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SERWIS ELEKTRONICZNY - RADIOELEKTRYKA SOSNOWIEC POLSKA |
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NIEZALEŻNA DZIAŁALNOŚĆ BADAWCZO - NAUKOWA KLIKNIJ NA OPIS DOKUMENTU |
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SMD transmitter circuit schematic smd fm transmitter Frequency Modulation Modulation is done by superimposing an audio signal from the electret mic onto the DC bias applied to V1. R3 and C1 form a low-pass filter to prevent RF from feeding back to the mic. R3, R4 and R2 form a votage divider for the audio. Transmitter output stage The output of the oscillator is fed through C9 to the Q2 emitter-follower. The output of Q2 drives the antenna through C11. The Q2 emitter-follower it ensures that the oscillator is not loaded down by the impedance of the antenna and it provides power gain to drive the antenna. |
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SMD Transmitter layout The figure below shows the layout of the PCB and it uses surface-mounted devices like resistors and capacitors (non-polar devices). All the caps are size 0805 and all resistors are size 1206. use through-hole components for Q1, Q2, IC1 and V1. You can use an SOT-89 device for IC1 and an SOT-23 device for V1. Use MPSH10 or a transistor equivalent. |
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Here you can learn how to solder smd chips smd transmitter layout The inductor A coil would consist of two or three turns of wire but for this schematic we will use an inductor with loops of copper on the PCB. Such flat spiral inductor are common at these frequencies. One formula for flat spiral inductors is: flat spiral inductors formula where L = inductance in uH r = radius of coil (outer radius + inner radius divided by 2 ) inches N = number of turns d = depth of coil (outer radius minus inner radius) inches Tuning range While commercial FM band goes from about 88 MHz to 108 MHz, the L and C values used in this design allow tuning up to 100 MHz. |
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Transmitter testing You will need a portable FM radio and an assistant. First, find an empty spot on the FM dial and set your radio about 30 feet away (9 meters). The radios volume control should not be set too high to prevend feedback. Next, power-up your transmitter and talk to yourself as you adjust the frequency with the trim-pot. When your assistant hears you, your transmitter is tuned. |
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You might have to adjust the radios tuner slightly for best reception. Have fun with it but remember that using the transmitter as a bugging device may not be legal in your country. To use the circuit as a wireless microphone, increase the value of R3. The transmitter range is about 100 feet (30 meters) inside a building. Our Pen Bug projects have been very popular. The idea of being able to hide a transmitter in a pen is very appealing. In an effort to reduce the size of this design, we have used surface-mount components. PEN BUG CIRCUIT Topside of Pen Bug MkIV Our first Pen Bug circuit appeared in our book: "More FM Bugs" and the Mkll modification was presented in "Security Devices." The only difference between these was the antenna arrangement. In the Mkl version the antenna was wound in a spiral and fitted into the barrel of the pen. |
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In the Mk II version a tank circuit was added and the coil of the tank circuit used as an active antenna. Our Pen Bug MkIII out-performed the earlier models where the antenna was longitudinally wound and coupled between two stages via the emitter of the oscillator. Firstly, the thought of using the coil in the tank circuit for transmitting RF was a little far fetched, but we used it as an example for those who were interested in experimenting with our circuits. |
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