[Elecraft] IMD

[Sean Baxter]

Jerry:
Assuming there is not a conspiracy of radio manufacturers trying to pawn off
cheap radios on unsuspecting amateurs, I have the following questions
concerning IMD.

How bad is the difference between -21 verse -30 dB IMD?  Can someone
actually hear a difference on the signal?  Is it wider or more distorted
at -21 dB?  Maybe it costs many dollars more to engineer and manufacture a
radio with -30 dB IMD specifications?  If no one can hear the difference but
only seen on a scope, then who cares?  I do not know maybe Gary or Wayne
would know the answer.

Sean

----- Original Message -----
From: "Jerry T Dowell" <jerryandgail@earthlink.net>
To: <elecraft@mailman.qth.net>
Sent: Sunday, February 10, 2002 7:47 PM
Subject: [Elecraft] IMD


> 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
>
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