Hi Jerry,
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John Z and I did several simulations of the QDX PA.? The most significant issue is the end-of-transmission spike when the energy in L502 is dissipated by the off state of all four BS170s.? You could email John Z (KJ4A) to see if you can get a copy of his last model.? I have attached one of the models I modified for testing.
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Of note is the power reduction that occurs if L502 is removed.? I did several bench tests on one of my QDX radios to see which solution provided the least power reduction.? The result was that the Zeners provided the lowest power reduction.? John mentioned that the Zeners may not protect the MOSFETs in real-world situations.? I did not have the means to simulate or beach test his points.? The net summation for me is giving up the 1/4 watt to protect the BS170s with the comutation diode solution is the best choice.? Note that there is NOT an end-of-transmission spike for the QMX; it is only the QDX series.? The key shaping circuit allows for a soft landing at the end of the key down.
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I suggest you look up past posts from John Z (there are many) or PM me if you want more information.
73
Evan
AC9TU
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