Re: Full QSK for a uBITX

Jerry Gaffke

I haven't watched the video but "the bottom end dropped out" suggests to me that he is operating at too low a frequency for his particular PIN diode.
The idea of a PIN diode is that it need only be forward biased for part of the waveform, for the remainder of the waveform there are residual charge carriers available in the diode to carry the current.  Minimum frequency is determined by the "charge carrier life time", which is the average amount of time that those charge carriers remain available.  Take a look at the spec's for NXP's BAP64Q attenuator.  PIN diode attenuators get used successfully in many many places.  In a normal diode such as the 1n4148, there are virtually none of these charge carriers available once the diode gets reverse biased.

Jerry, KE7ER

On Thu, Aug 8, 2019 at 03:00 AM, Tom, wb6b wrote:
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.  

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