4 Watt FM TRANSMITTER
This is a small but quite powerful FM transmitter having three RF stages
incorporating an audio preamplifier for better modulation. t has an output power
of 4 Watts and works off 12-18 VDC which makes it easily portable. It is the
ideal project for the beginner who wishes to get started in the fascinating
world of FM broadcasting and wants a good basic circuit to experiment with.
Modulation type: ........ FM
Frequency range: .... 88-108 MHz
Working voltage: ..... 12-18 VDC
Maximum current: ....... 450 mA
Output power: ............ 4 W
As it has already been mentioned the transmitted signal
is Frequency Modulated (FM) which means that the carrier’s amplitude stays
constant and its frequency varies according to the amplitude variations of the
audio signal. When the input signal’s amplitude increases (i.e. during the
positive half cycles) the frequency of the carrier increases too, on the other
hand when the input signal decreases in amplitude (negative half-cycle or no
signal) the carrier frequency decreases accordingly. In figure 1 you can see a
graphic representation of Frequency Modulation as it would appear on an
oscilloscope screen, together with the modulating AF signal. The output
frequency the transmitter is adjustable from 88 to 108 MHz which is the FM band
that is used for radio broadcasting. The circuit as we have already mentioned
consists of four stages. Three RF stages and one audio preamplifier for the
modulation. The first RF stage is an oscillator and is built around TR1. The
frequency of the oscillator is controlled by the LC network L1-C15. C7 is there
to ensure that the circuit continues oscillating and C8 adjusts the coupling
between the oscillator and the next RF stage which is an amplifier. This is
built around TR2 which operates in class C and is tuned by means of L2 and C9.
The last RF stage is also an amplifier built around TR3 which operates in class
C the input of which is tuned by means of C10 and L4. From the output of this
last stage which is tuned by means of L3-C12 is taken the output signal which
through the tuned circuit L5-C11 goes to the aerial.
The circuit of the preamplifier is very simple and is built around TR4. The
input sensitivity of the stage is adjustable in order to make it possible to use
the transmitter with different input signals and depends upon the setting of
VR1. As it is the transmitter can be modulated directly with a piezoelectric
microphone, a small cassette recorder etc. It is of course possible to use an
audio mixer in the input for more professional results.
First of all let us consider a few basics in building
electronic circuits on a printed circuit board. The board is made of a thin
insulating material clad with a thin layer of conductive copper that is shaped
in such a way as to form the necessary conductors between the various components
of the circuit. The use of a properly designed printed circuit board is very
desirable as it speeds construction up considerably and reduces the possibility
of making errors. Smart Kit boards also come pre-drilled and with the outline of
the components and their identification printed on the component side to make
construction easier. To protect the board during storage from oxidation and
assure it gets to you in perfect condition the copper is tinned during
manufacturing and covered with a special varnish that protects it from getting
oxidised and also makes soldering easier. Soldering the components to the board
is the only way to build your circuit and from the way you do it depends greatly
your success or failure. This work is not very difficult and if you stick to a
few rules you should have no problems. The soldering iron that you use must be
light and its power should not exceed the 25 Watts. The tip should be fine and
must be kept clean at all times. For this purpose come very handy specially made
sponges that are kept wet and from time to time you can wipe the hot tip on them
to remove all the residues that tend to accumulate on it.
DO NOT file or sandpaper a dirty or worn out tip. If the tip cannot be cleaned,
replace it. There are many different types of solder in the market and you
should choose a good quality one that contains the necessary flux in its core,
to assure a perfect joint every time.
DO NOT use soldering flux apart from that which is already included in your
solder. Too much flux can cause many problems and is one of the main causes of
circuit malfunction. If nevertheless you have to use extra flux, as it is the
case when you have to tin copper wires, clean it very thoroughly after you
finish your work. In order to solder a component correctly you should do the
following:
- Clean the component leads with a small piece of emery paper.
Bend them at the correct distance from the component’s body and insert the
component in its place on the board.
- You may find sometimes a component with heavier gauge leads than usual, that
are too thick to enter in the holes of the p.c. board. In this case use a mini
drill to enlarge the holes slightly.
- Do not make the holes too large as this is going to make soldering difficult
afterwards.
- Take the hot iron and place its tip on the component lead while holding the
end of the solder wire at the point where the lead emerges from the board. The
iron tip must touch the lead slightly above the p.c. board. - When the solder
starts to melt and flow wait till it covers evenly the area around the hole and
the flux boils and gets out from underneath the solder. The whole operation
should not take more than 5 seconds. Remove the iron and allow the solder to
cool naturally without blowing on it or moving the component. If everything was
done properly the surface of the joint must have a bright metallic finish and
its edges should be smoothly ended on the component lead and the board track. If
the solder looks dull, cracked, or has the shape of a blob then you have made a
dry joint and you should remove the solder (with a pump, or a solder wick) and
redo it.
- Take care not to overheat the tracks as it is very easy to lift them from the
board and break them.
- When you are soldering a sensitive component it is good practice to hold the
lead from the component side of the board with a pair of long-nose pliers to
divert any heat that could possibly damage the component.
- Make sure that you do not use more solder than it is necessary as you are
running the risk of short-circuiting adjacent tracks on the board, especially if
they are very close together.
- When you finish your work cut off the excess of the component leads and
clean the board thoroughly with a suitable solvent to remove all flux residues
that may still remain on it.
This is an RF project and this calls for even more care during soldering as
sloppiness during construction can mean low or no output at all, low stability
and other problems. Make sure that you follow the general rules about electronic
circuit construction outlined above and double-check everything before going to
the next step. All the components are clearly marked on the component side of
the P.C. board and you should have no difficulty in locating and placing them.
Solder first of all the pins, and continue with the coils taking care not to
deform them, the RFC’s, the resistors, the capacitors and finally the
electrolytic and the trimmers. Make sure that the electrolytic are correctly
placed with respect to their polarity and that the trimmers are not overheated
during soldering. At this point stop for a good inspection of the work done so
far and if you see that everything is OK go on and solder the transistors in
their places taking grate care not to overheat them as they are the most
sensitive of all the components used in the project. The audio frequency input
is at points 1 (ground) and 2 (signal), the power supply is connected at points
3 (-) and 4 (+) and the antenna is connected at points 5 (ground) and 6 (signal).
As we have already mentioned the signal you use for the modulation of the
transmitter could be the output of a preamplifier or mixer or in case you only
want to modulate it with voice you can use the piezoelectric microphone supplied
with the Kit. (The quality of this microphone is not very good but it is quite
adequate if you are interested in speech only.) As an antenna you can use an
open dipole or a Ground Plane. Before you start using the transmitter or every
time you change its working frequency you must follow the procedure described
below which is called alignment
If you expect your transmitter to be able to deliver its maximum output at
any time you must align all the RF stages in order to ensure that you get the
best energy transfer between them. There are two ways to do this and it depends
if you have a SWR meter or not which method you are going to follow. If you have
a SWR meter turn the transmitter on, having connected the SWR meter in its
output in series with the antenna, and turn C15 in order to tune the oscillator
to the frequency you have chosen for your broad casts. Then start adjusting the
trimmers C8,9,10,12 and 11 in this order till you get the maximum output power
in the SWR meter. For those who don’t have a SWR meter there is another method
which gives quite satisfactory results. You only have to build the little
circuit in Fig. 2 which is connected in the out put of the transmitter and in
its output (across C16) you connect your multi-tester having selected a suitable
VOLTS scale. You tune C15 in the desired frequency and then adjust the other
trimmers in the same order as it is described above for the maximum output in
the multitester. The disadvantage of this method is that you do not align the
transmitter with a real antenna connected in its output and it may be necessary
to make slight adjustments to C11 and C12 for a perfect antenna match.
Do not forget to align your transmitter every time you change your aerial or
your working frequency.
WARNING: In every transmitter there are present apart from the main output
frequency various harmonics that usually have a very short range. In order to
make sure you haven’t tuned on one of them do the tuning as far as possible
from your receiver, or use a Spectrum Analyser to see your output spectrum and
make sure that you tune your transmitter on the right frequency.
Smart kits are sold as stand alone training kits.
If they are used as part of a larger assembly and any damage is caused, our
company bears no responsibility.
While using electrical parts, handle power supply and equipment with great care, following safety standards as described by international specs and regulations.
CAUTION
All the RF kits are sold for experimental and laboratory use only. Their
possession and use are limited by laws which vary from state to state. Please
get information about what you can or can not do in your area and stay within
the legal limits. Make sure you do not become a nuisance to others with your
experiments. Smart Kit has no responsibility whatsoever for any misuse of its
products.
- Check your work for possible dry joints, bridges across adjacent tracks or
soldering flux residues that usually cause problems.
Check again all the external connections to and from the circuit to see if there
is a mistake there.
- See that there are no components missing or inserted in the wrong places.
- Make sure that all the polarised components have been soldered the right way
round.
- Make sure that the supply has the correct voltage and is connected the right
way round to your circuit.
- Check your project for faulty or damaged components.
If everything checks and your project still fails to work, please contact your
retailer and the Smart Kit Service will repair it for you.