Lon,
One factor oftentimes neglected when reporting diode-switched, QSK
performance is the degree of IMD product produced. For example, the Alpha
86 had approximately 10-15 dB worse 3rd order IMD than the comparable Alpha
87A and 89 as a result of non-optimized PIN diode biasing. The RF deck and
components off all three Alpha amps are nearly identical.
Paul, W9AC
----- Original Message -----
From: "k5jv" <k5jv@kingwoodcable.com>
To: "Amp Reflector" <amps@contesting.com>; "W5PVR" <alpha76@triton.net>
Sent: Friday, December 18, 2009 12:56 PM
Subject: [Amps] QSK switching time
> Greetings to all,
>
> I would like to share a great discussion that I was evolved in
> this
> morning on an 80 meter net about QSK, with some of my own comments.
>
> The basic argument is that because PIN diodes switch faster than PN
> diodes, PIN's must make a better QSK switch. Well, do they? If you
> take
> the entire QSK circuit into consideration, you will see that overall
> switching time is not determined by the diodes, but rather by a sequence
> of,
> time consuming, switches, the bias switching transistors being the major
> consideration.
>
> Both types of diodes switch in microseconds, with Pin's being
> noticeably faster. Meaningful QSK switching times are measured in
> milliseconds, not microseconds (a magnitude of 1000, or so). Since a
> diode
> switch can not switch any faster than it's bias is switched, the QSK
> switching speed is really controlled by the switching time of the bias
> switching transistors. These transistors are many times slower than
> either
> type of diode. Therefore, unless you just like to spend more money than
> you
> need to, PIN's are not necessary (an expense of $5 vs $200 for a set of
> diodes).
>
> The other factor that is really misunderstood is SWR with a QSK
> system.
> The basic argument here is that diode QSK will not handle SWR. Not so, at
> lease with in reason. The maximum SWR that a QSK board can operate with,
> without self destruction, is determined by the value of reverse bias
> placed
> to the diodes during transmit. This is usually a fairly low DC value,
> sometimes as low as 250 volts, and seldom over 400 volts. The HeathKit
> boards and early Ameritron boards had switching transistors that were
> fairly
> reliable, at these voltages. The switching transistors in the current
> boards will work reliable with 400 volts reverse bias, most of the time.
> However, do not try to increase reverse bias with these transistors. I
> have
> had them literally explode off the board with only 600 volts reverse bias.
> I replace all four switchers with 1200 volt devices. These devices switch
> reliable with 900 volts reverse bias applied (the highest I have tested).
>
> With these new switching transistors installed, along with the
> above mentioned PN diodes, and 900 volts reverse bias, I have run
> repeated,
> two hour long tests, with 2500 watts and SWR of 4 - 5 to 1 with
> absolutely
> no failures. (yes, I did reverse the SWR) In, fact, taking frequent
> temperature readings, on 160, 80, 40, and 20 meters, with an inferred,
> LASER
> guided, thermometer, I found that the PN diodes actually run between one
> and
> three degrees C cooler then PIN's.
>
> Not a week goes by that I do not buy one, or two, of these
> boards,
> sometimes the complete stand alone units, that the owner can not make
> work,
> or in unsatisfied with. I do not understand why anyone would remove one
> of
> these boards from an amp. With just a little updating, they are extremely
> reliable and a pleasure to operate on any mode.
>
>
> 73 de Lon, K5JV
>
>
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