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Show WBFM TX V7 circuit

2017-08-07 18:48  
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The following shows the WBFM TX V7 circuits. The circuit is very simple, and very practical. Recommend Duokanjibian The schematic in order to master this principle. I hope to be helpful to your work.The frequency of the oscillator is determined by L1 and the 22pf trimmer capacitor and functions in the range of about 76MHz to 119MHz using the PCB I have made.


TR1 = BC547 ? TR2 = BC547 ? ? TR3 = 2N3866 or 2N4427

TR2 (BC547 ) is both biassed and directly connected to the Emitter of TR1, which is a little unconventional in a VHF circuit. I needed to get a good input to TR2 and cut down on components. There are already far too many coils as it is in this circuit. Remember that the BC547 is an audio transistor but works well at VHF. The inductor in the Emitter of TR2 helps to extend the response a little to give a bit more signal to drive the final power amplifier transistor (TR3). TR2 gives no voltage gain; it is current we need to drive TR3. We already have enough volts from the oscillator.The collector of TR3 (2N4427) has a big (by QRP standards) choke to pass the supply DC, but presents a high impedance to RF. The RF signal is then matched to 50-Ohms with the 15p, L5 and 56p. The 1nf cap simply blocks the supply voltage that would otherwise pass to the antenna. L5 and 56pf form a low-pass filter that helps to block harmonics present in the output signal. L6 and 47pf are added to further reduce the harmonic levels. This filter is an absolute MUST for all transmitters if one does not wish to offend every other user of the radio spectrum. L5 and L6 have also been positioned on the PCB so that there is a little coupling between them. This coupling serves to cancel out any residual signals, not within the passband of the filter, that may be present at the input to L5. It is this effect that was responsible for the unexpected cleanliness of the first prototype, and a little layout experimenting has now reduced the 2nd and 3rd harmonics to -60dBc at all supply voltages. With 150mW output, this corresponds to 3rd harmonic of 150 nano-watts and a 2nd harmonic level of just 50 nano-watts.


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