A good example of where this comes into play is
early in the morning on 75 meter SSB during the
winter months. During the longpath opening at
my sunrise, DX signals are typically right at the
noise floor, whereas the big guns here in
California just sit on the peg of my S meter. In this
environment, the guys who are running setups
with mediocre IMD can wipe out the entire DX
window for everyone else, whereas the guys with
clean rigs can park 3 to 4 KHz from a weak
longpath signal without inflicting significant
damage.
This is a pet peeve of mine as some of the
more well endowed big guns ought to know
better, but don't really seem to care if they
have wide signals (I sometimes wonder if they
don't do it on purpose just to kick a little more
sand in everyone elses face).
73 de Mike, W4EF...........................................
----- Original Message -----
From: "Tom Rauch" <w8ji@contesting.com>
To: <amps@contesting.com>; "Zyg Skrobanski" <af4mp@mindspring.com>
Sent: Tuesday, March 04, 2003 6:52 PM
Subject: Re: [Amps] Why hasn't solid state replaced tubes?
> > Looking at the ARRL graphs of transceiver IMD testing, and comparing
> > different rigs by their 3rd order products, if one rig has poor 3rd IMD
> > results it does not necessarily follow (according to the graphs) that
the
> > higher order products are also going to be worse. For example, for the
> 160
> > meter result at -4 KHz, the TS2000 has a spur at -55 dB, and the
FT1000MP
> > is at -50 dB. Both rigs are below 60 dB at 10 KHz. So, if the 3rd
order
> > distortion is not audible (not that I thought it would be;) and its
value
> > does not indicate the amount of the higher order IMD, then what is the
> > purpose of having the 3rd order IMD specification?
>
> That's right, and that's an important observation most people miss Zyg.
>
> The important thing is how the rig rolls off on higher order products, not
> the close-in stuff. I can hear a modestly strong IC756 or TS2000 on SSB
> 15kHz away on a quiet band (my noise floor does not move my S meters
much).
>
> It is the stuff that stick out 10kHz that is annoying.
>
> The ARRL and everyone else should start testing rigs in a meaningful
manner,
> instead of nearly useless two-tone tests. The FCC now requires landmobile
> radios to be tested with voice modulation and a peak sample-and-hold
> spectrum display used. That would be a useful test for ham gear. A
bandwidth
> test would weed out keyclicks and splatter.
>
> > That I understand, what I don't understand is how one rig is deemed to
be
> > "better" than another relative to transmitter IMD, when graphs show that
> > each has its advantage but on different bands.
>
> You're right. The only thing I care about is the bands I work weak signals
> on. Those are the rigs I notice. If I worked ten meters I suppose I'd
> dislike some other models.
>
> The real important points are radios really are not tested in any
meaningful
> way, and what data we get isn't easily understood. I'm just as perplexed
as
> anyone else when trying to decide what to buy. You can't actually tell how
> anything is until you use the rig for a year (or more) how it is. It took
me
> almost two years to get the first keyclick report on my FT1000D, and darn
if
> the guy wasn't right. The thing clicked like heck, and no one told me for
> almost two years.
>
> Of course I learned a new 775DSP was a problem in two days, but that was a
> receiver issue (and a 300 watt transmitter spike that showed on my power
> meters and dumped the grid protect over and over in my triode amplifier).
>
> 73 Tom
>
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