[Elecraft] IMD

[Jerry T Dowell]

For many years now, I have been concerned by the performance of modern
transceivers with respect to intermodulation distortion on SSB. To me, from
listening around the phone bands and reading on the subject, the performance
of almost all modern amateur equipment seems unacceptable. I would like to
solicit opinions on this ..... I may be way off base.

For most gear on the market, 3rd order products are only down about 19 to 24
dB from single tone in a two-tone test. (The ARRL quotes dB down from PEP
output, but professionals rate it with respect to single tone. Apparently
the ARRL doesn't want people to realize how bad the rigs really are.)
Examples: FT-1000MP MkV :  -21 dB, (in Class A mode: -39 dB);
IC 756 PROII: -23 dB;  Elecraft K2:  -23 dB, etc.  The best I have seen
reported was the JST-245 at -33dB. Even my beloved Drake TR-7 is -27 dB for
3rd order, but the 5th order is the same magnitude as the 3rd order, which
is why I really do not care to operate SSB with it unless I have to. At
least Yaesu has taken a step in the right direction with their Class A mode.

In "Solid State Radio Engineering" by Krauss, Bostian and Raab, it is stated
that any less than -30 dB from single tone is unacceptable. "Single Sideband
Systems and Circuits" by the Collins fellas (1987) also takes that viewpoint
and shows some nice spectral displays of the differences between the KWM-380
(-24 dB) and the HF-8010 (-50dB) with voice drive.

How difficult is it to engineer in better distortion performance with solid
state devices? I realize that it is not too important for something like the
K2 running a few watts and would indeed affect the cost and power
requirements. However, now that the 100W amp is looming, it becomes just
another transceiver to spread Imod products around the phone bands. Could
something be done to improve the K2 in this regard, say by changing the
transmit gain distribution, adding a stage or two, etc? Just thinking ahead
of possible mods. Especially if the K2 is to become the primary station rig,
as it will at AI6L.

Am I too unrealistic here and too much still hung up on the better than -40
dB performance of the 200V and some of the old Collins gear?

73,   Jerry   AI6L