To: <amps@contesting.com>
Date: Thu, 15 Jun 2000 20:27:50 -0700
From: dan hearn <dhearn@ix.netcom.com>
Subject: [AMPS] Power Rectifiers as R.F. Switches
dan hearn <dhearn@ix.netcom.com> schreib:
>I had a number of QSO's with K5CX on both SSB and CW before he became a
>silent key. He was using a home built QSK diode switcher on a 3-1000
>linear amp. Instead of a power PIN diode, he used a Diodes Inc. 6A6
>power rectifier. He tested this circuit at powers up to 2kw and
measured
>no change in the 3rd and 5th order products and harmonic content using
>his HP 141T spectrum analyzer. Jim sent me a circuit diagram and page
of
>notes on how it operates. I have not met him personally but he seemed
to
>be a skilled engineer. One of his projects was a 600w power fet linear
>which sounded great.
> I have jpegs of the 2 pages and am willing to share them with anyone
>who would like to try the circuit if I don't get buried in requests,Hi.
>I believe I saw somewhere in one of the ham magazines info on using
>something like a 1N4004 as a 2m r.f. switch.
> I am sure there are people on this reflector who can tell us about
the
>pro's and cons of this and possibly have tried it. I checked the Diodes
>Inc. web page and found that DigiKey stocks these devices as well as
>some with higher voltage ratings in the same series. They are epoxy
>cased jobs and should be pretty inexpensive, I think. I should mention
>that Jim used sheet copper mounting plates on each end of the diodes.
>
>73, Dan, N5AR
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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