It was five years
ago when I did an attempt to build my first fm-transmitter. It ended in
a giant faillure. The only thing it did was interferring with our
tv-set. Looking back it was due to the lack of information I had. A
schematic was my only help. Now, five years later, I know a lot more
about electro-technics. So I searched for a schematic of a stable,
tested fm-transmitter with a far reach. I will put all information
you'll have to know in my page. I made drawings to make things clearer.
As said before: I'm still building it, so I will add information every
time I made progress. It would be wise for you out there not to start
building untill I'm ready and have tested it. It has been succesfully
built before, but my succes will give you a double security. I remind
you of the fact that I can also fail.
Intro
Building a
good fm-transmitter(88-110Mhz) begins with getting a good schematic. You
don't have to understand the precise working of the transmitter to build
it. But some basic information won't harm. A transmitter alone is, as
you probably know, is not enough to start your radio-station. In the
simplest form you need 4 things. First an input device such as an
amplifiler you also use with your home-stereo.
You can also use a
walkman. Details about input-devices in the page: "Input". Second you
need a regulated power-supply. In this case a 14-18 Volts/2,5-3,5
Ampere. One of the most influencial things you need is antenna and
coax-cable. More about this later on. And finally the transmitter
itself. You can devide the transmitter in two main parts: the oscilator
and the amplifiler. The oscilator converts electric sound information
into electromagnetic waves. The amplifiler gives these waves
a bigger
amplitude.
Building
It's stable and has
output of 15-18 watts. This enough to terrorize your wide surroundings
at the fm-band.
The most often used
technique to connect the components to each other is soldering them on a
double sided copper-board. Another way is connecting the components
floating. It is cheaper but very tricky. Below you see the copper-board
layout(PCB). I designed it looking closely at the root scheme.
Full size PCB:
PCBGRID.GIF
To get this
pattern in copper surface you use a acid bath. Use a water-resistant
permanent marker to paint your own copper-board black in the pattern the
shown above. Color the back side ompletely black. The grid-squares are
0,5*0,5 cm each.
When the acid has
eaten the non-painted copper away you must remove the complet thin layer
of black paint with sandpaper. Don't remove too much copper with
it.
So, now you have the surface to solder the electric
components on.
Now a few basic rules for good soldering:
1.
Use a special electronics-solderingrod with a slim top.
2. Use
soldering-metal with an anti-oxidant-fluid core.
3. Don't heat the
components! Heat the connection-point on your PCB.
4. Make sure that
the surface is not too smooth.
5. Don't use too much metal.
6.
Don't let the soldering metal form a bridge beetween two
copper-surfaces.
7. If you're smart you start from the middle of your
prepaired board.
In this way you'll have enough space.
Below
the schematic. The yellow lines are pieces of copperboard that devide
the transmitter in 3 parts. This is essential. Without them, internal
interferrence will ruin your signal.
This
picture is the coldwater-version... For the detailled version:
PCBCOMPLETE.GIF

Components
For
readable version:
COMPON.GIF
There
are some components that need extra attention. Transistors usually have
3 or 4 different
wires comin' out. If you connect these wires in the
wrong way the transmitter won't work. It may even explode. The picture
below shows how to prevent from such an event.
You can find
the numbers and letters back in the soldering schematic.
Coils
also require extra attention. You can buy the coils trough ferrite in
the shop, but the other ones have to be made yourself. Use 1mm AgCu
wire. A coil like 7x/d=10mm/l=15mm, goes round 7 times, has an diameter
of 10 millimeter and is long 15 millimeters. The best way to make a coil
is to bend it around a pencil or other cilindrical shaped object tight.
The diameter of the object is always d-coil minus 1 mm. In this case
9mm. As I said: bend the wire round (in this case 7times) with the
revolves tight together. To get the desired length stretch the coil when
still around the pencil
If you decide to build the transmitter
and buy the parts, this list will be handy:
COMPON.DOC
READ THIS E-MAIL I
RECEIVED
erwin.huybreghts@verhaert.com
Hello,
just to give
some input: I have built the 15W FM transmitter you describe about 4.5
years ago.
The PCB lay out and component selection is still the same
as it was then and after some modifications, I had an average output
power of 16.8W @ 98.6 MHz (measured with Rhode and Schwarz equipement).
You will need additional filtering on the power lines otherwise a stable
power supply for the modulating circuit cannot be guaranteed. The legs
of the modulating diode are, at best, kept long for extra capacitance.
This to make sure you fall within the FM band because before I did that,
I had
problems falling withing the 88-108 MHz. I was actually
interfering with the police and fire brigade radio bands (Belgium). Of
course, this is not the intention. I also advice you and readers to
carefully check the orientation of the BLY88 because my professor blew
one up due to lack of specification and inclarities in the datasheets
(the actual pin out of the component changed a few years ago, resulting
in a swapped emitter and collector - no good if you position it wrong!!!
(the white cap flies of)). You will also need to play with the spacing
between the windings of the different coils in order to get a good
coupling between the different stages. I short circuited parts of the
coils and made them smaller than specified to have near-optimal
coupling. I also added extra ferrite bead coils for extra decoupling of
the power lines, and used a very good shielding. Above 16.8W there is
coupling (primarily through the air) between the output and the
modulating/input stage and oscillation occurs. So for I have not found
any other solution than lower the output power. Both extra
decoupling
and extra shielding had no effect (my transmitter is built
into a fully closed aluminium box with seperating plates that are fully
connected to the case or ground plane on the PCB, except from where
tracks run (0.5mm spacing provided)). Also, use a good heat sink for the
last power stage!!!
I hope this information will be usefull. If you
have any questions, please ask.
Kind
regards,
Erwin
Erwin Huybreghts
Electronic
Engineer
Space applications and space instruments
division
Verhaert D&D
Hogenakkerhoekstraat 9
9150
Kruibeke
Belgium
Tel.: +32 (0) 3/250.14.50
Fax.: +32 (0)
3/253.14.64