Which boards are you referring to and what are the specific changes?
I tried a 1500W QSK unit ages ago that was a piece of crap and have stayed
far away since.
Id want one that can run cool and reliable at 2500W into those VSWR's which
is the minimal overhead margin Id accept for 1200-1500W.
Carl
KM1H
----- 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
>
>
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|