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Short
Wave AM Transmitter
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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:
- It
provides the DC path for extending +Vcc supply to
transistor T2.
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It amplifies the audio signals obtained from speaker.
- It
injects the audio signal into the high frequency carrier
signal for modulation.
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. |
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