> 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
|