Short Wave AM Transmitter |
The main feature of this trans- mitter is that it is free from the
LC (inductor, capacitor) tuned circuit and operates on a fixed frequency of 12 MHz which
is extremely stable. An LC based tuned circuit is inherently unstable due to drift of
resonant frequency on account of temperature and humidity variations. The circuit is very
simple and uses only a few components. The figure shows the complete circuit diagram of
the transmitter. Resistors R1 and R2 are used for DC biasing of transistor T1. The
capacitor C1 provides coupling between the speaker and the base of transistor T1.
Similarly, resistors R3, R4 and R5 provide DC bias to transistor T2. Resistor R5 also
provides negative feedback which results in higher stability. The oscillator section is a
combination of transistor T2, crystal Xtal, capacitor C2 and resistors R3, R4 and R5. The
crystal is excited by a portion of energy from the collector of transistor T2 through the
feedback capacitor C2. Thus the oscillator circuit generates the carrier frequency at its
fundamental frequency of 12 MHz. Any crystal having the frequency in short wave range can
be substituted in this circuit, although the operation was tested with a 12MHz crystal.
Transistor T1 serves three functions:
The loudspeaker converts the voice message into the electrical signal which is amplified by transistor T1. This amplified audio signal modulates the carrier frequency generated by transistor T2. The amplitude modulated output is obtained at the collector of transistor T2 and is transmitted by a long wire antenna into space in the form of electromagnetic waves. The transmitted signals can be received on any short wave receiver without distortion and noise. The range of this transmitter is 25 to 30 metres and can be extended further if the length of the antenna wire is suitably increased along with proper matching. |