Thursday, July 31, 2008
BR1 = Bridge Rectifier, 100V - 3A
C1 = 2200 ìF, 63V
IC1 = LM317, adjustable regulator
C2 = 0.1 ìF
V = Meter, 30V,
Ri = 85 ohm
C3 = 1 ìF, 40V
TR1 = Transformer, 25V, 2A Plug = 3-wire plug & cord
R1 = 1K8, 5%
S1 = On-Off toggle switch
R2 = 220 ohm, 5%
D1 = 1N4001
R3 = 27K, 5%
Fuse = 110V, 500mA, slow-blow
P1 = 5K, potentiometer FuseHolder, wire, solder, case, knob for P1
P2 = 10K, 10-turn trim-pot Red & Black Banana Jacks
L1 can be selected for the application. A 470μH coil works on lower frequencies and lie in AM, for shortwave try a 20μH coil. This unit can be powered by a 9 volt alkaline battery. If a power supply is used, bypass the power supply with a 0.04μF capacitor to prevent noise pickup. The antenna used on this circuit is a standard 18-inch telescoping type, but a thick piece of copper, bus-bar, or piano wire will also work fine.
Wednesday, July 9, 2008
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.