위의 회로에서 주목해야 할 5 점 :
1. 탱크 회로는 고정 39p를 가지며 2-10p 트리머에 의해 조정된다.
코일은 밴드에서 원하는 위치와 트리머 좋은 곡 위치를 얻기 위해 뻗어있다.
2. 마이크 결합 22N 세라믹이다.
3-4kHz에서의 용량 성 리액턴스는 약 4K이고베이스에 1M 의해 언급 된 바와 같이 오디오 입력 스테이지는, 상당히 높은이 값은 충분하다.
3. 47K베이스 바이어스 저항에 의해 언급 한 바와 같이,베이스가 낮은 임피던스로 오디오 무대와 오실레이터 사이의 1U가 필요하다.
4. 전력 레일에 걸쳐 22N은 레일을 유지하는 데 필요한 "타이트." 를 100MHz에서의 임피던스는 옴보다 하나 더 적으며 거대 발진기의 성능을 향상시킨다.
5. 탱크 회로의 코일 공심으로 에나멜 전선의 권선 (5)이다. 이 PC 보드에 만든 코일보다 훨씬 더 나은 및 구입 인덕터보다 저렴합니다. 장거리의 비밀 발진기 단계에서 높은 활동이다. (그것을 가로 질러 코일과 콘덴서로 구성) 탱크 회로 인해 "붕괴 자기장"로 알려진 효과로 공급 전압보다 높은 전압을 생성 할 것이고 코일 붕괴 및 커패시터의 역 전압을 통과 할 때 발생한다. 안테나는이 점에 접속되고, 그것은이 높은 파형을 수신하고, 전자기파 등의 분위기에 에너지를 전달한다.
TRANSISTOR CIRCUITS
The next progressive step is to add a transistor to give the electret microphone more sensitivity. The electret microphone contains a Field Effect Transistor and you can consider it to be a stage of amplification. That's why the electret microphone has a very good output.
A further stage of amplification will give the bug extremely good sensitivity and you will be able to pick up the sound of a pin dropping on a wooden floor.
Many of the 1 transistor circuits over-drive the microphone and this will create a noise like bacon-and-eggs frying. The microphone's used by Talking Electronics require a load resistor of 47k for a 6v supply and 22k for a 3v supply. The voltage across the microphone is about 300mV to 600mV. It will produce an audio waveform of about 2-20mV.
Only a very simple self-biasing common-emitter stage is needed for the audio amplifier. This will give a gain of approx 70 for a 3v supply. The next circuit shows this audio amplifier, added to the previous transmitter circuit. This circuit is the best design using 2 transistors on a 3v supply. The circuit takes about 7mA and produces a range of about 200 - 400metres.
2 Transistor FM Transmitter
Five points to note in the circuit above:
1. The tank circuit has a fixed 39p and is adjusted by a 2-10p trimmer. The coil is stretched to get the desired position on the band and the trimmer fine tunes the location.
2. The microphone coupling is a 22n ceramic. This value is sufficient as its capacitive reactance at 3-4kHz is about 4k and the input to the audio stage is fairly high, as noted by the 1M on the base.
3. The 1u between the audio stage and oscillator is needed as the base has a lower impedance as noted by the 47k base-bias resistor.
4. The 22n across the power rails is needed to keep the rails "tight." Its impedance at 100MHz is much less than one ohm and it improves the performance of the oscillator enormously.
5. The coil in the tank circuit is 5 turns of enameled wire with air core. This is much better than a coil made on a PC board and is cheaper than a bought inductor. The secret to long range is high activity in the oscillator stage. The tank circuit (made up of the coil and capacitors across it) will produce a voltage higher than the supply voltage due to the effect known as "collapsing magnetic field" and this occurs when the coil collapses and passes its reverse voltage to the capacitor. The antenna is also connected to this point and it receives this high waveform and passes the energy to the atmosphere as electromagnetic radiation.
When the circuit is tightly constructed on a PC board, the frequency will not drift very much if the antenna is touched. This is due to the circuit design and layout as well as the use of large-value capacitors in the oscillator. If low value capacitors are used, the effect of your body has a greater effect on changing the frequency.
