Clifton never worked as an operator in a multi-multi contest station! ;-)
His description is accurate when describing how it responds to a wanted
signal.
However the voltage hitting the diodes will come from the sum of all signals
hitting the front end of the radio.
At a multi-multi station, you have 6 stations running 1500w in close
proximity of each other. In fact it is sometimes 12 (considering you have a
Run and a S&P on each band). There can be a lot of RF in the air, most of
the time.
This can cause these diodes to slightly forward bias, which in turn causes
IMD in the receiver.
Most people will never find themselves in this situation, so it is pretty
much a non-issue for most.
Our contest club tries to avoid radios and amplifiers with pin diode
switching.
73 - Rick, DJ0IP
(Nr. Frankfurt, Germany)
-----Original Message-----
From: TenTec [mailto:tentec-bounces@contesting.com] On Behalf Of Al Gulseth
Sent: Tuesday, May 17, 2016 4:11 AM
To: tentec@contesting.com
Cc: Gary J FollettDukes HiFi
Subject: Re: [TenTec] QSK switching diodes in Scout
Clifton Labs has a writeup which might be of interest in this regard:
<snip> It's commonly understood that the 1N4007 has an special junction
structure resembling a PIN diode and it has, of course, been widely used as
a "poor man's PIN diode" in RF power switching. Elecraft, for example, uses
1N4007 diodes for transmit/receive switching in both its 10 watt and 100
watt option K2 transceiver.
Used as a quasi-PIN diode for RF power switching, the 1N4007 takes advantage
of the lengthy reverse recovery period, which prevents it from rectifying
the high frequency signal it is switching. (This is because the reverse
voltage half-cycle period of, say, a 3.5 MHz signal is much shorter (140 ns)
than the reverse recovery time. Hence, if forward biased with DC, any
reverse bias from RF negative half-cycles don't last long enough to
neutralize the 1N4007's excess minority carriers. Hence, the 1N4007 stays
forward biased and presents a low impedance to the full RF cycle. There's a
minimum frequency for this effect which we see from the trace below. With a
bias current in the
60 mA range from the 8012B pulse generator, we see the following voltage
waveform. <snip>
http://www.cliftonlaboratories.com/diode_turn-on_time.htm
73, Al
On Mon May 16 2016 8:59:45 pm Gary J FollettDukes HiFi wrote:
> Interesting, i did not know that!
>
> Thanks!
>
> > On May 16, 2016, at 6:28 PM, Stuart Rohre <rohre@arlut.utexas.edu>
wrote:
> >
> > The silicon diodes signal path in the Scout are made with die
> > similar to PIN diodes, or identical.
> >
> > -Stuart Rohre
> > K5KVH
> > _______________________________________________
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> > TenTec@contesting.com
> > http://lists.contesting.com/mailman/listinfo/tentec
>
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