Topband: IMD Problems with Alpha 87A

[Tom Rauch]

> The problem is only present in the Operate mode when the
pin diode T/R switching is used. In order to isolate the
problem Alpha suggested several things. First, they
recommended that we check to see if there was any RF
traveling in the coax shield.  They recommended we put a
common mode filter in the line, which we did without
success. They also recommended putting a highpass filter in
the line to see if that would help.  BINGO - That fixed the
problem.  I happened to have an ICE 402X highpass filter
laying around and that did the job.

Terry,

Common mode on the coax shield makes no sense, so it doesn't
surprise me it wasn't a cure. If common mode was getting
into the ETO internals, they would have a MAJOR connector
mounting or cable grounding error! That's not likely.

The real problem is nearly all PIN diodes do not behave like
PIN diodes on 160 meters. Carrier lifetime is too short and
the PIN diode acts like a regular diode instead of a linear
"resistor". There isn't anything wrong with that *IF* the
diode has enough forward bias current and voltage to keep
the diode in conduction. A good example of this are diodes
used in our receivers, which despite Internet claims of
better performance by changing diodes really contribute no
detectable distortion, IM, or noise. The manufacturers use
enough bias current, so a regular diode works just as well
as a PIN diode. (Both Sherwood Engineering and myself have
independently confirmed common expensive PIN diode swaps do
nothing except lighten your wallet in transceivers.) The ETO
is another matter. Because there are very few diodes expense
and labor is less an issue. You could easily use diodes with
long carrier lifetime (in the tens of microseconds), but HV
LF diodes are on the expensive side. They are about 10-20
dollars each wholesale last I knew.

If your ETO is hosing up the RX on a BC station, I would
also be suspect of what it does to your RX during contests!
The accumulated level of BC stations or even 160 meter
signals alone is probably producing a problem you are not
aware of. My first choice would be to fix the bias problem,
rather than hide the problem with a filter. The amplifier
should have less IM or harmonic distortion contribution than
the receiver you use, not more!

In LF QSK switches I've designed, I always use nearly
maximum rated current and dissipation in the RX diodes. You
might look at modifying the ETO to do the same. As a second
option, you should consider inserting a high pass filter in
the diode path. Then you can use a low power filter. I'm
sure there is an insertion point available. Almost certainly
a bias current correction (increase) would be much easier
and have the advantage of improving in-band IM or harmonic
distortion. Maybe ETO could help with that.

73 Tom