I see that the info I had on this subject is now posted on the UC Davis
Sonic Server in 2 forms. Tnx to Skipp. It is at
http://sonic.ucdavis.edu/so02000.html .To make this complete, here is
additional info from Dave W8NF and from an earlier post by myself in
case you missed them. 73, Dan, N5R.
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I have considered power diodes, too.
For switching, you really can use any diode. However, diodes which are
not optimum will lead to distortion or excess power dissipation (DC or
RF). A PIN diode has an extremely long reverse recovery time. What
that means is that when you turn it "on" you don't have to use as much
forward current as the peak RF you intend for it to carry. The ratio of
the DC "on" current to the peak RF current you're switching is a
function of the diode and frequency. The higher the frequency, the
better they work. That's why you are more likely to have distortion
problems on 160 meters than higher. If you look at the specifications
of PIN diodes, they'll sometimes include an "on" resistance spec - that
essentially derives from the reverse recovery time issue.
Power rectifiers also have a long reverse recovery time - which suggests
their use in this application. That would give them the same advantage
as the PINs with respect to using low DC current to turn them "on". In
the "off" position, I wonder if the junction capacitance would be low
enough. A PIN diode has a large gap (the intrinsic region) between the
P and N areas - which leads not only to the long reverse recovery time,
but also to a very small "off" state capacitance. The power rectifier
possibly would not have the low capacitance. But perhaps it would be
low enough for a 50 ohm system at under 30 MHz? I would have to gaze at
datasheets, which are all in storage a number of miles from here in a
box labelled by the movers as something other than "data sheets", I'm
sure.
Fast recovery rectifiers are out of the question - they purposely reduce
the recovery time, and it's that long recovery time we're after. A
Schottky rectifier is a nice efficient, low voltage drop rectifier, but
would not work so well in the place of a PIN.
Regards,
Dave W8NF
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Let me state here that I have not tried the circuit and cannot
> > guarantee that you will not smoke your transceiver. I am confident of
> > K5CX skills and would try it with no hesitation. I would probably use my
> > scope to see how much R.F. gets thru to the xcvr input terminals
> > (replaced with a 50 ohm resistor). I might even run it into the
> > terminals of an external Palomar pre amp for safety (set for low gain so
> > as to not decrease the dynamic range of my xcvr.
> > Thanks to Skip for stepping forward to help and I hope to hear from
> > those who try the circuit. 73, Dan, N5AR
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