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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