Date: Fri, 22 May 2015 22:40:53 +0930
From: "Leigh Turner" <invertech@frontierisp.net.au>
To: <amps@contesting.com>
Subject: Re: [Amps] SPE 2K RTTY Duty Cycle / spectral cleanliness
All good valid points that you make here Kevin.
There is also an equal fascination / obsession in some ham radio quarters
with IM3 and levels! We are talking here of HF amateur / ham band
transmitters NOT professional transmitters where spectral cleanliness is
more important.
It's the higher order IM stuff / crud that has to be curtailed...
The modern trend towards Adaptive pre-distortion techniques works well to
clean up mediocre PA transistors and sort of turn a sow's ear into a silk
purse.
Leigh
VK5KLT
## Nonsense Leigh. Where in the commercial world, do you have 10,000 brain
dead
contesters screaming their lungs out on crowded HF bands..for 48 hrs non stop ?
## it would be different if we were all on channels, spaced every 4 khz apart.
4 khz
spacing is what all the telcos here in NA used, back in the analog cxr days..b4
the
conversion to digital. It was all SSB, stacked nose to tail, every 4 khz, then
the entire
mess, called baseband..was stuffed into the microwave TX. IMD was important
in that case, since one klystron typ had anywhere from 240-1200 channels. If
the tube
went bad, and imd degraded, the TX imd would affect all 1200 channels.
## marine, aircraft, broadcast etc, are all assigned channels. In the ham
radio world, we have
vfos. One night a group of us were on 75m, with a rag chew session going
on..during a major contest.
Some bozo starts up with the cq contest stuff..... 2 khz above us. A half hr
later, another bozo starts up
exactly 500 hz below us! We were all running 1.5- 2.5 kw output into typ 75m
ants. We ask the bozos
if they can hear us. Sure, I can hear you...you are all S9++. WTF, then why
not stay at least 3 khz away.
## half the problem is stations way too close to each other to begin with.
The rest of the problem is
junk SS xcvrs, with sky high, higher order imd...fed into linear amplifiers.
The amplifier just amplifies
a high imd xcvr = total disaster. IMD3 and IMD5 is a meaningless spec.
Whats important is
IMD 7-9-11-13-15. A static two tone imd test is equally useless. Everything
is in a static condition,
not a dynamic condition. The various voltages, including bias, alc,
collector, B+ etc, dont budge.
## two tone testing isnt even allowed nor used in the commercial world. By
juggling the tone spacing, you
can easily reduce IMD3 and IMD5 by quite a bit. We don’t tx 2 tones....we
use ssb.
## as noted in my previous posts, marine and aircraft SSB IMD specs are WAY
more stringent than typ
amateur radio junk. The marine stuff all runs on 12 vdc too. Somebody
mentioned the FT-990. Take a
look at the yaesu 767GX from years ago. It had superb imd. But it used 200
watt devices...run at 100w out.
## that doesnt cost much more. The power supply, LP filters, cooling, auto ant
tuner, etc, etc, are all rated for 100w.
The finals are rated for 200-300 watts.... but run at 100w. Use loads of
feedback, and you too can meet the FCC
marine spec for SSB TX imd. No pre-distortion nor class A blast furnace
tricks required.
## we have regs for harmonic suppression etc, etc, but nothing for TX imd.
Too bad, cuz until we do, we will
continue to get crap TX imd. The kenwood 870, when run at 100w out...has bad
imd..like –30db pep. Back it down
to just 85w out, and imd cleans up a whole bunch, huge improvement. Reduce it
down to just 50 watts, and its
superb. I gave up on the 100w xcvrs years ago. Buy a 200w xcvr...then run it
at 100w, or whatever is required to drive
your amp. Remember all the xcvrs with key clix ? Same deal, no regulations
in place.
later.... Jim VE7RF
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