The transmitter circuit is made up of two building blocks - the 303MHz RF oscillator and the 32 kHz crystal controlled oscillator.
The 303MHz oscillator consists of a self-oscillating circuit made up of the coil on the PC board and a 9p (9 puff) capacitor (actually 4p and 5p in parallel).
The circuit starts-up by the transistor producing noise. This rising-and-falling signal on the collector is passed to the parallel tuned circuit (the tank circuit) and the base sees a very smooth sine wave at a frequency of 303MHz.
This sine wave is then amplified by the transistor and this is how the 303MHz frequency is generated.
Now we come to the purpose of the 15microhenry choke on the tank circuit.
When the circuit oscillates, it takes a larger and small amount of current.
This current passes through the choke and the turns produce a back- emf or back voltage that fights against the flow (change) in current. The end effect is a voltage created at the point where the choke is connected to the track-work on the board. This effectively allows the track work to produce a waveform and since the frequency of this wave is very high, a percentage of the energy is radiated into the air as electromagnetic energy.
The choke allows the track-work to effectively rise and fall while providing a very low resistance path for the flow of current during certain parts of the cycle.
The second building block is the crystal oscillator.
This is made up of a two-stage DC coupled amplifier with feedback via the 2n2 and crystal.
If the crystal is removed, the oscillator is seen as producing very narrow spikes with a frequency determined by the 2n2.
When the crystal is added, the frequency increases (because the effect of the 2n2 and crystal in series creates a lower capacitance than 2n2) and as it rises, the amplitude of the feedback signal increases until it reaches a maximum at the resonant frequency of the crystal. The crystal exhibits the lowest impedance (the highest capacitance) at the resonant frequency. This is how the circuit stabilizes at the frequency of the crystal.
When the device is turned on, the 150k on the base of the second transistor turns the transistor on.
The third transistor has 0.65v on the collector and the base also receives very close to 0.65v, via the 220k resistor.
The third transistor is not fully turned on and it produces a small amount of noise. This noise is passed to the second transistor and appears on the collector. The collector passes this noise to the base of the third transistor and the noise very rapidly increases to a maximum.
It comes to a point where the waveform above is generated and the reason why the spikes are so narrow is easy to explain.
When the middle transistor changes from an OFF state to an ON state, the capacitor will be partially charged and the voltage on the end connected to the base of the third transistor will drop about 6v and put a negative voltage on the base of the third transistor. This will keep it off and the middle transistor will be kept ON via the 150k base-resistor.
The capacitor will gradually charge in the opposite direction via the 220k and 150k and when the base of the third transistor sees about 0.6v, it begins to turn ON.
This causes the middle transistor to turn OFF and the collector voltage rises. This causes the capacitor to charge and create a current-flow in the base of the third transistor. Both transistors are now turned ON and the capacitor charges very quickly via the 12k and base-emitter junction of the third transistor. This creates the very narrow high-period in the waveform.
When the push button is pressed, the circuit produces a 303MHz carrier with a 32 kHz tone.
The receiver detects the 32 kHz and turns on a SOUND chip.
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