ONE TRANSISTOR

FM TRANSMITTER

INSTRUCTION SHEET

 

Price - Stock No 140

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Schematic Diagram




PCB Layout

 The transmitter antenna is a short piece of insulated wire included in the kit and connected to the PCB by soldering to a Circuit-board Pin at the antenna point on the PCB.   Premium transmission often requires the antenna to be moved around to suit the location of the Receiver and other conditions.

 Generally the wire antenna supplied is fine but lengthing and/or shortening the length of the antenna could be tried if reception is poor.

Some suggestions are:

1.  Tune the approximate frequency to a clear part, of tile dial around 100 Mhz and try transmitting. Tune the receiver rather than the transmitter. Move the aerial around to get the strongest signal. 

2.  Try a length of wire attached to the antenna to see if that improves the periormance remember that there is a premium lenath for the antenna. A longer wire doesn't autornaticaliv work better. Try hanging the antenna straight up or down. Generally the transmittincr antenna works better if it is in roughly the same plane as the receiver antenna eq. it you are tuning on a car radio, or a portable with a vertical antenna the transmitter will probably work better if the antenna is hung vertically,

The Variable Tuning Capacitor- in the Radio Mic. is fairly delicate so avoid unnecessary adjustaient. Tune the receiver instead.

Technology and Notes

This circuit is basically an oscillator which runs at around 100 MHz.  The most important parts of the oscillator are the transistor Q1 and the tuned circuit, which comprises the inductor Ll and the variable capacitor CV1.

When the battery is first connected, a brief surge of current flows from the collector to the emitter of Q1, causing an oscillating (i.e: alternating) current to flow back and forth between Ll and CV1. An oscillating voltage therefore appears at the junction of Ll and CV1. The frequency of the oscillation depends on the values of Ll and CV1, so that varying the value of CV1 tunes the oscillations to the exact frequency required.

The oscillating current would very soon decay to nothing, but for the fact that the oscillating voltage is fed back via C4 to the emitter of Q1. This makes the base-emitter current of the transistor vary at the oscillation frequency and hence current flowing in the tuned circuit and maintaining the oscillations. Some of the energy in the oscillating electric and magnetic fields of the tuned circuit is radiated as radio waves.

But how do sound waves arriving at the microphone vary the frequency of these waves? They can do this because the frequency of the oscillations depends on the TOTAL capacitance in the oscillator circuit. Although CV1 accounts for the major part of this capacitance, other parts of the circuit also make minor contributions. In particular, the capacitance between the base and the collector of Ql has a small but noticeable effect on the oscillation frequency. This capacitance, which is known as the 'junction capacitance', is not fixed but varies when the voltage on the base of the transistor varies. Sound waves hitting the microphone induce a voltage that varies in time with the sound and this voltage is applied via C1 to the base of Q1, thereby frequency modulating the transmitter.

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