Tom says:
>Now I could have changed the PIN diodes and did all other kinds of
>nonsense that doesn't help, but I dislike that approach.
Now if the PIN diodes were correctly chosen in the first place (long
recovery time) and are biased correctly, their contribution to the rx IMD
performance will be negligible - in spite of various rumours.
>I shorted the active PIN diodes out, and measured the IMD
>change....zero.
Exactly what should happen!
>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.
At a guess, the gate protection diode wasn't reverse biased when the noise
blanker is off. Pretty good way to screw IMD performance.
>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.
I wouldn't put any amplifier in without some filtering in front - well,
maybe an 807 running 60 or 100 mA as a cathode follower, but that sort of
power handling capability. But when you talk of 20 cent FETs, how much are
2N5109s, which are really good linear CATV transistors? Probably don't cost
any more. Way to go, Tom.
> 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.
There is an argument that inclusion of an attenuator is done because you
can't design front ends properly. I suppose you can argue that where you're
shaving every penny, it may allow acceptable performance at slightly lower
cost, but I argue that is marginal. Certainly for a 'top of the line rig',
it shouldn't be that much of a consideration. Generally speaking, I think
that you can do as well by having a RF amp that is switched in and out,
because the basic IMD limited dynamic range required appears (at least over
here) to be around 90 to 100dB. The attenuator enables you to set where that
dynamic range sits with respect to the noise floor, but unless you have that
amount of IMD limited dynamic range, you will suffer, especially on 3.5 and
7 MHz CW. Incidentally, CW is somewhat more demanding because the noise
floor is lower, as a result of the narrower bandwidth. And the lids are
fewer........
But in the final analysis, just as used grey matter between the ears of the
operator is worth at least 10dB, so it is in using a receiver....
73
Peter G3RZP.
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