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An FM Radio Tuner
Card for your PC
Fancy an FM radio receiver inside your PC?
This simple circuit plugs into a spare slot on
your PC’s motherboard and is tuned using an
on-screen display.
Why on earth would you want to fit
an FM receiver into your PC?
Well, why not? If you’re the type
who enjoys music while you work or
while you take a break, this FM radio
receiver is only a couple of mouse
clicks away. It’s a mono-only design
but when you’re working you’re not
likely to notice the difference, espe
cially when using low-cost multimedia loudspeakers.
A PC-controlled FM receiver has
several advantages. It’s convenient to
use, there are no batteries to go flat
and, because it fits inside the PC and
is software controlled, you don’t have
to worry about a case, a tuning knob
or an external power supply.
Unlike most other PC-controlled
tuners, this circuit includes its own
on-board audio amplifier and this is
capable of driving a set of external
speakers to good volume. In other
words, this circuit operates independently of the sound card. This
means that the sound card can be
Design by MARK ROBERTS
18 Silicon Chip
FEATURES
•
•
•
•
•
•
Full 88-108MHz FM band
coverage
Fully self-contained on one ISA
card
Computer controlled via your
screen
3 preset memories for your
favourite FM stations
Slider volume control
Does not need a sound card –
fully independent operation.
used for other purposes while the FM
receiver is operating.
The circuitry for the PC FM Tuner is
The circuit is built on an ISA card which plugs into a spare slot on your PC’s
motherboard. Note that several changes were made to the PC board layout after
this photo was taken.
memory presets (1, 2 or 3). The receiver also “remembers” the last station it
was tuned to when it was turned off.
built on an expansion card and plugs
into a standard ISA slot on the your
PC’s motherboard. A single on-board
link sets the address to either 300H
or 301H. Note that the card isn’t Plug
and Play (PnP), so if you have other
non-PnP cards in your system you may
have to manually juggle the resources
to suit.
The screen grab on the facing page
shows the display that’s used to
“drive” the FM tuner. There’s really
not much to it! The top half consists
of a linear dial scale to show the tuned
frequency (88-108MHz), while the bottom half carries the controls – an On/
Off button, a Mute button, a volume
control slider, three memory preset
buttons and a tuning knob.
You tune the unit by dragging the
tuning “knob” with the mouse, or you
can click anywhere on the circumference of the knob to tune to that spot.
Alternatively, you can tune the unit
Block diagram
by clicking the up and down arrows.
In addition, up to three stations can
be stored in memory by clicking the
‘Memory’ button (so that it displays
‘WR’) and then clicking one of the
Fig.1 shows the block diagram of
the PC FM Tuner. It’s built around a
Philips TDA7000 FM radio IC, which
is virtually a complete FM radio on a
Fig.1: the block diagram
of the PC FM Tuner.
Most of the hard work
is done by the Philips
TDA7000 IC.
JUNE 1999 19
lator so that the received deviation is
always less than ±15kHz.
In effect, the recovered audio signal
is compressed to reduce its dynamic
range.
Although this isn’t desirable in a
hifi FM tuner, the results are still quite
good and this technique considerably
simplifies the filtering circuitry that
would otherwise be required.
Basically, the technique trades
dynamic range for lower audio distortion. In fact, the distortion is typically
less than 2.3% at ±75kHz deviation,
so your favourite FM station will still
come in loud and clear.
Circuit details
Fig.2: inside the Philips TDA7000 FM receiver IC. This device is virtually
a complete FM tuner on a single chip. Also shown are external components to make a full working FM receiver (we did just that in the November 1992 issue of SILICON CHIP). This time, though, it is teamed with other
components to make the PC-based tuner.
single chip. It drives an LM386 audio
amplifier stage via an 8-step analog
multiplexer, the latter providing the
volume control function.
The multiplexer is controlled by the
data on the PC bus and this in turn is
controlled by the software. The PC bus
also controls a D/A converter stage
to provide the tuning voltage to the
TDA7000 chip. This tuning voltage
is applied to a varicap diode.
In addition, the PC bus controls a
muting switch which connects to the
muting circuit of the TDA7000. This
allows the receiver to be muted when
tuning between stations by setting the
Muting button on the control panel to
the ‘on’ position.
The TDA7000 chip
Fig.2 shows the various circuit
blocks inside the TDA7000, as well as
the external parts required to make a
complete FM tuner.
Unlike many other FM tuners, this
design is easy to align since only the
local oscillator (ie, the VCO) requires
adjustment. This is done by ‘tweaking’
the coil across the VCO, so that the
20 Silicon Chip
tuner covers the desired frequency
range.
The TDA7000 IC more or less
functions as a conventional superheterodyne tuner. This means that the
incoming FM signal is mixed with a
local oscillator signal (from the VCO)
to produce an intermediate frequency
(IF). This IF signal is then filtered to
remove any mixer artefacts and then
demodulated to recover the desired
audio signal.
There’s just one deviation from normal practice. Virtually all FM broadcast receivers use an IF of 10.7MHz
whereas the TDA7000 uses an IF of
just 70kHz. So why does it do this?
The answer is that an IF of 70kHz
can be filtered using standard active
op amp circuits instead of coils and
ceramic filters. Normally though, a
low IF results in high distortion levels
when used with wideband deviation
FM (broadcast band FM has a maximum deviation of ±75kHz).
The TDA7000 overcomes the problem by employing a clever trick. What
happens is that the recovered audio is
also used to modulate the local oscil-
Take a look now at the complete
circuit diagram shown in Fig.3. You
can easily discover the functions of
the main ICs by relating them back to
the block diagram.
As mentioned above, the TDA7000
IC (IC5) is really the heart of this
circuit. The incoming RF signal is
picked up by the antenna and fed to
the TDA7000’s internal mixer (pins 13
& 14) via a bandpass filter, consisting
of two 27pF capacitors (C4 & C5) and
inductor L1. Its job is to filter signals
that lie outside the desired tuning
range and thus eliminate interference.
Varicap diode D1 and inductor L2
are used to tune IC5’s internal voltage
controlled oscillator (VCO) so that
the receiver covers the FM broadcast
band. The tuned frequency in turn
depends on the voltage applied to
D1, which varies its capacitance
ac
cordingly. This tuning voltage is
derived from the D/A converter (IC4).
The recovered audio signal appears
on pin 2 of IC5 and is fed via a lowpass filter (R5 & C8) to the top of a
resistive divider network (R15-R22).
The filter stage, in conjunction with
the divider resistance, provides the
necessary 50µs de-emphasis for the
recovered audio signal.
The eight steps in this divider are
in turn fed to the X0-X7 inputs of IC3,
the 4051 analog multiplexer. This IC is
controlled by the signals on its three
binary control inputs, designated A,
B & C (pins 11, 10 & 9). In operation,
the three binary control signals select
which of the eight input channels is
switched through to the output at
pin 3.
In essence, IC3 functions as a single-pole 8-position switch. It selects
one of eight input levels and applies
Fig.3: the circuit might look complex but it
is based on just a few ICs and a sprinkling
of other components. Best of all, it's easy to
build and get going!
JUNE 1999 21
Parts List
1 double-sided PC board, code
06106991
1 3.5mm mono PC-mount mono
speaker socket
1 PC jumper link
1 100mm length 0.7mm
enamelled copper wire
1 backplane bracket plus
rightangle brackets (see text)
2 750mm length light-duty insulat-
ed hookup wire (for antenna)
1 PC FM Tuner software utility
(download Pcfmtune.zip from
www.siliconchip.com.au)
1 2.2µF 16VW electrolytic (C28)
11 0.1µF MKT polyester
(C2, C3, C11, C12, C17, C20,
C21, C23, C25, C26, C27)
1 0.1µF ceramic (C31)
1 .0047µF ceramic (C6)
1 .0039µF ceramic (C1,C13)
1 .001µF ceramic (C8)
2 330pF ceramic (C15,C18)
1 270pF ceramic (C9)
1 220pF ceramic (C19)
2 100pF ceramic (C14, C16)
2 27pF ceramic (C4, C5)
1 4.7pF ceramic (C10)
Semiconductors
1 74LS273 octal D-type flipflop
(IC1)
1 LM386 audio amplifier (IC2)
1 4051 4-channel analog
multiplexer (IC3)
1 MAX504 D/A converter (IC4)
1 TDA7000 FM receiver (IC5)
1 LM7805 5V regulator (IC6)
2 74LS05 hex inverters (IC7,IC8)
1 74LS32 quad 2-input OR gate
(IC9)
2 BC548 NPN transistors (Q1,Q4)
2 BC327 PNP transistors (Q2,Q3)
1 BB809 varicap diode (D1)
Resistors (0.25W, 1%)
4 82kΩ (R1,R2,R10,R11)
1 47kΩ (R7)
2 33kΩ (R4,R8)
1 22kΩ (R5)
2 10kΩ (R12,R15)
1 6.8kΩ (R16)
1 5.6kΩ (R17)
5 2.7kΩ (R3,R9,R14,R18,R20)
1 2kΩ (R19)
2 1.5kΩ (R6,R21)
1 1.2kΩ (R22)
1 1kΩ (R13)
1 10Ω
Capacitors
1 470µF 10VW electrolytic (C24)
1 220µF 16VW electrolytic (C22)
3 10µF 16VW electrolytic
(C7,C29, C30)
the switched audio output to pin 3 of
the LM386 audio amplifier stage (IC2).
IC2 operates with an AC gain of
20 by virtue of its internal feedback
components. The amplified output
appears at pin 5 and is coupled to the
loudspeaker via a 470µF capacitor.
Control circuitry
IC1 and IC4 are the main control
circuits for the tuner. The data on the
ISA bus is generated by the software
and is applied to data inputs D0-D7 of
IC1, a 74LS273 octal D-type flipflop.
This device contains eight identical
D-type flipflops and functions as a
buffer stage for the data lines.
Inverter stages IC7 & IC8, together
with OR gates IC9a & IC9b, form a
hardware decoder which sets the I/O
address of the card.
22 Silicon Chip
Note: a kit of parts for this project is
available from Jaycar Electronics.
The kit includes all parts including a
PC board with plated-through holes,
but does not include the backplane
connector or the software.
This decoder monitors the A0-A9
address lines of the ISA bus and, when
the correct address (either 0300H or
0301H) is present, pulls pin 9 of IC7d
high. This in turn switches pin 9 of OR
gate IC9c low, which means that signals on the IOR (Input/Output Read)
line are applied to pin 12 of IC9d.
Provided that the AEN (address enable) line is low, this signal also appears
on pin 11 and is used to clock IC1.
In other words, the AEN and IOR
lines decide when the address is accessed. Each time pin 11 of IC9d goes
high, the data on the D0-D7 inputs is
latched into IC1 and appears at the
Q0-Q7 outputs.
Outputs Q0-Q2 of IC1 are used to
control the D/A converter (IC4), which
in turn produces the tuning control
voltage for the varicap diode (D1). IC4
is a MAX504 10-bit D/A converter.
The serial data on Q0 of IC1 (as generated by the software) is fed into pin
2 (DIN), while Q2 and Q1 drive the
clock (CLK) and chip select (CS-bar)
inputs respectively.
The analog voltage output appears
at pin 12 (VOUT) of IC4 and is applied to the varicap diode via a 47kΩ
resistor.
The next three ‘Q’ outputs from IC1
(Q3, Q4 & Q5) are fed to the binary
control inputs (pins 11, 10 & 9) of IC3.
These lines switch the multiplexer to
select one of eight volume levels, as
described previously.
Transistors Q1 and Q2 form the
muting switch and are controlled by
the Q6 output of IC1. When Q6 of
IC1 is high, both transistors are on
and pin 1 (Mute) of the TDA7000 is
connected to the +5V rail (Vcc) via a
10kΩ resistor. This turns the muting
circuit in IC5 off. Conversely, when Q6
is low, transistors Q1 & Q2 are off and
the muting circuit turns on.
Output Q7 of IC1 controls transistors Q4 & Q3 to provide on/off
switching. When Q7 is high, Q4 turns
on and provides base current for Q3.
Thus, Q3 also turns on and connects
the +12V line from the ISA bus to the
input of 3-terminal regulator IC6. IC6
in turn provides a regulated 5V rail to
power the circuit.
If Q7 subsequently goes low (ie, if
the on/off button on the software-generated control panel is clicked to ‘off’),
Q4 and Q3 both turn off. As a result,
no power is applied to the input of
the regulator and so the circuit shuts
down.
Construction
All of the parts for the FM radio
(except for the loudspeaker), are fitted
to a PC board coded 06106991. Fig.4
shows how the parts are fitted.
The prototype was built on a double-sided board with plated-through
holes. If your board doesn’t have plated-through holes, it’s simply a matter
of soldering all component leads on
both sides of the board.
You will also have to fit vias (links)
to the unused holes, to connect tracks
on one side of the board to their corresponding tracks on the other. But
more on this in a moment.
Before starting construction, inspect
the board carefully to ensure that it
has been correctly etched. This done,
start the assembly by installing all the
Capacitor Codes
Fig.4: the parts layout for the FM Tuner Card. This is a double-sided PC board
– the component side is shown in grey and the underside in blue. If you don’t
have a plated-through board, the points marked solely with a dot must be fitted
with “pin throughs” (or vias) and you must solder the component leads on both
sides of the board (see text).
resistors, the capacitors and the ICs.
Table 1 shows the resistor colour
codes, while Table 2 shows the codes
for the MKT polyester and ceramic capacitors. It’s also a good idea to check
each resistor on a digital multimeter,
just to make sure of its value.
Note particularly that a 0.1µF ceramic capacitor is in
stalled on the
copper side of the board, directly
beneath IC1.
Keep all capacitor leads as short as
possible and don’t forget to solder all
component leads on both sides if the
board doesn’t have plated-through
holes (this includes the ICs).
All the ICs can be directly soldered
to the PC board. Take care to ensure
that they are all oriented correctly and
don’t get them mixed up.
Next, install the varicap diode (D1),
the four transistors and the 3-terminal
Value
IEC Code EIA Code
0.1µF
100n
104
.0047µF
4n7
472
.0039µF
3n3
392
.001µF
1n0
102
330pF
330p
331
270pF
270p
271
220pF
220p
221
100pF
100p
101
27pF
27p 27
4.7pF
4p7
4.7
regulator. Once again, take care not to
get the transistors mixed up and watch
their orientation.
In particular, note that Q3 and Q4
face in opposite directions. The regula
tor is mounted with its leads bent at
rightangles, as shown in the photo.
Now for the two inductors (L1 and
L2). These are both made by winding
0.7mm enamelled copper wire (ECW)
onto a 4mm former (eg, a 4mm drill
bit). L1 consists of six closely-spaced
turns, while L2 consists of five turns,
evenly spaced to form a coil 8mm long
(this coil is later adjusted during the
alignment procedure).
After winding each coil, slide it off
the drill bit, scrape away the enamel
from its leads and push it all the
way down onto the PC board before
soldering.
The 3.5mm audio socket and
the backplane bracket can now be
installed. You can either make up
a couple of rightangle brackets to
attach the backplane bracket, or you
can salvage a backplane bracket with
integral attaching points from an old
Table 1: Resistor Colour Codes
No.
4
1
2
1
2
1
1
5
1
2
1
1
1
Value
82kΩ
47kΩ
33kΩ
22kΩ
10kΩ
6.8kΩ
5.6kΩ
2.7kΩ
2kΩ
1.5kΩ
1.2kΩ
1kΩ
10Ω
4-Band Code (1%)
grey red orange brown
yellow violet orange brown
orange orange orange brown
red red orange brown
brown black orange brown
blue grey red brown
green blue red brown
red violet red brown
red black red brown
brown green red brown
brown red red brown
brown black red brown
brown black black brown
5-Band Code (1%)
grey red black red brown
yellow violet black red brown
orange orange black red brown
red red black red brown
brown black black red brown
blue grey black brown brown
green blue black brown brown
red violet black brown brown
red black black brown brown
brown green black brown brown
brown red black brown brown
brown black black brown brown
brown black black gold brown
JUNE 1999 23
Sorting out I/O and resource problems . . .
Fig.5: click on Start>Control Panel>System>Device
Manager to bring up this window, showing which
devices are installed and any problems (indicated by a
yellow question mark).
Fig.6: if you double-click the Computer icon in Fig.5
above, then select Input/output (I/O), you’ll get a
complete listing of all I/O addresses being used and the
hardware that’s using them. By selecting the other
buttons at the top of the window, you can also find
which IRQs are being used and by what, which memory
the devices are using and also which devices are using
direct memory access (DMA) channels. The Reserve
Resources tab allows you to allocate resources for legacy
cards if necessary (ie, non-PnP cards) to avoid conflicts.
24 Silicon Chip
Fig.7: from Fig.5, double click the device you're
interested in, then click the Resources tab and it will tell
you which interrupt request (IRQ) and I/O range is being
used by that device and, most importantly, if there are
any device conflicts. In this case, we’re in the clear.
Fig.8: if necessary, you can manually change the
resources allocated to existing cards – just select the
setting you wish to change in Fig.7, then click the Change
Setting button and enter in the new values . One bonus is
that you get to see immediately if you have entered values
which conflict with other devices. If so, change the values
to something that doesn’t cause problems!
Fig.9: this is the “receiver” that pops up on your screen when you load the
software. At this stage it is not turned on – clicking the “power” button will do
that for you. The other controls are a slide volume control, three memory preset
buttons, a “rotary” tuning knob and a pair of “click and hold” tuning buttons.
expansion card. A hole will have to
be drilled in the bracket, to align with
the audio output socket. Once all the
parts have been fitted, you will notice
that there are quite a few vacant holes.
If you don’t have a plated-through
board, what you have to do now is
install “pin-throughs” at each of these
hole locations. These can be made
from tinned copper wire and are soldered to both sides of the PC board.
Now would also be a good time to
check that all component leads are
soldered to their pads on the top of
the PC board. Don’t neglect this step
– one missed solder connection on the
top of the board is enough to stop the
circuit from working.
Strictly speaking, you only have to
solder those pads on the top of the
PC board that have tracks running to
them. However, by soldering all the
pads, you can be sure of not missing
any.
Finally, connect an antenna by soldering a 750mm length of light-duty
hook-up wire to the PC board. The
antenna lead is then fed through the
backplane connector via a small hole
drilled adjacent to the antenna connection point.
Software
The software for this project can be
downloaded from the SILICON CHIP
website, www.siliconchip.com.au
The download is free and the file you
want is called Pcfmtune.zip (it will be
at or near the end of the downloadable
software listing).
You’ll find it by clicking the “Software Downloads” link on the home
page. If you don’t have Internet access, you can buy the software on two
floppy disks from Silicon Chip Publications for $12, including postage).
Unzip the file after downloading,
then install the software by running
setup.exe. Assuming you’re running
Windows 95/98, this will install
the files in a folder called ‘vhf’ and
install the necessary entries in your
Start menu.
Installing the card
You will need to set the I/O address
of the card before installing it in the
computer. In most cases, the default
address of 300H should work just fine.
This is set be installing the jumper
across pins 2 & 3. If you strike problems, try the alternative 301H address
setting (ie, jumper pins 1 & 2).
Neither of the available addresses
should cause any conflicts with commercial expansion cards. If you do
strike problems, you can check the
resources that are being used via the
System Properties utility in Windows
95/98.
To view these, double click the
System icon in Control Panel, click the
Device Manager tab and double-click
on Computer at the top of the list of
devices.
You can now check the I/O addresses that are in use by selecting the
Input/Output (I/O) button – see Fig.6.
If you do find a card that occupies the
300H/301H address space, try chang
ing the resources assigned to that card.
To do this, double-click the device
in the Device Manager list, click the
Resources tab, click on the resource
setting you wish to change (in this
case, the Input/Output Range) and
click the Change button – see Fig.7.
Note that if the card isn’t a plug
Fig.10: clicking on the little button at
bottom left of the dial scale brings up
this “about” box which, among other
things, tells you the voltage being
applied to the varicap diode to tune
the station being listened to at that
time.
and play type, it will also often be
necessary to change its configuration
using the software setup disk that
came with it. Having said all that,
we don’t expect too many problems
with resource conflicts. In nearly all
cases, is should simply be a matter of
plugging it in.
By the way, don’t forget to connect
a loudspeaker. As mentioned at the
start, the audio output from the PC
FM tuner isn’t directed through the
sound card, so you can’t rely on its
loudspeakers.
If you have a spare pair of multimedia loudspeakers, try plugging them
Fig.11: this screen grab shows the
contents of the vhf.ini file which
records the preset channels, their
volumes and the mute status. If you
really wanted to, you could alter the
data using a text editor and the FM
Receiver would respond next time it
is turned on. But why bother when the
software does it all for you anyway?
JUNE 1999 25
directly into the PC FM tuner’s audio
output socket. If you get sound (mono)
through both loudspeakers, you’re in
business. If not, you will need a suitable mono-to-stereo adapter socket.
A word of warning here – do not
use a conventional (unshielded) loudspeaker in close proximity to your
computer’s monitor. If you do, it could
magnetise the internal shadow mask
and cause strange colour patches.
Always use properly designed multimedia speakers if you want them on
the desk.
Test & alignment
top
Fig.12: two patterns are required for this double-sided PC board. The
pattern above is for the top (component) side while the pattern below is
the ‘normal’ copper side. If etching your own board for this project, great
care will need to be taken to ensure that the two patterns line up correctly
on the blank board. The easiest way to do this is with some form of pin
registration on the board and through the film patterns.
26 Silicon Chip
Now run the software (Start, Programs, FM Receiver, FM Receiver).
The first thing you should see is the
FM Receiver image on screen (see
Fig.9) but you shouldn't hear anything
yet, because you haven’t turned the
“receiver” on.
Move your mouse pointer to the
“on” button and click it. The FM
receiver “dial” now lights up and
the power button illuminates green.
Now you should hear some sound
coming from your speaker(s). Clicking
on the “mute” button should quieten
inter-station noise.
Click the mute back off and try tuning in some stations. If you’re within
about 20-30km of some reasonably
strong FM stations, you should be able
to pick them up.
Sweep through the entire frequency
range and keep a record of the stations
you hear and their locations on the
‘dial’. You will need to know which
stations are on which frequencies – in
many cases, FM stations broadcast
their frequency as part of their callsign
or station promotions.
If the indicated station frequencies
are higher than they should be, spread
the turns on inductor L2 to decrease
its inductance. Conversely, if the indicated frequencies are too low, push
the turns closer together.
Basically, it’s just a matter of adjusting L2 so that you can tune right
across the FM broadcast band (from
88-108MHz) with the stations in the
correct locations on the dial. Be sure
to make only small adjustments to L2
at any one time before re-checking the
frequency range.
Make sure too that the computer is
switched off each time you remove
and replace the tuner card, to avoid
possible damage to this card or to the
SC
motherboard.
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