Greetings ###
I continued to seek a frequency doubler - and started experimenting with emitter-coupled pairs to make the full-wave rectifier. After several promising circuits, I discovered this circuit from Didier, KO4BB. Scroll down to find Low phase noise common base frequency doubler
I encourage builders to check out his entire web site ko4bb.com
My work is pushing me towards, simple, but good, battery powered circuits -- this implies keeping current low to conserve battery power. This low phase noise doubler draws 3.4 mA quiescent - and delivers 6 - 6.4 dBm output power at 7 MHz from an input power amplitude of 10 dBm at 3.5 MHz. Fantastica.
Above — The doubler designed by Didier, K04BB. I used 2N4401 BJTs that came from the same reel, but did not match them. The 200 Ω emitter degeneration resistors were 1% tolerance parts.
Q1 and Q2 provide a series pass voltage regulator with Q2 as the feedback amplifier. The Q2 BJT also serves as the voltage reference -- and provides temperature compensation. Ideal for a field mobile radio where temperature changes happen. The bias to the emitter- coupled pair is only 0.85 VDC. I added a 100 µF electrolytic capacitor to the base of Q1 to boost DC filtration.
I designed my own output network, and because the emitter-coupled BJT pair provides such a high voltage amplitude, I eliminated an active buffer stage. This in turn, helps keep current draw low. A simple 32 turn to 5 turn ferrite transformer goes into a low pass filter plus a final -4 dB 50 Ω pad.
The proof of how accurately the input signal gets full-wave rectified is demonstrated by the amplitude of the 3.5 MHz input signal at the output.
Above — Before I tested the output signal in a spectrum analyzer, I connected it to a 50 Ω input 'scope channel. Even here, you may see this doubler functions fairly well. This is the best 3.5 MHz doubler I've built. Big thanks to Didier, K04BB for sharing his design -- it's a keeper.
Best!
