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4 watts FM Transmitter Introduction

2016-04-04 09:55  
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 This article is 4 watts FM transmitter introduction. To more effectively grasp this principle, the proposal combine text read schematics. The circuit is simple but effective.


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.

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 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.

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.

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