Hi
The current task I have trouble with is building a bjt frequency doubler. Unfortunately I haven't been able to find a lot of practical hands-on design information but the circuit I built seems to be considered a working one in general. If someone knows a hands-on design guide somewhere please point me to it.
Right now the input is 13.5 MHz 1 V p-p and the output is supposed to be 27 MHz but I don't get anything usable out of it. The best I see is a distorted and much attenuated input frequency. First I had fixed resistors for the base bias whose values worked ok in a previous non-multiplier amp but when it made trouble I also tried 10k, 25k and 50k trimmers. The inductor is self-wound and should be in the right ballpark, however I also exchanged the resonant capacitor for slightly different values just to make sure. I did use a bread-and-butter 2222 transistor on purpose; although it might not be the best choice the working frequency is still way below the fT of 300 MHz.
Where's the pitfall? There must be one but I just don't see it.
Regards
Martin
Frequency Multiplier is basically to create Nth harmonics from a active device by proper biasing.So the circuit should create large harmonic contents by choosing a heavy Nonlinear Region of its Operating Point -for instance exponential region- of a Vbe transfer curve of a BJT or anything else such as.Even a diode can create harmonics but the second ( and other higher degree) harmonics cannot be higher than fundamental.The wanted Nth harmonic is filtered -for instance by a tank circuit as in your schematic- and the others are eliminated.I have checked your tank circuit and I couldn't find 27MHz resonance frequency ( I assume 12.75uH // 39pF).FFT is what I can use right now and sure enough the second harmonic is there about 8 dB below the first. However I may not have correctly understood the concept of a frequency multiplier. I thought the tank circuit's purpose was to select the second harmonic (in this case) over the others. Is that a wrong assumption? Because if it did then I'd expect the second to stand out from the others.
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