Re: Full QSK for a uBITX

Tom, wb6b

On Thu, Aug 8, 2019 at 12:14 AM, kh6sky wrote:
W2AEWs video on PIN diode switching
Very good video.

Also, from the video, it looks like if you use PIN diodes as a variable attenuator the design will need to carefully consider the forward current through the PIN diode throughout the full RF cycle. It looks like it requires a careful balance to keep the PIN diode acting as a "linear" current controlled resistor rather than reverting to not conducting on half the RF cycle and introducing severe waveform distortion.  

You could see that on his scope display where, as he lowers the current, the amplitude of the positive half of the waveform decreased for a while. But, soon, rather dramatically, "the bottom end dropped out" and the scope was displaying a very distorted waveform. This must limit the useful range of PIN diodes as variable attenuators. Many designs do use multiple PIN diodes to improve the attenuation obtainable. Just looks like PIN diode variable attenuators might be dancing closer to the edge of distortion than I'd imagined. Maybe I'll stick to using them in step attenuators. 

Tom, wb6b

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