Topband: BDR redux

[Doug Waller]

> Two diodes back-to-back in front of all the filters would absolutely kill
> reception if the signals were limited in the diodes! The IM and general
> "crud" would be horrible.

If all signals entering the antenna input of a receiver were weak signals,
including noise, then receiver specs would be of little concern.  Audio gain
could be turned wide open for easy copy.  Presiently RF Limiting Amplifiers
are used to compress strong signals while amplifying weak signals at UHF and
Microwave.  The output is filtered and usable.  Let's apply this technology
to 160M receivers.
>
> A soft limiter after the filters, if kept well out of limiting on
background
> noise, is useful. I use that on my R4C's to prevent "ear blasting" because
I
> have to run gain almost full open for weak signals when listening in quiet
> directions.

I run the MP's audio near wide open into the clipping diodes (use 1K series
resistor to limit current).  The following SCAF (switched capacitor audio
filter, old technology chips) almost completely removes clipping hash, as
well as MP audio amp hiss.  Switching the filter out yields an ugly audio
mess, indicating that the diodes are hard clipping everything but weak
signals & weak background noise.  It may be possible that only a SCAF filter
can clean up diode hard clipped audio.

> 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.

There is a surprising reserve of weak signal performance in the FT-1000MP,
and probably most other receivers.  Applications of inventive design, such
as you have done with the R4C, should make alot of this hidden performance
available.
>
> This means no matter what direction I listen (noise changes in each
> direction) I don't have to adjust anything except volume. I haven't moved
> any control on the R4C's except volume and tuning in the past three years,
> summer, winter, fall and spring and every direction and band. Other guest
> ops, who seem to do OK here, always strongly agree. The comment is always
> "signals jump out of the noise" and are easy to copy. The reason for that
is
> the AGC slopes on the low end of threshold, becomes stiff at about 15 dB
out
> of noise floor, and the whole thing hard clamps AFTER all the filters to
> prevent ear damage.

You have spectacular reception capabilities that can not be attributed to
phased, staggered Beverages and quiet location alone.  For topband
reception, no stone must be left unturned.  As for 160 M high performance
receivers, many yearn for a simple, well designed receiver that will get the
job done.  No bells, whistles, FM or digital modes... and affordable.  A
modified MP or a receiver kit could do the trick!
>
> IMO too much limiting on signals at too low a threshold is a VERY bad
thing,
> as is anything that makes me have extra knobs to fiddle with as receive
> direction changes and noise floor changes.
>
> 73 Tom
>
>
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