Hi Jon,
> >In any properly designed receiver, the IMD performance of the RF amp will
> >be a relatively small contribution. The mixer, and occasionally, high
> >frequency crystal filters are the major causes of IMD. Switching off the
> >RF amp reduces signal levels into the mixer, and thus mixer IMD. Getting
> >the performance in an RF amp is relatively easy, especially if you use
> >noiseless feedback and a good linear transistor running well in Class A
> >with plenty of current.
> I stand corrected. And now that I think about it, I don't know why I
> rushed to agree with Tom. If the IM distortion was caused just be the
> preamps in rigs like the FT-1000D and mixers are hard to overdrive, then I
> would expect my FT-840 to have better receiver IM performance (no preamp).
Doesn't matter if you agree or not, or why anyone guesses they did
what they did on any rig. A guess is just a guess.
The fact of the matter, however, is I actually measured the internal
performance stage by stage in my FT-1000. I had to, or else toss it
out like I did the 775. When I first tried to operate a CW contest
with it, it was useless. It sounded like listening to the 40 meter
novice band in the 60's with the BFO turned off. Bleep bloop beep
everywhere, like a cat running across a dozen telegraph keys.
Now I could have changed the PIN diodes and did all other kinds of
nonsense that doesn't help, but I dislike that approach. So I simply
drove the receiver with two generators and looked at the IMD level
stage by stage with a selective level meter. I shorted the active PIN
diodes out, and measured the IMD change....zero.
It doesn't matter what anyone guesses or estimates, the vast
majority of the IMD was from the gate of a dual gate MOSFET left
hanging on the IF chain ahead of the narrow filter IF mixer and just
after the roofing filter IF. Removing forward bias from that transistor
when the noise blanker was off improved IMD about 40 dB.
The second largest problem was the RF amp, so much that I use a
pair of push-pull CATV bipolar transistors in a "patched-in" preamp.
IMO, four twenty cent FET's in push-pull parallel in a broad-band
amplifier (before any filtering takes place) is not the way to go,
although everyone is free to think it is.
Finally, my point got lost in reflector IMD. I never said mixers in
theory were non-problems. My point was simply that mixers (or
other cycle by cycle non-linear devices) are not always the weak
link nor do they often set the IMD limits in our equipment. There
are countless examples where they are not problems, especially in
amateur equipment.
Now the mixer did set the limit in the R4C. The second mixer was
horrid, it suffered from all sorts of basic design flaws mainly
because the injection frequency was many times lower than the IF
frequency. But that was not a cycle by cycle problem, it was a
transfer function problem between the mixer signal input and IF
output.
A typical solid state rig on transmit is a good example. By simply
replacing the driver transistors in my 751A, the transmit IMD
improved about ten dB.
Those are facts, not guesses.
> It doesn't. And also if mixers can't be over-driven easily (as Tom
> suggests) then why do they include additional fron end attenuation levels
> on rigs like the FT-1000D?
Maybe so people can give more accurate reports than with the
guess meter? Or overcome the crummy IMD performance cause by
the noise blanker downstream? Or simply remove the preamp and
get rid of any problem there?
The fact the receiver has an attenuator that replaces the preamp
means nothing, except what we assume it to mean in our own
minds. None of use were privy to the decision why an attenuator
was included, and it certainly would or could reduce overload of any
stage from the preamp to the audio output, be there for
convenience, marketing, or any other reason or reasons. It could
be there simply because Kenwood has one. We have no idea.
> Shame on me, I should have thought my post through more thoroughly before
> responding!
Second guessing without data always is a risk, no matter how
carefully we guess.
73, Tom W8JI
w8ji@contesting.com
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