gKit 32. TWO STAGE FM TRANSMITTER
This Kit is a powerful two stage, 9V FM transmitter (Tx) with a range of up to 1 kilometer in the open. It uses an RF transistor in its output stage. Distance of transmission is critically dependent on the operating Conditions (in a building or out on the open), type of aerial used (single wire or dipole), operating voltage (12V is better than 6V) and if the circuit is peaked for maximum performance. The kit is constructed on a single-sided printed circuit board (PCB). It has a silk screen overlay on top to aid construction. On the bottom there is a solder mask to help in soldering. Protel Autotrax and Schematic were used to design the board. ASSEMBLY INSTRUCTIONS Components may be added to the PCB in any order. It is usually easier to add the lowest height components first then proceed to the taller components. The electret microphone should be inserted with the pin connected to the metal case connected to the negative rail (that is, to the ground or zero voltage side of the circuit.) This is marked with a '-' sign at the MIC on the circuit board printed overlay. To save space all the resistors must be inserted standing up on one end. Be careful to get the transistors around the correct way. Note three points: - the two 100n capacitors on the PCB have 0.2" and 0.1" spacing. We have put in components with the correct spacing for ease of construction. - we have supplied prewound, pre-soldered enamel 6 turn & 8 turn wire coils. The legs should solder directly to the pads of the PCB. However, pay special attention to the solder joint in case all the enamel has not been removed. You may have to solder off or scrape off more enamel from the ends of the two enamelled coils. - a connection (or tap) is required from the middle of the L1 tinned copper wire coil to the pad marked TAP next to the coil. Solder a piece of wire to the top of the middle turn as shown on the overlay. Then solder the other end to the pad immediately next to the L1 coil. Spread out the coils of L1 about 1 mm apart. Make sure none of the loops of the coil are touching the loop next to it. CIRCUIT DESCRIPTION The circuit is basically a radio frequency (RF) oscillator that operates around 100 MHz. Audio picked up and amplified by the electret microphone is fed into the audio amplifier stage built around the first transistor. Output from the collector is fed into the base of the second transistor where it modulates the resonant frequency of the tank circuit (L1 coil and the red trimcap) by varying the junction capacitance of the transistor. Junction capacitance is a function of the potential difference applied to the base of the transistor T2. The tank circuit is connected in a Hartley oscillator circuit. The final stage built around T3 amplifies the output RF signal. Let us look at the individual blocks of the circuit more closely: The electret microphone: an electret is a permanently charged dielectric. It is made by heating a ceramic material, placing it in a magnetic field and then allowing it to cool while still in the magnetic field. It is the electrostatic equivalent of a permanent magnet. In the electret microphone a slice of this material is used as part of the dielectric of a capacitor in which the diaphram of the microphone forms one plate. Sound pressure moves one of its plates. The movement of the plate changes the capacitance. The electret capacitor is connected to an FET amplifier. These microphones are small, have excellent sensitivity, a wide frequency response and a very low cost. First amplification stage: this is a standard self-biasing common emitter amplifier. The 22n capacitor isolates the microphone from the base voltage of the transistor and only allows alternating current signals to pass. Oscillator stage: every transmitter needs an oscillator to generate the RF carrier waves. The tank circuit, the transistor and the feedback capacitor are the oscillator circuit here. An input signal is not needed to...
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