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Wednesday, July 9, 2008

AM DSB Transmitter for HAMS

This circuit of AM transmitter is designed to transmit AM(amplitude modulated) DSB (double side Band) signals. A modulated AM signal consists of a carrier and two symmetrically spaced side bands. The two side bands have the same amplitude and the carry 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 in each side band
In this transmitter we remove the carrier and transmits only the two side bands. The effective output of the circuit is three times that of an equivalent AM transmitter.

Opamp IC741 is added here as a microphone amplifier to amplify the audio signals from the condenser mike. The output of the Opamp is fed to the double balanced modulator build around In4148 diodes. The modulation level can be adjusted with the help of preset VR1.
The carrier using cristal wired around BC548 transistor T2. The carrier is further amplified bt transistor T1, which also acts as a buffer between modulator. The working frequencey of the transmitter can be changed by using the cristals of the diffreent frequencies. For multi-frequency operation, selection of different cristals can be made using a selector switch. The level of the carrier coupled to the DEM(double Balanced Modulator) cna be adjusted with the help of preset VR2.

The output of the DBM contains only the product (of audio and carrier) frequencise. The DBM supress both the input signals anf produce the double side band supressed 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 leckage. By adjusting the 100-ohm preset(VR2) and the trimmer (c7) you can nullify the carrier leckage.

To receive DSb signals you need a Beat Frequency Oscillator(BFO) to reinsert the missing carrier. If you dodn't have a BFO, or want to transmit only AM signal, adjust preset VR2 to leak some carrier so thar 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 libnear amplifiers. Transisor 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 antienna.

All coils are to be wound on ferrite balun cores(same ast used in TV balun transformer of size 1.4cm x 0.6 cm) using 24 SWG enameled copper wire. proper heat-sink should be provided for SL100B transistor used as final power amplifier.

Range of the order of a few kilometers can be easily achieved by proper choice of the site, type of antenna (such as a resonant half-wave dipole of lenght 10 meters for 7.08 Mhz frequency) and proper matching of transmitter to the antanna. Use good quality shielded wire of short length to connect the crystals.

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