[ICOM] Truth in advertising?

[Alex Eban]

...truth, there is! It's only that it's selective! They don't tell you
relative to what the measurement is made. Once upon a time there was
something called shape factor of a filter which told you how wide the filter
was at the -6dB and -60dB points and then you knew. KVG, blessed their
memory, gave for the ham filters measurements at -6 and -90 dB points and it
was usually below the 2:1 ratio relative to the -6dB width. It was also know
as skirt selectivity. That was one of the reasons radios at that time didn't
use LNA's and settled for a noise figure of around 8 to 10 dB, but with a
tremendous dynamic range. Later, when the noise figure became a fetish,
LNA's came in, destroyed the dynamic range and didn't add much, since with
the normal level of atmospheric noise the improved noise figure in
practically worthless.
		Soooo, now they come back and place the selectivity back
where it belongs: as close up front as practical, right after the mixer. The
only bad thing is that the LNA still ruins the dynamic range and that's the
reason most radios have a disable (LNA bypass) switch on them. I would
choose a radio with a high IF and with the best available filter wired after
the mixer. After that one cleaned up the signal, all that remains for the
later filters to do is to reduce the wideband noise generated by the IF
amplifiers, before they reach the detector.	This is why I like the
TS-140: it's designed along these lines, wit a 45MHz first IF. No better
gain and selectivity distribution scheme ever came up to this very day,
except perhaps the Hughes military radio with a 109.35MHz first IF (13kHz
wide)!
Alex	4Z5KS