Topband: BDR redux

[K9AY]

from W8JI

> My (totally redesigned) R4C's absolutely blow every other receiver here
out
> of the water because the AGC has a slope at low end, and does not HARD
clamp
> the audio level. The harder part of the AGC comes in about 10-15 dB out of
> noise floor, and the hard limiting comes in before my ears get hurt.
-----

Tom's note illustrates why AGC action in our receivers requires our
attention -- right after phase noise, IMD dynamic range and key clicks.
Simple first-order AGC loops (or 2nd order with AIP and similar schemes) may
not be sufficient for maximum readability of signals.

Some years ago, I got two lessons that woke me up to this issue -- My main
radio was an IC745 with an AGC threshold that was basically at the noise
floor (ugh!).  I acquired a basket-case FT757GXII and fixed it up -- not a
great radio, but it sounded a zillion times better than the '745 on 160M. I
figured out that it had a fairly high AGC threshold, so the noise wasn't
pumped up, masking weak signals.

The second lesson came from a simple homebrew QRP rig. I used a one-chip FM
receiver for CW reception, with some circuit tricks to keep its IF strip
linear for 30-40 dB before it went into limiting. This simple radio was a
joy to use, since it had both the "transparent" sound of no AGC and a
limiter to keep loud signals from blasting me away.

Now I need to decide if I want to open up my '765s and modify them for an
ideal keying waveform, lower phase noise, optimum RF/IF/AF gain
distribution, and variable-threshold dual-slope AGC plus limiting!!!

73, Gary
K9AY