This circuit of AM transmitter is designed to transmit AM (amplitude modulated) DSB (double sideband) signals. A modulated AM signal consists of a carrier and two symmetrically spaced side bands. The two side bands have the same amplitude and carry the same information. In fact, the carrier itself conveys or carries no information. In a 100% modulated AM signal 2/3rd of the power is wasted in the carrier and only 1/6th of the power is contained in each side band.
In this transmitter we remove the carrier and transmit only the two side bands. The effective output of the circuit is three times that of an equivalent AM transmitter.
Opamp IC 741 is used here as a microphone amplifier to amplify the voice picked up by the condenser microphone. The output of opamp is fed to the double balanced modulator (DBM) built around four 1N4148 diodes. The modulation level can be adjusted with the help of preset VR1.
The carrier is generated using crystal oscillator wired around BC548 transistor T2. The carrier is further amplified by transistor T1, which also acts as a buffer between the carrier oscillator and the balanced modulator. The working frequency of the transmitter can be changed by using crystals of different frequencies. For multi-frequency operation, selection of different crystals can be made using a selector switch. The level of the carrier coupled to the DBM can be adjusted with the help of preset VR2.
The output of the DBM contains only the product (of audio and carrier) frequencies. The DBM suppresses both the input signals and produces double side band suppressed carrier (DSBSC) at its output. However, since the diodes used in the balanced modulator are not fully matched, the output of the DBM does
contain some residual carrier. This is known as carrier leakage. By adjusting the 100-ohm preset (VR2) and trimmer (C7) you can anull the scarier leakage. To receive DSB signals you need a beat frequency oscillator to reinsert the missing carrier. If you don’t have a beat frequency oscillator, or want to transmit only AM signal, adjust preset VR2 to leak some carrier so that you can receive the signals on any ordinary radio receiver. In AM mode 100% modulation can be attained by adjusting presets VR1 and VR2.
The DSBSC signal available at the output of the balanced modulator is amplified by two stages of RF linear amplifiers. Transistor 2N2222A (T3) is used as an RF amplifier, which provides enough signal amplification to drive the final power amplifier around transistor SL100B. The output of the final power amplifier is connected to the antenna.
All coils are to be wound on ferrite balun core (same as used in TV balun transformer of size 1.4 cm x 0.6 cm) using 24SWG enameled copper wire. Proper heat-sink should be provided for SL100B transistor used as final power amplifier.
Range of the order of a few kilometres can be easily achieved by proper choice of site, type of antenna (such as a resonant half-wave dipole of length 20 metres for 7.05 MHz frequency) and proper matching of transmitter to the antanna. Use good-quality shielded wire of short length to connect the crystals.