[AMPS] Re: Ferrite Rod for 6M Amp

[Tom Rauch]

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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