Page 1
GET IN TUNE WITH THIS
TEACHING RESOURCES
SCHEMES OF WORK
DEVELOPING A SPECIFICATION
COMPONENT FACTSHEETS
HOW TO SOLDER GUIDE
FM RADIO KIT
Version 2.2
Page 2
FM Radio Teaching Resources
www.kitronik.co.uk/2157
Index of Sheets
TEACHING RESOURCES
Index of Sheets
Introduction
Schemes of Work
Answers
The Design Process
The Design Brief
Investigation / Research
Developing a Specification
Design
Design Review (group task)
Soldering in Ten Steps
Resistor Values
Capacitor Basics
Ceramic Disc Capacitors
How Does a Radio Work?
Instruction Manual
Evaluation
Packaging Design
ESSENTIAL INFORMATION
Build Instructions
Checking Your FM Radio PCB
Adding an On / Off Switch
Designing the Enclosure
Improving the Radio Tuning
How the FM Radio Kit Works
Online Information
Page 3
FM Radio Teaching Resources
www.kitronik.co.uk/2157
Introduction
About the project kit
Both the project kit and the supporting material have been carefully designed for use in KS3 Design and Technology
lessons. The project kit has been designed so that even teachers with a limited knowledge of electronics should have
no trouble using it as a basis from which they can form a scheme of work.
The project kits can be used in two ways:
1. As part of a larger project involving all aspects of a product design, such as designing an enclosure for the
electronics to fit into.
2. On their own as a way of introducing electronics and electronic construction to students over a number of
lessons.
This booklet contains a wealth of material to aid the teacher in either case.
Using the booklet
The first few pages of this booklet contains information to aid the teacher in planning their lessons and also covers
worksheet answers. The rest of the booklet is designed to be printed out as classroom handouts. In most cases all of
the sheets will not be needed, hence there being no page numbers, teachers can pick and choose as they see fit.
Please feel free to print any pages of this booklet to use as student handouts in conjunction with Kitronik project
kits.
Support and resources
You can also find additional resources at www.kitronik.co.uk. There are component fact sheets, information on
calculating resistor and capacitor values, puzzles and much more.
Kitronik provide a next day response technical assistance service via e-mail. If you have any questions regarding this
kit or even suggestions for improvements, please e-mail us at:
Alternatively, phone us on 0845 8380781.
Page 4
FM Radio Teaching Resources
Hour 1
Introduce the task using ‘The Design B
rief’ sheet. Demonstrate a built unit. Take students through the
Hour 2
Develop a specification for the project using the ‘Developing a Specification’ sheet.
Hour 3
Read ‘Designing the Enclosure’ sheet. Develop a product design using the ‘Design’ sheet.
Hour 4
Using cardboard
, get the students to model their enclosure design. Allow them to make alterations to
Hour 5
Split the students into groups and get them to perform a group design review using the ‘Design Review
’
Hour 6
Using the ‘Soldering in T
en S
teps’ sheet
, demonstrate and get students to practice
soldering. Start the
Hour 7
Build
the electronic kit using the ‘Build Instructions’.
Hour 8
Complete the build of the electronic kit. Check the completed PCB and fault find i
f required using the
Hour 9
Build the enclosure
. Hour 10
Build the enclosure
.
Hour 11
Build the enclosure.
Hour 12
Using the ‘Evaluation’ sheet, get the students to evaluate their final product and state where
www.kitronik.co.uk/2157
Schemes of Work
Two schemes of work are included in this pack; the first is a complete project including the design & manufacture of
an enclosure for the kit (below). The second is a much shorter focused practical task covering just the assembly of
the kit (next page). Equally, feel free to use the material as you see fit to develop your own schemes.
Before starting we would advise that you to build a kit yourself. This will allow you to become familiar with the
project and will provide a unit to demonstrate.
Complete product design project including electronics and enclosure
design process using ‘The Design Process’ sheet.
Homework: Collect examples of radio and audio products. List the common features of these products
on the ‘Investigation / Research’ sheet.
Resource: Sample of radios and audio products.
Homework: Using the internet or other search method, find out what is meant by ‘design for
manufacture’. List five reasons why design for manufacture should be considered on any design project.
Homework: Complete design.
their design if the model shows any areas that need changing.
sheet.
‘Resistor Value’ and ‘Capacitor Basics’ worksheets.
Homework: Complete any of the remaining resistor / capacitor tasks.
‘Checking Your FM Radio PCB’ section and the fault finding flow chart.
Homework: Read ‘How the FM Radio Kit Works’ sheet.
improvements can be made.
AdditionalWork
Package design for those who complete ahead of others.
Page 5
FM Radio Teaching Resources
Hour 1
Introduction to the kit demonstrating a buil
t unit. Using
the ‘Soldering in T
en Steps’ sheet
, practice
Hour 2
Build the kit using the ‘Build In
structions’.
Hour 3
Build the kit using the ‘Build Instructions’.
Hour 4
Check the completed PCB and fault fi
nd if required using ‘Checking Y
our
FM Radio
PCB’ and fault finding
www.kitronik.co.uk/2157
Electronics only
soldering.
flow chart.
Answers
Resistor questions
1st Band 2nd Band Multiplier x
Brown Black Yellow
Value
100,000 Ω
Green Blue Brown
Brown Grey Yellow
Orange White Black
Value
180 Ω
3,900 Ω
47,000 (47K) Ω
1,000,000 (1M) Ω
1st Band 2nd Band Multiplier x
Brown Grey
Orange White
Yellow Violet
Brown Black
560 Ω
180,000Ω
39Ω
Brown
Red
Orange
Green
Capacitor Ceramic Disc values
Printing on capacitor Two digit start Number of zero’s Value in pF
222 22 00 2200pF (2.2nF)
103 10 000 10000pF (10nF)
333 33 000 33000pF (33nF)
473 47 000 47000pF (47nF)
Page 6
FM Radio Teaching Resources
www.kitronik.co.uk/2157
The Design Process
The design process can be short or long, but will always consist of a number of
steps that are the same on every project. By splitting a project into these
clearly defined steps, it becomes more structured and manageable. The steps
allow clear focus on a specific task before moving to the next phase of the
project. A typical design process is shown on the right.
Design brief
What is the purpose or aim of the project? Why is it required and who is it
for?
Investigation
Research the background of the project. What might the requirements be?
Are there competitors and what are they doing? The more information found
out about the problem at this stage, the better, as it may make a big
difference later in the project.
Design Brief
Investigation
Specification
Design
Specification
This is a complete list of all the requirements that the project must fulfil - no
matter how small. This will allow you to focus on specifics at the design stage
and to evaluate your design. Missing a key point from a specification can
result in a product that does not fulfil its required task.
Build
Evaluate
Design
Develop your ideas and produce a design that meets the requirements listed
in the specification. At this stage it is often normal to prototype some of your
ideas to see which work and which do not.
Improve
Build
Build your design based upon the design that you have developed.
Evaluate
Does the product meet all points listed in the specification? If not, return to the design stage and make the required
changes. Does it then meet all of the requirements of the design brief? If not, return to the specification stage and
make improvements to the specification that will allow the product to meet these requirements and repeat from
this point. It is normal to have such iterations in design projects, though you normally aim to keep these to a
minimum.
Improve
Do you feel the product could be improved in any way? These improvements can be added to the design.
Page 7
FM Radio Teaching Resources
www.kitronik.co.uk/2157
The Design Brief
A manufacturer has developed a circuit for an FM radio. The circuit
has been developed to the point where they have a working
Printed Circuit Board (PCB). The manufacturer would like ideas for
how this PCB could be housed to create a final product.
The manufacturer has asked you to do this for them. It is important
that you make sure that the final design meets all of the
requirements that you identify for an FM radio.
Complete Circuit
A fully built circuit is shown below.
Page 8
FM Radio Teaching Resources
www.kitronik.co.uk/2157
Investigation / Research
Using a number of different search methods, find examples of similar products that are already on the market. Use
additional pages if required.
Name………………………………………………… Class………………………………
Page 9
FM Radio Teaching Resources
Requirement
Reason
Example:
It must be easy to access the
Example:
So that they can easily be changed when they become flat.
www.kitronik.co.uk/2157
Developing a Specification
Using your research into the target market for the product, identify the key requirements for the product and
explain why each of these is important.
Name……………………………………………………… Class………………………………
batteries.
Page 10
FM Radio Teaching Resources
www.kitronik.co.uk/2157
Design
Develop your ideas to produce a design that meets the requirements listed in the specification.
Name……………………………………………… Class………………………………
Page 11
FM Radio Teaching Resources
Comment
Reason for comment
Accept or Rej
ect
www.kitronik.co.uk/2157
Design Review (group task)
Split into groups of three or four. Take it in turns to review each person’s design against the requirements of their
specification. Also look to see if you can spot any additional aspects of each design that may cause problems with the
final product. This will allow you to ensure that you have a good design and catch any faults early in the design
process. Note each point that is made and the reason behind it. Decide if you are going to accept or reject the
comment made. Use these points to make improvements to your initial design.
Page 12
FM Radio Teaching Resources
www.kitronik.co.uk/2157
Soldering in Ten Steps
1. Start with the smallest components working up to
the taller components, soldering any interconnecting
wires last.
2. Place the component into the board, making sure
that it goes in the right way around and the part sits
flush against the board.
3. Bend the leads slightly to secure the part.
4. Make sure that the soldering iron has warmed up
and if necessary, use the damp sponge to clean the
tip.
5. Place the soldering iron on the pad.
6. Using your free hand, feed the end of the solder onto
the pad (top picture).
7. Remove the solder, then the soldering iron.
8. Leave the joint to cool for a few seconds.
9. Using a pair of cutters, trim the excess component
lead (middle picture).
10. If you make a mistake heat up the joint with the
soldering iron, whilst the solder is molten, place the
tip of your solder extractor by the solder and push
the button (bottom picture).
Solder joints
Good solder joint Too little solder Too much solder
Page 13
FM Radio Teaching Resources
www.kitronik.co.uk/2157
Resistor Values
A resistor is a device that opposes the flow of electrical current. The bigger the value of a resistor, the more it
opposes the current flow. The value of a resistor is given in Ω (ohms) and is often referred to as its ‘resistance’.
Identifying resistor values
Band Colour 1st Band 2nd Band Multiplier x
Silver
Gold
Black 0 0 1
Brown 1 1 10
Red 2 2 100
Orange 3 3 1000
Yellow 4 4 10,000
Green 5 5 100,000
Blue 6 6 1,000,000
Violet 7 7
Grey 8 8
White 9 9
100 10%
Example: Band 1 = Red, Band 2 = Violet, Band 3 = Orange, Band 4 = Gold
The value of this resistor would be:
2 (Red) 7 (Violet) x 1,000 (Orange) = 27 x 1,000
= 27,000 with a 5% tolerance (gold)
= 27KΩ
10
Tolerance
5%
1%
2%
Too many zeros?
Kilo ohms and mega
ohms can be used:
1,000Ω = 1K
1,000K = 1M
Resistor identification task
Calculate the resistor values given by the bands shown below. The tolerance band has been ignored.
1st Band 2nd Band Multiplier x
Brown Black Yellow
Green Blue Brown
Brown Grey Yellow
Orange White Black
Value
Page 14
FM Radio Teaching Resources
10 12 15 18 22 27 33 39 47 56 68 82
10 11 12 13 15 16 18 20 22 24 27 30
33 36 39 43 47 51 56 62 68 75 82 91
www.kitronik.co.uk/2157
Calculating resistor markings
Calculate what the colour bands would be for the following resistor values.
Value
180 Ω
3,900 Ω
47,000 (47K) Ω
1,000,000 (1M) Ω
What does tolerance mean?
Resistors always have a tolerance but what does this mean? It refers to the accuracy to which it has been
manufactured. For example if you were to measure the resistance of a gold tolerance resistor you can guarantee
that the value measured will be within 5% of its stated value. Tolerances are important if the accuracy of a resistors
value is critical to a design’s performance.
Preferred values
There are a number of different ranges of values for resistors. Two of the most popular are the E12 and E24. They
take into account the manufacturing tolerance and are chosen such that there is a minimum overlap between the
upper possible value of the first value in the series and the lowest possible value of the next. Hence there are fewer
values in the 10% tolerance range.
1st Band 2nd Band Multiplier x
E-12 resistance tolerance (± 10%)
E-24 resistance tolerance (± 5 %)
Page 15
FM Radio Teaching Resources
www.kitronik.co.uk/2157
Capacitor Basics
What is a capacitor?
A capacitor is a component that can store electrical charge (electricity). In many ways, it
is like a rechargeable battery.
V
C
Filling a capacitor with charge
A good way to imagine a capacitor is as a bucket, where the size of the base of the
bucket is equivalent to the capacitance (C) of the capacitor and the height of the bucket
is equal to its voltage rating (V).
The amount that the bucket can hold is equal to the size of its base multiplied by its
height, as shown by the shaded area.
R
R
BATTERY
V
BATTERY CAPACITOR
When a capacitor is connected to an item such as a battery, charge will flow from the battery into it. Therefore the
capacitor will begin to fill up. The flow of water in the picture above left is the equivalent of how the electrical
charge will flow in the circuit shown on the right.
The speed at which any given capacitor will fill depends on the resistance (R) through which the charge will have to
flow to get to the capacitor. You can imagine this resistance as the size of the pipe through which the charge has to
flow. The larger the resistance, the smaller the pipe and the longer it will take for the capacitor to fill.
C
Emptying (discharging) a capacitor
Once a capacitor has been filled with an amount of charge, it will retain this charge until it is
R
connected to something into which this charge can flow.
The speed at which any given capacitor will lose its charge will, like when charging, depend on
the resistance (R) of the item to which it is connected. The larger the resistance, the smaller the
pipe and the longer it will take for the capacitor to empty.
Maximum working voltage
Capacitors also have a maximum working voltage that should not be exceeded. This will be
printed on the capacitor or can be found in the catalogue the part came from. You can see
that the capacitor on the right is printed with a 10V maximum working voltage.
Page 16
FM Radio Teaching Resources
1F = 1,000mF
1F
1F = 1,000,000,000nF
1F = 1,000,000,000,000pF
www.kitronik.co.uk/2157
Ceramic Disc Capacitors
Values
The value of a capacitor is measured in Farads, though a 1 Farad capacitor
would be very big. Therefore we tend to use milli Farads (mF), micro Farads
(F), nano Farads (nF) and pico Farads (pF). A F is a millionth of a Farad, 1F
= 1000 nF and 1nF = 1000 pF.
The larger electrolytic capacitors tend to have the value printed on the side of
them along with a black band showing the negative lead of the capacitor.
Other capacitors, such as the ceramic disc capacitor shown on the right, use a
code. They are often smaller and may not have enough space to print the
value in full, hence the use of the 3-digit code. The first 2 digits are the first
part of the number and the third digit gives the number of zeros to give its
value in pF.
Example: 104 = 10 + 0000 (4 zero’s) = 100,000 pF (which is also 0.1 F)
Work out what value the four capacitors are in the table below.
= 1,000,000F
Printing on capacitor Two digit start Number of zero’s Value in pF
222
103
333
473
Page 17
FM Radio Teaching Resources
www.kitronik.co.uk/2157
How Does a Radio Work?
For around 100 years, radio stations have been transmitting signals which people have been able to receive and
listen to. Early radio transmissions were AM and were received either on simple crystal radios or radios that used
vacuum tubes (valves).
AM radio
AM radio work by combining the audio signal
which you wish to transmit with a higher
frequency carrier signal. AM stands for
‘Amplitude Modulation’ as it is the amplitude of
the carrier signal that is used to carry the audio
signal. This can be seen in the diagram to the
right. The receiver is tuned into the frequency of
the carrier frequency. It then removes the carrier
to recover the audio signal, which you can then
listen to.
FM radio
FM radio came along after AM radio and
offered a much improved signal quality. FM
stands for ‘Frequency Modulation’. As the
name suggests, instead of altering the
amplitude of the carrier signal, FM radio works
by changing the frequency of the carrier signal
(increasing it or decreasing it) to reflect the
audio signal you wish to transmit. This can be
seen in the diagram to the right.
Audio signal
Radio carrier signal Carrier signal combined with audio signal
Audio signal
Radio carrier signal Carrier signal combined with audio signal
Transistor radios
Transistors started to be used in radio during
the middle of the last century. Transistors have
many advantages over vacuum tubes including
being cheaper, smaller and requiring less
power. As a result of this, they helped bring
radio ownership to the masses.
Page 18
FM Radio Teaching Resources
Point to include
:
Point to include
:
Point to include
:
Point to include
:
www.kitronik.co.uk/2157
Instruction Manual
Your radio is going to be supplied with some instructions. Identify four points that must be included in the
instructions and give a reason why.
Reason:
Reason:
Reason:
Reason:
Page 19
FM Radio Teaching Resources
Good aspects of the design
Areas that could be improved
www.kitronik.co.uk/2157
Evaluation
It is always important to evaluate your design once it is complete. This will ensure that it has met all of the
requirements defined in the specification. In turn, this should ensure that the design fulfils the design brief.
Check that your design meets all of the points listed in your specification.
Show your product to another person (in real life this person should be the kind of person at which the product is
aimed). Get them to identify aspects of the design, which parts they like and aspects that they feel could be
improved.
Improvements
Every product on the market is constantly subject to redesign and improvement. What aspects of your design do you
feel you could improve? List the aspects that could be improved and where possible, draw a sketch showing the
changes that you would make.
Page 20
FM Radio Teaching Resources
Requirement
Reason
www.kitronik.co.uk/2157
Packaging Design
If your product was to be sold in a high street electrical retailer, what requirements would the packaging have? List
these giving the reason for the requirement.
Develop a packaging design for your product that meets these requirements. Use additional pages if required.
Page 21
GET IN TUNE WITH THIS
ESSENTIAL INFORMATION
BUILD INSTRUCTIONS
FINDING
MECHANICAL DETAILS
WORKS
Version 2.
2
FM RADIO KIT
CHECKING YOUR PCB & FAULT-
HOW THE KIT
Page 22
FM Radio Essentials
PCB Ref
Value
Colour Bands
R1 100k Brown, black,
yellow
R2 & R9
220k
Red, red
, yellow
R3
56k Green,
blue, orange
R5 & R11
0Ω Black
R6
6.8k Blue, grey, red
R8
3.3k
Orange, orange, red
R10 100Ω
Brown, black, brown
R12 470Ω
Yellow, purple, brown
R13 10k Brown, black, orange
PCB Ref
Value
Text
C1, C
2, C14
100nF
104
C3, C4
22pF 22 C5, C6 470nF
474
C7 470pF 471
C8 1nF
102
C9 33pF 33
PLACE
RESISTORS
1
SOLDER THE CERAMIC DISC CAPACITORS
2
SOLDER THE ELECTROLYTIC CAPACITORS
3
www.kitronik.co.uk/2157
Build Instructions
Before you start, take a look at the Printed Circuit Board (PCB). The components go in the side with the writing on
and the solder goes on the side with the tracks and silver pads.
Start with the eleven resistors:
The text on the PCB shows where R1, R2 etc go.
Ensure that you put the resistors in the right place.
The ceramic disc capacitors should be soldered into the board. There are a lot of these
so be careful to put them all in the correct place. The capacitors can be identified by
the text printed on them (see close up image below right). To allow for easier soldering
wide footprints have been used on the board, the smaller capacitors do not need to be
pushed firmly in to the PCB.
The other six capacitors are electrolytic capacitors. The capacitor C10 is marked 100µF,
place this one first. Make sure that the device is the correct way around. The capacitors
have a ‘-’ sign marked on them, which should match the same sign on the PCB. The
other 5 are all marked 1µF and should go in the spaces marked C11, C12, C13, C15 and
C16.
Page 23
FM Radio Essentials
PCB Ref
Value
R4
100k
R7
10k
SOLDER THE TIMING CRYSTAL
4
5
SOLDER THE ANTENNA
6
SOLDER THE
BATTERY CAGE
7
SOLDER THE SPEAKER
8
Value
www.kitronik.co.uk/2157
This is a very small metal cylinder with two very thin legs protruding from one end. Gently place this
component into the holes in the PCB where it is marked ’XTAL1’. The legs are fragile so care should be taken
when putting them through the holes in the PCB.
SOLDER THE POTENTIOMETERS
The two potentiometers are different so make sure that you check which is which
before soldering them into position. When placing the potentiometer into the board
make sure the knob is pointing outwards from the PCB.
Strip one end of the thin white wire and solder it into the hole on the PCB labelled ‘Antenna’. The longer the antenna
wire is up to 56cm the better it will perform. It is not a problem to have an antenna longer than 56cm as it won’t
degrade the reception. Keep the off cut for the speaker.
Next look for the two holes labelled ’Power’. Thread the wires from battery cage through the
strain relief hole near these holes then solder the red wire into the hole marked ‘red’ and the
black wire into the hole marked ‘black’.
Finally, cut the wire left over from the antenna connection in half and strip the ends. Thread both
lengths of wire through the strain relief hole marked ‘Speaker’ and solder one of the wires to
each of the two holes, it doesn’t matter which one goes in either hole.
Solder the other end of the speaker wires to the terminals on the speaker. Again it doesn’t
matter which way around these go.
Page 24
FM Radio Essentials
1
2
3
www.kitronik.co.uk/2157
Checking Your FM Radio PCB
Check the following before you connect any batteries to your radio.
Check the bottom of the board to ensure that:
All holes (except the four large mounting holes) are filled with the lead of a component.
All these leads are soldered.
Pins next to each other are not soldered together.
Check the top of the board to ensure that:
The negative markings on the electrolytic capacitors line up with the same markings on the PCB.
The red wire on the PP3 lead is connected to the power connector labelled ‘Red’ and the black wire on the
PP3 lead is connected to the power connector labelled ‘Black’.
Additionally check that the resistors and capacitors are in the correct place if your board does not work.
Adding an On / Off Switch
If you wish to add a power switch, don’t solder both ends of the battery clip directly into the board, instead:
Solder one wire from the battery cage to the PCB, either black to ‘-’ or red to ‘+’.
Solder the other wire from the battery cage to the on / off switch.
Using a piece of wire, solder the remaining terminal on the on / off switch to the remaining
power connection on the PCB.
Page 25
FM Radio Essentials
www.kitronik.co.uk/2157
Designing the Enclosure
(All measurements are in mm)
Mounting the PCB to the
enclosure
The drawing to the left
shows how a hex spacer
can be used with two bolts
to fix the PCB to the
enclosure.
Your PCB has four
mounting holes designed
to take M3 bolts.
Page 26
FM Radio Essentials
circumference = 12cm
www.kitronik.co.uk/2157
Improving the Radio Tuning
Tuning the radio using the potentiometer can be a tricky task. This is because one turn of the potentiometer covers
the whole frequency range. This means that only very small adjustments of the potentiometer are required to tune
into the different stations.
Large Dial
One way to improve the tuning mechanism is to attach a large dial to the end of the potentiometer. Look at the two
dials below. As the smaller dial has a circumference six times smaller than the large dial, it would take six time the
travel on the larger dial to achieve the same degree of change than if you used the smaller dial. This would therefore
make fine tuning the radio six times easier.
Dial / Pulley
circumference = 12cm
45 degree adjustment = 1.5cm travel
Dial / Pulley
circumference = 2cm
45 degree adjustment = 0.25cm travel
Using a Pulley to give Mechanical Gearing
Another solution is to use a pulley system to improve the tuning mechanism. Take the system shown below. In this
arrangement the larger pulley would be attached to the tuning potentiometer and the smaller dial would be used to
make the tuning adjustments. As the smaller dial (pulley) has a circumference six times smaller than the large pulley,
it would take six full rotations of the smaller dial to give one full rotation of the larger pulley. This would therefore
make fine tuning the radio six times easier than if the small pulley had been connected directly to the tuning
potentiometer.
Potentiometer
Dial
circumference = 2cm
Pulley
Using both a Pulley System and a Large Dial
Of course and even better solution would be to add a large dial in front of, but on the same shaft that the small
pulley/dial in the above diagram is attached to. That way you would get the benefit of both systems this would give
thirty six times better adjustment.
Page 27
FM Radio Essentials
www.kitronik.co.uk/2157
How the FM Radio Kit Works
To aid the description of the circuit we have split it into two parts. The first deals with the decoding of the FM radio
signal and the second deals with the audio amplification.
Radio decoding
For the radio circuit to work it needs a stable power supply. To provide the stable power supply a voltage regulator is
used. This provides a 3.3V output for the circuit to run from. Capacitors C15 and C13 help the regulator to output a
steady voltage by removing noise from its input and output.
The key component of the radio circuit is the Si4820/24-A10 chip. This chip is used to receive radio signals via an
‘antenna’ and then decodes this signal to recover the audio signal it carries. To recover the radio signal the chip
needs to generate a timing signal to match the radio signal to. This is achieved using the timing crystal XTAL1 and the
two capacitors C3 and C4.
The resistor network composed of R2, R3, R4, R8, R9 and R10 controls the range or ‘band’ of frequencies that the IC
is able to tune into. Different countries use different bands for FM radio and these values have been chosen for best
performance in the UK and Europe. R4 is a potentiometer which can be turned to swap from station to station.
L1 and C9 help to filter out unwanted signals received by the antenna. C10 and C14 help to keep the tuning signal to
the IC steady to prevent fluctuations in power from causing the IC to lose the station it has been tuned to.
The ESD protection diode protects the IC in the event of a static shock being delivered to the antenna. Static shocks
are harmless to people but can damage the silicon inside the IC. The ESD protection diode redirects any such shock
harmlessly to the ground rail.
C1 controls the reset timing of the IC which in this case only applies when switching on the device as this pin is held
low by R1. C6 takes any DC bias out of the output signal from the radio IC and R5 and R11 just acts as a wire links to
“jump” over tracks on the PCB.
Page 28
FM Radio Essentials
www.kitronik.co.uk/2157
Audio amplification
The audio amplification is performed by another Integrated Circuit or IC called an NCP2890-D. Inside the NCP2890-D
are lots of transistors, which are connected together to allow the small input signal to be amplified into a more
powerful output that can drive a speaker.
All amplifiers need to use feedback to ensure the amount of gain stays the same. This allows the output to be an
exact copy of the input just bigger. The gain is the number of times bigger the output is compared to the input, so if
an amplifier has a gain of 10 and there is 1 volt on the input there will be 10 volts on the output. Before looking at
how the feedback works, we first need to understand how a standard amplifier works. An operational amplifier has
two inputs, these are called the inverting (-) and non-inverting (+) inputs. The output of the operational amplifier is
the voltage on the non-inverting input less the voltage on the inverting input multiplied by the amplifiers gain. In
theory an operational amplifier has unlimited gain so if the non-inverting input is a fraction higher than the inverting
input (there is more + than -) the output will go up to the supply voltage. Change the inputs around and the output
will go to zero volts. In this format the operational amplifier is acting as a comparator, it compares the two inputs
and changes the output accordingly.
With an infinite gain the amplifier is no good to
amplify audio, which is where the feedback comes in.
By making one of the input a percentage of the output
the gain can be fixed, which allows the output to be a
copy of the input but bigger. Now when the two
inputs are compared and the output is adjusted,
instead of it going up or down until it reaches 0 volts
or V+, it stops at the point when the two inputs match
and the output is at the required voltage.
Looking at the circuit diagram for the audio amplifier
you can see R6 connecting the pin ‘OutA’ back into the
input pin ‘INM’. This is the feedback for the amplifier.
Input
X10 gain
Output
Amplifier
90%
10%
Page 29
FM Radio Essentials
www.kitronik.co.uk/2157
Audio amplification continued
The output of the NCP2890-D uses two op-amps that are connected in a differential configuration. The second
amplifier produced a 'mirror' (or reverse) of what the first op-amp produces. The output of one feeds OutA and the
other feeds OutB. This configuration allows twice the power to be generated compared to amplifiers using a single
op-amp configuration.
The rest of the components are needed as follows:
R7 is the volume control potentiometer. C5 and R12 filter the input signal. C7 increases bass response by allowing it
to bypass R13. C11 and 12 keep the power supply to the IC smooth.
Page 30
This kit is designed and manufactured in the UK by Kitronik
Online Information
Two sets of information can be downloaded from the product page where the kit can also be reordered from. The
‘Essential Information’ contains all of the information that you need to get started with the kit and the ‘Teaching
Resources’ contains more information on soldering, components used in the kit, educational schemes of work and so
on and also includes the essentials. Download from:
www.kitronik.co.uk/2157
Every effort has been made to ensure that these notes are correct, however Kitronik accept no responsibility for
issues arising from errors / omissions in the notes.
Kitronik Ltd - Any unauthorised copying / duplication of this booklet or part thereof for purposes except for use
with Kitronik project kits is not allowed without Kitronik’s prior consent.
Loading...