I understand what you are saying, but the RF amplifier should be conjugate
matched to 50 ohms anyway, in order to have uncondisional stability. I am
not sure what the noise figure of this system is, but it seems that the gain
distribution is such that most of the gain is in the 2nd I.F. strip anyway.
Even so, under 30MHz, in most areas the excess environmental noise is in the
15dB region so a receiver with a 12dB noise figure does just fine.
I remember the old Allied Model 2568 CB radio. This thing had quite a bit of
RF gain and relatively low I.F. gain. As soon as you connected an antenna,
it sounded like an FM unit. The problem with that design is that the AGC
voltage was derived from the RF stage with its relatively low selectivity,
in addition to the I.F. strip. Strong off channel signals would capture the
AGC loop and desense the whole system. Remember the old term "bleed over"?
You do have a good point about keeping the RF gain ahead of the mixer as low
as possible, since any gain ahead of the 1st mixer degrades the dynamic
range by that same amount. In my last contract with Motorola, we were using
mixers that had an IP3 of +40dBm so we were able to get away with having
some gain ahead of that mixer.


IMD is minor compared to the negative effects of high input impedance
of the RF preamp, which is almost always too high. Reducing the 1st RF
gain (A = 5 to 10) will reduce the input impedance and make it less
vulnerable to noise -- especially the white background noise. You can
compensate by increasing gain in the IF stages. Then realign your
input matching network and =blammo=, weak signals are solid with
much less noise.




......., and a diode attenuator is used in the
front end. I have found that IMD performance can be improved by changing
that general purpose switching diode in the front to a 1N5767 PIN diode.


As long as the original isn't used for overload protection, I don't
see any problem with that. But then I don't see any advantage, either.
The band is just too noisy for it to make any significant improvement.
Still, I've got a crusty old Diamond 60 laying around somewhere, so
I'll give it a try anyway.