
red color due to the copper leaf used for the electrode. The
silver color is due to the use of aluminum foil as the electrode.
The device on the left has a height of 10 mm, is 5 mm thick, and
is rated 100pF. The device in the middle has a height of 10 mm,
5.7 mm thickness, and is rated The device on the right has a
height of 24 mm, is 10 mm thick, and is rated 10000pF. These
devices have no polarity.
17
This capacitor is used when a higher tolerance is necessary
than polyester capacitors offer. Polypropylene film is used for
the dielectric. It is said that there is almost no change of
capacitance in these devices if they are used with frequencies of
100KHz or less. The pictured capacitors have a tolerance of
±1%.
From the left in the photograph Capacitance: 0.01 µF
(printed with 103F) [the width 7mm, the height 7mm, the
thickness 3mm]Capacitance: 0.022 µF (printed with 223F) [the
width 7mm, the height 10mm, the thickness 4mm]Capacitance:
0.1 µF (printed with 104F) [the width 9mm, the height 11mm, the
thickness 5mm] When I measured the capacitance of a 0.01 µF
capacitor with the meter which I have, the error was +0.2%.
These capacitors have no polarity.
2.4
Variable Capacitors
Variable capacitors are used for adjustment etc. of frequency
mainly. On the left in the photograph is a "trimmer," which uses
18
ceramic as the dielectric. Next to it on the right is one that uses
polyester film for the dielectric.
The pictured components are meant to be mounted on a printed
circuit board.
When adjusting the value of a variable capacitor, it is advisable
to be careful. One of the component's leads is connected to the
adjustment screw of the capacitor. This means that the value of
the capacitor can be affected by the capacitance of the
screwdriver in your hand. It is better to use a special screwdriver
to adjust these components.
photograph
are
variable
Pictured in the upper left
capacitors
with
the
following
specifications: Capacitance: 20pF (3pF - 27pF measured)
[Thickness 6 mm, height 4.8 mm]Their are different colors, as
well. Blue: 7pF (2 - 9), white: 10pF (3 - 15), green: 30pF (5 35), brown: 60pF (8 - 72). In the same photograph, the device
on the right has the following specifications: Capacitance: 30pF
(5pF - 40pF measured) [The width (long) 6.8 mm, width (short)
4.9 mm, and the height 5 mm] The components in the
photograph on the right are used for radio tuners, etc. They are
called "Varicons" but this may be only in Japan. The variable
capacitor on the left in the photograph, uses air as the dielectric.
It combines three independent capacitors. For each one, the
capacitance changed 2pF - 18pF. When the adjustment axis is
turned,
the
capacitance
of
all
3
capacitors
change
simultaneously. Physically, the device has a depth of 29 mm,
and 17 mm width and height. (Not including the adjustment rod.)
19
There are various kinds of variable capacitor, chosen in
accordance with the purpose for which they are needed. The
pictured components are very small. To the right in the
photograph is a variable capacitor using polyester film as the
dielectric. Two independent capacitors are combined. The
capacitance of one side changes 12pF - 150pF, while the other
side changes from 11pF - 70pF. Physically, it has a depth of
11mm, and 20mm width and height. (Not including the
adjustment rod.) The pictured device also has a small trimmer
built in to each capacitor to allow for precise adjustment up to
15pF.
20
3.
Coils
A coil is nothing more than copper wire wound in a spiral. This
symbol
is used to indicate a coil in a circuit diagram.
Inductance value is designated in units called the Henry(H). The
more wire the coil contains, the stronger its characteristics
become. The inductance value can become quite large. If a coil
is wound around an iron rod, or ferrite core (strengthened with
iron powder), the inductance of the coil will be greatly increased.
Coils used in typical electric circuits varely widely in values,
ranging from a few micro-henry (µH) to many henry (H). Coils
are sometimes called "inductors." Inductance is the measure of
the strength of a coil. Capacitors have capacitance, resistors
have resistance, and Inductors (coils) have inductance. When
alternating current flows through a coil, the magnetic flux that
occurs in the coil changes with the current. When a second coil
is put close to the first coil (with the changing flux), alte rnating
voltage is caused to flow in the second coil by an effect known
as
"mutual
induction."
Mutual
inductance
(inductance)
is
measured in units of the Henry. The changing magnetic flux in a
coil affects itself as well as other coils. This is called self
induction, the degree of this self induction is called Self
Inductance. Self inductance is a measure of a coil's ability to
establish an induced voltage as a result of a change in its
current. Self inductance is commonly referred to as simply
"inductance," and is symbolized by "L". The unit of inductance is
the Henry (H).
21
The definition of "Henry" is "When a current of 1 ampere flows
through a given coil in 1 second such that 1 volt is induced to
flow in a second coil, the mutual inductance between the c oils is
said to be 1 Henry." The definition of self inductance is the
same, except that the 1 volt is induced in the first coil; there is
no second coil.
Characteristic of coils When wire is coiled, it takes on various
characteristics that are different from straight wire. Below I will
explain some of the characteristics coils that I know. Current
Stabilization Characteristic When current begins to flow in the
coil, the coil resists the flow. When current decreases, the coil
makes current continue to flow (briefly) at the previous rate. This
is called " Lenz's law ".The direction of induced current in a coil
is such that is opposes the change in the magnetic field that
produced it.
This characteristic is used for the ripple filter circuit of a power
supply where it transforms alternating current(AC) to direct
current(DC).When a rectifier is used to make DC from AC, the
output of the rectifier without a ripple filter circuit is ripple
current. Ripple current is DC that has a large AC component. A
ripple filter circuit often combines coil and capacitors. The coil
resists the change of current and capacitors supplement the flow
of current by discharging into the circuit if the input voltage
drops. Thus, clear, ripple-free DC is obtained from the ripple
22
filter circuit. Resistor is used instead of coil in simple ripple filter
circuit. Mutual induction
23
4.
Diodes
A diode is a semiconductor device which allows current to flow
through it in only one direction. Although a transistor is also a
semiconductor device, it does not operate the way a diode does.
A diode is specifically made to allow current to flow through it in
only one direction. Some ways in which the diode can be used
are listed here . A diode can be used as a rectifier that converts
AC (Alternating Current) to DC (Direct Current) for a power
supply device.
Diodes can be used to separate the signal from radio
frequencies
Diodes can be used as an on/off switch that controls current.
This symbol
is used to indicate a diode in a circuit diagram.
The meaning of the symbol is (Anode)
(Cathode). Current
flows from the anode side to the cathode side.
Although all
diodes operate with the same general principle, there are
different types suited to different applications. For example, the
following devices are best used for the applications noted.
4.1
Light emitting diode
The circuit symbol is
.This type of diode emits light when current
flows through it in the forward direction. (Forward biased.)
24
4.2
Variable capacitance diode
The circuit symbol is
.The current does not flow when
applying the voltage of the opposite direction to the diode. In
this condition, the diode has a capacitance like the capacitor. It
is a very small capacitance. The capacitance of the diode
changes when changing voltage. With the change of this
capacitance, the frequency of the oscillator can be changed.
25
The graph on the right shows the electrical characteristics of a
typical diode. When a small voltage is applied to the diode in
the forward direction, current flows easily. Because the diode
has a certain amount of resistance, the voltage will drop slightly
as current flows through the diode. A typical diode causes a
voltage drop of about 0.6 - 1V (V F ) (In the case of silicon diode,
almost 0.6V)
This voltage drop needs to be taken into
consideration in a circuit which uses many diodes in series.
Also, the amount of current passing through the diodes must be
considered.
When voltage is applied in the reverse direction
through a diode, the diode will have a great resistance to current
flow. Different diodes have different characteristics when
reverse-biased. A given diode should be selected depending on
how it will be used in the circuit. The current that will flow
through a diode biased in the reverse direction will vary from
several mA to just µA, which is very small. The limiting voltages
and currents permissible must be considered on a case by case
basis. For example, when using diodes for rectification, part of
the time they will be required to withstand a reverse voltage. If
the diodes are not chosen carefully, they will break down.
26
4.3 Rectification/Switching/Regulation
Diode
The stripe stamped on one end of the diode shows indicates the
polarity of the diode. The stripe shows the cathode side. The top
two devices shown in the picture are diodes used for
rectification. They are made to handle relatively high currents.
The device on top can handle as high as 6A, and the one below
it can safely handle up to 1A. However, it is best used at about
70% of its rating because this current value is a maximum
rating. The third device from the top (red color) has a part
27
number of 1S1588. This diode is used for switching, because it
can switch on and off at very high speed. However, the
maximum current it can handle is 120 mA. This makes it well
suited to use within digital circuits. The maximum reverse
voltage (reverse bias) this diode can handle is 30V. The device
at the bottom of the picture is a voltage regulation diode with a
rating of 6V. When this type of diode is reverse biased, it will
resist changes in voltage. If the input voltage is increased, the
output voltage will not change. (Or any change will be an
insignificant amount.) While the output voltage does not
increase with an increase in input voltage, the output current
will. This requires some thought for a protection circuit so that
too much current does not flow. The rated current limit for the
device is 30 mA.
28
Generally, a 3-terminal voltage regulator is used for the
stabilization of a power supply. Therefore, this diode is typically
used to protect the circuit from momentary voltage spikes. 3
terminal regulators use voltage regulation diodes inside.
4.4
Diode bridge
Rectification diodes are used to make DC from AC. It is possible
to do only 'half wave rectification' using 1 diode. When 4 diodes
are combined, 'full wave rectification' occurrs. Devices that
combine 4 diodes in one package are called diode bridges. They
are used for full-wave rectification. Physically, it is 7 mm high,
and 10 mm in diameter. The flat device on the left has a current
29
limit of 4A. It is has a thickness of 6 mm, is 16 mm in height, and
19 mm in width.The photograph on the right shows a large, highpower diode bridge. It has a current capacity of 15A. The peak
reverse-bias voltage is 400V. Diode bridges with large current
capacities like this one, require a heat sink. Typically, they are
screwed to a piece of metal, or the chasis of device in which
they are used. The heat sink allows the device to radiate excess
heat. As for size, this one is 26 mm wide on each side, and the
height of the module part is 10 mm.
4.5
Light Emitting Diode ( LED )
Light emitting diodes must be choosen according to how they
will be used, because there are various kinds. The diodes are
available in several colors. The most common colors are red and
green, but there are even blue ones. The device on the far right
30
in the photograph combines a red LED and green LED in one
package. The component lead in the middle is common to both
LEDs. As for the remaing two leads, one side is for the green,
the other for the red LED. When both are turned on
simultaneously, it becomes orange. When an LED is new out of
the package, the polarity of the device can be determined by
looking at the leads. The longer lead is the Anode side, and the
short one is the Cathode side. The polarity of an LED can also
be determined using a resistance meter, or even a 1.5 V battery.
When using a test meter to determine polarity, set the meter to a
low resistance measurement range. Connect the probes of the
meter to the LED. If the polarity is correct, the LED will glow. If
the LED does not glow, switch the meter probes to the opposite
leads on the LED. In either case, the side of the diode which is
connected to the black meter probe when the LED glows, is the
Anode side. Positive voltage flows out of the black probe when
the meter is set to measure resistance.
It is possible to use
an LED to obtain a fixed voltage. The voltage drop (forward
voltage, or V F ) of an LED is comparatively stable at just about
2V.
31
5.
Transistors
The transistor's function is to amplify an electric current. Many
different kinds of transistors are used in analog circuits, for
different reasons. This is not the case for digital circuits. In a
digital circuit, only two values matter; on or off. The amplification
abilitiy of a transistor is not relevant in a digital circuit. In many
cases, a circuit is built with integrated circuits(ICs). Transistors
are often used in digital circuits as buffers to protect ICs. For
example, when powering an electromagnetic switch (called a
'relay'), or when controlling a light emitting diode. (In my case.)
Two different symbols are used for the transistor.
PNP type
and NPN type
The name (standard part number) of the transistor, as well as
the type and the way it is used is shown below.
2SAXXXX PNP type high frequency 2SBXXXX PNP type low
frequency 2SCXXXX NPN type high frequency 2SDXXXX NPN
type low frequency
The direction of the current flow differs between the PNP and
NPN type.When the power supply is the side of the positive
(plus), the NPN type is easy to use.
32
6.
Resistors
The resistor's function is to reduce the flow of electric current.
This symbol
is used to indicate a resistor in a circuit
diagram, known as a schematic. Resistance value is designated
in units called the "Ohm." A 1000 Ohm resistor is typically
shown as 1K-Ohm ( kilo Ohm ), and 1000 K-Ohms is written as
1M-Ohm ( megohm ). There are two classes of resistors; fixed
resistors
and the variable resistors . They are also classified
according to the material from which they are made. The typical
resistor is made of either carbon film or metal film. There are
other types as well, but these are the most common. The
resistance value of the resistor is not the only thing to consider
when selecting a resistor for use in a circuit. The "tolerance" and
the electric power ratings of the resistor are also important. The
tolerance of a resistor denotes how close it is to the actual rated
resistence value. For example, a ±5% tolerance would indicate a
resistor that is within ±5% of the specified resistance value. The
power rating indicates how much power the resistor can safely
tolerate. Just like you wouldn't use a 6 volt flashlight lamp to
replace a burned out light in your house, you wouldn't use a 1/8
watt resistor when you should be using a 1/2 watt resistor. The
maximum rated power of the resistor is specified in Watts.
2
Power is calculated using the square of the current ( I ) x the
resistance value ( R ) of the resistor. If the maximum rating of
the resistor is exceeded, it will become extremely hot, and even
burn. Resistors in electronic circuits are typicaly rated 1/8W,
33
1/4W, and 1/2W. 1/8W is almost always used in signal circuit
applications.
When
powering
a
light
emitting
diode,
a
comparatively large current flows through the resistor, so you
need to consider the power rating of the resistor you choose.
6.1
Rating electric power
For example, to power a 5V circuit using a 12V supply, a threeterminal voltage regulator is usually used. However, if you try to
drop the voltage from 12V to 5V using only a resistor, then you
need to calculate the power rating of the resistor as well as the
resistance value. At this time, the current consumed by the 5V
circuit needs to be known. Here are a few ways to find out how
much current the circuit demands. Assemble the circuit and
measure the actual current used with a multi-meter. Check the
component's current use against a standard table. Assume the
current consumed is 100 mA (milliamps) in the following
example. 7V must be dropped with the resistor. The resistance
value of the resistor becomes 7V / 0.1A = 70(ohm). The
consumption of electric power for this resistor becomes 0.1A x
0.1A x 70 ohm = 0.7W. Generally, it's safe to choose a resistor
which has a power rating of about twice the power consumption
needed.
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6.2
Resistance value
As for the standard resistance value, the values used can be
divided like a logarithm. For example, in the case of E3, The
values [1], [2.2], [4.7] and [10] are used. They divide 10 into
three, like a logarithm. In the case of E6 : [1], [1.5], [2.2], [3.3],
[4.7], [6.8], [10]. In the case of E12 : [1], [1.2], [1.5], [1.8], [2.2],
[2.7], [3.3], [3.9], [4.7], [5.6], [6.8], [8.2], [10]. It is because of
this that the resistance value is seen at a glance to be a discrete
value. The resistance value is displayed using the color code (
the colored bars/the colored stripes ), because the average
resistor is too small to have the value printed on it with numbers.
You had better learn the color code, because almost all resistors
of 1/2W or less use the color code to display the resistance
value.
6.3
Fixed Resistors
A fixed resistor is one in which the value of its resistance cannot
change.
6.4
Carbon film resistors
This is the most general purpose, cheap resistor. Usually t he
tolerance of the resistance value is ±5%. Power ratings of 1/8W,
1/4W and 1/2W are frequently used. Carbon film resistors have
a disadvantage; they tend to be electrically noisy. Metal film
35
resistors are recommended for use in analog circuits. However, I
have never experienced any problems with this noise. The
physical size of the different resistors are as follows.
This resistor is called a Single-In-Line(SIL) resistor network. It is
made with many resistors of the same value, all in one packag e.
One side of each resistor is connected with one side of all the
other resistors inside. One example of its use would be to
control the current in a circuit powering many light emitting
diodes (LEDs). In the photograph on the left, 8 resistors are
housed in the package. Each of the leads on the package is one
resistor. The ninth lead on the left side is the common lead. The
face value of the resistance is printed. ( It depends on the
supplier. ) Some resistor networks have a "4S" printed on the
top of the resistor network. The 4S indicates that the package
contains 4 independent resistors that are not wired together
inside. The housing has eight leads instead of nine. The internal
wiring of these typical resistor networks has been illustrated
below. The size (black part) of the resistor network which I have
36
is as follows: For the type with 9 leads, the thickness is 1.8 mm,
the height 5mm, and the width 23 mm. For the types with 8
component leads, the thickness is 1.8 mm, the height 5 mm, and
the width 20 mm.
6.5
Variable Resistors
There are two general ways in which variable resistors are used.
One is the variable resistor which value is easily changed, like
the volume adjustment of Radio. The other is semi-fixed resistor
that is not meant to be adjusted by anyone but a technician. It is
used to adjust the operating condition of the circuit by the
technician. Semi-fixed resistors are used to compensate for the
inaccuracies of the resistors, and to fine-tune a circuit. The
rotation angle of the variable resistor is usually about 300
degrees. Some variable resistors must be turned many times to
use the whole range of resistance they offer. This allows for very
precise
adjustments
of
their
value.
These
are
called
"Potentiometers" or "Trimmer Potentiometers."
37
In the photograph to the left, the variable resistor typically used
for volume controls can be seen on the far right. Its value is very
easy to adjust. The four resistors at the center of the photograph
are the semi-fixed type. These ones are mounted on the printed
circuit board. The two resistors on the left are the trimmer
potentiometers.
This symbol
diagram.
is used to indicate a variable resistor in a circuit
There are three ways in which a variable resistor's
value can change according to the rotation angle of its axis.
When type "A" rotates clockwise, at first, the resistance value
changes slowly and then in the second half of its axis, it
changes very quickly. The "A" type variable resistor is typically
used for the volume control of a radio, for example. It is well
suited to adjust a low sound subtly. It suits the characteristics of
the ear. The ear hears low sound changes well, but isn't as
sensitive to small changes in loud sounds. A larger change is
38
needed as the volume is increased. These "A" type variable
resistors are sometimes called "audio taper" potentiometers. As
for type "B", the rotation of the axis and the change of the
resistance value are directly related. The rate of change is the
same, or linear, throughout the sweep of the axis. This type
suits a resistance value adjustment in a circuit, a balance circuit
and
so
on.
They
are
sometimes
called
"linear
taper"
potentiometers. Type "C" changes exactly the opposite way to
type "A". In the early stages of the rotation of the axis, the
resistance value changes rapidly, and in the second half, the
change occurs more slowly. This type isn't too much used. It is a
special use. As for the variable resistor, most are type "A" or
type "B".
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7.
Soldering
The soldering is the basic work for electronic circuit engineering.
I will introduce the tools for soldering below. The sufficient
attention is necessary during work, because soldering handles a
high temperature. Pay attention to the handling of the soldering
iron sufficiently, because it becomes burn, fire more, carelessly.
7.1
Soldering iron
Soldering iron is a necessary instrument when you solder.
Solder is hardening in a normal temperature, but solder can melt
easily by using the soldering iron and the parts and wiring
materials can be fixed to the printed wiring board(PWB).The
important point is temperature of the soldering iron. For
soldering,
it
needs
to
become
the
temperature
of
the
object(PWB, parts, wire etc) to solder melting temperature.
However, the temperature of soldering iron must not be too high.
The electronic component gets damage with high temperature.
So, you need to solder in a short time. Sometimes, the loose
contact of soldering occurs. It is difficult to confirm only by
looking at. When the temperature of the object is not enough,
the loose contact will be occured. At the end of assembling of
the electronic circuit, you need to check the soldered contact
with circuit tester etc.
40
7.2
Electric power
There are various kind of soldering irons. I am using 3 kinds of
soldering irons. 25W type, 80W type, 15W type
7.3
The tip of iron
The soldering is done at the tip of iron. So, the tip of iron is very
important. There is the type that the tip of soldering iron is made
of copper stick. But I don't recommend that type. Because, the
copper stick rusts easily by heat and it becomes difficult to
convey heat. Also the tip of copper stick melts with solder. It
becomes difficult to solder. I recommend the one that is difficult
to convey heat. There are many shape of tips. The tip which fit
to the DIP type IC is used to remove the ICs. All of the solder on
the pins can be melt at same time then it easy to remove the
IC.I do not have such kind of soldering iron.
41
Usually the soldering iron is heated by electricity. However,
there is the soldering iron heated by gas. It is convenient to
carry.
7.4
Soldering iron stand
The soldering iron becomes high temperature. Therefore it can't
be placed on the desk directly. The stabilized soldering iron
stand is necessary.
When making the electronic circuit,
sometime I forgot the existence of soldering iron, because I
have devoted to the parts, wiring etc. It was serious when I
noticed, desk was burning. You need to choose the iron stand
with appropriate weight which can hold iron stably. Also you
need to choose the iron stand that fit the form of iron. Usually I
wipe the tip of iron with moistened sponge. Therefore I use the
iron stand with the place for sponge. This is your taste.
7.5
Solder
The solder is the alloy of lead and tin. As for good solder, the
containment rate of tin is high. The finish of soldering is
beautiful. The price is a little bit high. There are several kinds of
solder, solder wire( thread form solder ) is convenient for
electronic circuit making. This solder wire is doing the structure
of the pipe and flux is included inside. Flux melts together with
the solder and the solder becomes easy to attach to the
42
component leads. There are some thicknesses of solder wire. I
am usually using the one that diameter is 0.5 mm. The
containment rate of the tin is 60%.
7.6
Solder sucker
The failure of soldering occurs often. In this case, the part or the
wiring must be removed. I will introduce the instruments that can
be used for desoldering
7.7
Solder pump
This is the tool that can be absorbed the melted solder with the
repulsion power of the spring that was built in with the principle
of the piston. The usage is shown below. Push down the knob of
the upper part of the pump against to spring until it is locked.
Melt the solder of the part that wants to absorb solder with iron.
Apply the nozzle of the pump to the melted solder part. Push the
release knob of pump. Then the plunger of the pump is pushed
up with the power of spring and solder is absorbed inside the
pump. You need to do this operation quickly, otherwise the part
gets damage by the heat. A little practice is needed.
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7.8
De-soldering wire
This is made of thin copper net wire like a screen cable in a
coaxial cable. Like water inhales to cloth, the solder is absorbed
to the net wire by a capillary tube phenomenon. The usage is
shown below. Apply the de-soldering wire to the part that wants
to take solder. Apply the soldering iron from the top and Melt the
solder. The melted solder is absorbed to de-soldering wire with
a capillary tube phenomenon. At this time you absorb solder
while shifting de-soldering wire. When the solder can not be
removed in the once, remove repeatedly while shifting the desoldering wire. There are several kinds of width of de-soldering
wire. I am using the one with 2mm width.
44