|
*Just shake and wait till it builds up'' A full FM Transmitter with a range of 5km (in an obstructing area) and up to 10-15km in clear sight. The transmitter is based around 5 different stages (each being 1 of the 5 transistors). The first stage is an oscillator/modulator. It has two purposes. To generate the carrier and to apply the modulation to it. The modulating signal comes in through the 220nF capacitor. The 10k trimmer is used to tune the transmitter, by setting the DC biasing voltage for the 2x varicap diodes. The second and third stage act as both a voltage amplifier and as a tampon of a sort, to minimize oscillator destabilization, while adjusting the final stages. The fourth stage acts as a driver stage for the Power Amplifier (the last stage). To meet ITU-T regulations a 5 order LC filtering is applied to clear out any harmonics from the output, to minimize pollution of the EM spectrum. The antenna is 50ohm and must be matched, or the transmitter may misbehave.
Some tips:
1. Use very good regulation for the power supplies. There mustn't be any ripples, especially in the 9V supply! Place a 100nF capacitors to ground, near every collector inductors (170nH,150nH). Place a few more here and there along the supply rails. The more capacitors along the rail, the more overall stability.
2. The 50ohm antenna must be matched one way or another, whether by using a balun, or by matching the antenna itself. Nonetheless a 1:1 balun is recommended.
3. Shield every single stage from one another. Shield the first stage's LC tank circuit separately from the stage. Also shield the output filters.
4. Don't use any wirewound resistors. Only carbon.
5. All transistors must be rated above 100Mhz (obviously), but crucial.
|
