Topband: BDR redux

[Robert McGwier]

Tom:

I am attempting to capture good AGC action in the next release of
the SDR-1000 software (which is a major rework, using different tools,
libraries, programming style and VB is gone forever and utilizing the
much improved front end).

I do not understand what you mean by "slope at the low end".  Can you
also elaborate on what you mean by "harder part of the AGC".

Thanks for the constant stream of good stuff.

Bob
N4HY

-----Original Message-----
From: topband-bounces at contesting.com
[mailto:topband-bounces at contesting.com]On Behalf Of Tom Rauch
Sent: Friday, February 06, 2004 11:57 AM
To: on4kj; Doug Waller; topband at contesting.com; Bill Tippett
Subject: Re: Topband: BDR redux



>   Two diodes back to back in front of a receiver is'nt that a bit of a
> simple?

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.

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.

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.

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.

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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