[Amps] Pin Diodes

[Paul Christensen]

> 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