[Amps] Pin Diodes

[Joe Subich, W4TV]

John,

I would be interested in the carrier lifetime measurements on the
IN4007 when you get them (or if anyone else has similar numbers).

The biggest problem John Sheller had with the QSK1500 in the 1980s
was the high power switching diode (in series with the amplifier
output).  It needed to be able to handle better than 5 amps for 1500
watts output at 1:1 SWR.  If the SWR was such that the impedance went
down the diode current went up in a hurry.  The same current issues
apply to "conservative" designs (say 5 KW capability).

The other issue is sufficient "off" bias on the first diode in the
receive path so it does not break down with SWR on the high impedance
side.  300 volts is sufficient at 1500 watts and 1:1 SWR but the
requirement can become unmanageable at higher powers or with high
SWR.

73,

    ... Joe, W4TV



> -----Original Message-----
> From: amps-bounces at contesting.com
> [mailto:amps-bounces at contesting.com] On Behalf Of W0UN --
> John Brosnahan
> Sent: Wednesday, March 08, 2006 10:34 AM
> To: Paul Christensen
> Cc: amps at contesting.com
> Subject: Re: [Amps] Pin Diodes
>
>
> At 07:38 AM 3/8/2006, you wrote:
>
> >>I do quibble with your statement on speeds.   Vacuum relay speeds
> >>are measured in
> >>milli-second switching times whereas PIN diodes can be switched in
> >>micro-seconds.
> >
> >John,
> >
> >I agree with your statement.  I don't mean that all other factors
> >being equal, a vacuum relay can approach the microsecond switching
> >speeds of the PIN diode.  But as a practical matter as used in HF
> >amps, I think one would be hard-pressed to find a condition where
> >the QSK performance of the relay is inferior to that of the PIN
> >diode.  I don't contest, so the multi-million operations that you
> >guys see could certainly be a long-term factor.  But even that
> >limitation is the result of the hardware mechanics and not the
> >result of premature failure from hot-switching.  A well-designed QSK
> >circuit like Rich Measures' can avoid hot-switching at pretty much
> >any CW switching speed.  Although, high-speed data mode switching
> >may be a different matter.
> >
> >Using my IC-7800, I can run CW as fast as my WinKey chip will go -
> >perhaps 75 or 80 WPM and the 77Dx and 70V completely keep up when
> >I'm watching the waveform on a scope.  Moreover, I can hear between
> >keyed elements up to about 35WPM on IC-7800, but the T/R turnaround
> >time is limited by the transceiver and not the amp.
> >
> >That said, I did try Tom Rauch's QSK-5 a few months back.  It was
> >going to be used as part a silent T/R system for a Drake C
> >line.  While testing the QSK-5, I was very impressed with its
> >operation and fast switching time.
> >
> >At the end of the day, I think one has to make a decision as to
> >which evil is the lesser one to live with: (i) the mechanical
> >failure of the vacuum relay; or (ii) electrical failure of the PIN
> >diode resulting from lightning, SWR greater than 2:1, etc.  And,
> >when either of these switches does fail, which is less costly and
> >easier to replace, especially in the middle of a contest weekend?
> >
> >If I was a contester with predictable antennas, I would probably
> >want a PIN diode switch.  Since I am not a contester and some of may
> >antennas (e.g., G5RV) may present a 3:1 SWR, which is easily
> >compensated for by the amp's Pi-L network, I prefer a vacuum relay.
> >
> >Paul, W9AC
>
> Hi, Paul--
>
> I love how my 781s and 87As play together.  On CW we can hear the
> back-scatter echoes
> and turn the antennas as appropriate.  And even on SSB with PTT you
> can hear a slightly
> different "kerchunk" at the end of a transmission that indicates
> back-scatter.  Something
> that you will never hear with typical VOX delays.
>
> I am building some monoband amps and may use a different
> style of TR switch for
> these narrow-band amps. One with either 1/4 wave coax lines
> or lumped-element
> equivalents.  Something I have been doing since 1978 in my
> HF/VHF radars.  The
> 1N4007 is a PIN structure, and I think there are others,
> since the A6A was also
> mentioned as a candidate -- not sure of its package but there
> was also another
> diode used for 2.66 MHz sounding with similar nomenclature that was
> in a white,
> ceramic package that was about a 1/4 inch diameter sphere,
> and this one
> seemed to work well.  Anyway, I want to keep it CHEAP so I
> hope the 4007s
> will work -- just need to do some carrier lifetime measurements "one
> of these days".
>
> Alpha limited their PIN diode protection circuitry to a reflected
> power of 300 watts
> (rather than a VSWR) to make their warranty policy work out without
> undue expense.
> But Dick Ehrhorn's 87As have been running with a limit of 400 watts
> for some time
> (a decade maybe) with no ill effects.  So the design is a bit
> conservative as it should
> be in any product.
>
> The PIN diode vs. vacuum relay issue is a classic engineering
> tradeoff that can
> go either way depending on what factors you choose to
> consider.  Much like
> solid state amps vs. vacuum tube amps.  Depending on what
> questions you ask
> either one can be the "right" answer.
>
> 73  John  W0UN
>
>
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