> 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
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