76MHz to 119MHz Fm transmitter

Posted on Aug 7, 2012

TR1 (BC547) is an inverted Hartley oscillator which based upon an inductor fabricated on the PCB. This makes it megga-stable, and setable anywhere in the VHF FM band (76MHz to 119MHz) and the BB105 varicap makes it voltage tuneable over about 8MHz of that band. The inductor has one tapping for feedback and a second to feed an optional prescaler. TR2 is a buffer/amplifier and TR3 it the PA stage.

76MHz to 119MHz Fm transmitter
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A single CD4046 and crystal oscillator can be used as a very simple PLL frequency controller for this transmitter with the frequency being determined by the crystal. I used this technique with the V6 transmitter so you have circuits available to suit your needs. The V7b acts as a simple 1.375MHz to 1.6875MHz external VCO for just about any "Single modulus" synthesiser, with MC (pin 2) = +5 volts. With MC=Gnd then it acts as a 1.354MHz to 1.662MHz VCO. The final TX output is 64 times the synth frequency with MC=+5v (65 with MC=Gnd). This means that it will operate with my simple CMOS synthesiser and give final frequency steps of 50KHz with the correct reference crystal (6.4MHz). With a "Dual Modulus" synthesiser, such as the MC145152-2, then the V7b will form a dual modulus synthesiser and can be controlled in steps of typically 5KHz. Only problem is that the chip costs more than the complete simple CMOS synthesiser.

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