Date: Mon, 10 Apr 2017 00:14:39 -0700
From: Jim Brown <jim@audiosystemsgroup.com>
To: amps@contesting.com
Subject: Re: [Amps] rich Richard - tiny antenna
> It's almost as baffling as the fixation with IMD..
IMD is a VERY big deal if you care about not making a mess on the band
when you transmit. IMD ==> wide clicks on CW and splatter on SSB.
A disgustingly large number of SSB signals have almost as much splatter
in the bandwidth of their suppressed sideband, and an equal amount above
where the sideband filter in the rig cuts off.
K6XX taught me that a mis-tuned amp (or a solid state amp working into a
mismatch load) produces a lot more clicks and splatter than one that is
properly tuned. We're both serious contesters, and when I moved in 3
miles S of him, he made damn sure that I knew how to tune my Titan amps.
Bob is an engineer working at Elecraft, where he works as a production
engineer. As a result, we can work with 500 Hz of each other on CW at
legal limit and hear the other as simply another strong signal, and
easily work fairly weak signals. We're both running tube amps.
73, Jim K9YC
## Get your buddy K6XX to fix the lousy TX imd on the K3. Its on
a sweep tube level. -23dbc for IMD-3. The 10 watt version of that K3
is no better either. It needs a serious re-design of the entire pa section.
## I find it amusing that yaesu, kenwood, and Icom, all make marine
SSB xcvrs that easily meet the ITU / FCC TX IMD specs...and do it with
12 vdc..and even with less than 12 vdc available. The marine specs for IMD
are 20 db better than any of the current ham junk these days. They use loads
of
nfb, and dont run devices maxed out. They also dont have built in speech
processors,
nor do they have mic gain controls. Marine IMD specs go out way past the bw
of the
xcvr. IMD 3+ 5 is not the main issue. Its IMD 7-19 that is the issue. Its
all easily fixed.
## The pre-distortion tricks used in the latest ADAT 200w xcvr, put the IMD
at –71db pep.
Problem is, it still has a 20 msec delay, fine... until you try and monitor
yourself in the
headphones..sounds horrid. The sense loop can be pulled out of the
xcvr...and the sense signal can
instead come from any 1.5 kw amplifier. Folks typ see –51 db pep for IMD-3
on a
pair of 3-500Zs..which is superb. So the pre-distortion trick can be used
with any amp, but
you wont get the –71 db pep...like you can achieve, from the xcvr by itself.
I suspect that is because
you are not correcting in real time, but after the fact. The pre-distortion
appears to be able to make
a lousy amp pretty good for imd....and a clean amp even better.
## The max sense signal is 0 dbm. Their adat xcvr uses sma connectors for
the main sense input...and also the
-10dbm for the 200 watt pa section. If just the xcvr is used, they install a
loop between the
pair of sma connectors. If a kw amp is used, the loop is pulled, and the sense
if derived from the
kw amp. So the 1.5 kw has to be tapped off..and padded down to something less
than 0 dbm. It wont
work at all below –24 db, so there is a 24 db window it has to work with.
Jim VE7RF
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