[ICOM] 751A and DB from Heaven

[Adam Farson]

Hi John,

Thanks for the interesting comments from your end!  

In the AGC, a PIN-diode attenuator is located in the receiver's RF signal
path, at the output of the RF bandpass filters. The attenuator is controlled
by the AGC line. (I have sent you an excerpt of the IC-781 block diagram
showing this.) 

I agree with your remarks on the 756 auto-notch. I fitted my 756 with the
FL-222/223 narrow SSB filter pair, and found that the auto-notch, combined
with Twin PBT and the narrow filters, allowed pretty effective suppression
of multiple tones. The trick was to use Twin PBT to kill the strongest tone,
and thus minimise AGC swamping.

The 756Pro and Pro2 had comparable scope sensitivity (approx. -125 dBm for a
visible spike). I measured -131 dBm on my Pro3. The 10 db scope-attenuator
step is probably a decent compromise between the 756 and the 756Pro series,
reducing the "grass" level whilst retaining the ability to see relatively
weak signals.

-----Original Message-----
From: icom-bounces at mailman.qth.net [mailto:icom-bounces at mailman.qth.net] On
Behalf Of John Geiger
Sent: 21 September 2005 12:25
To: ICOM Reflector
Subject: Re: [ICOM] 751A and DB from Heaven

Hi Adam,

Those are interesting comments.  What is this:

Some receivers have an AGC-controlled passive attenuator in the RF input
signal path, as well as manually-switched pads. The IC-765 and IC-781 are
examples of this.

Not familiar with a AGC passive attenuator.

I agree that the post AGC autonotch is of pretty limited usefulness.
However, as an autonotch the one in the 756 does work better than the
autonotch in the FT100D or the TS2000 I have had recently.  The autonotch in
the TS2000 is at the IF level in the AGC loop, but it has trouble removing
all of the carrier.  There is a manual beat cancel you can use on CW and
SSB, and tune it like a manual notch, but it is at the AF level, so again
the receiver is still being affected, even though you can't hear the signal.

Yes the scope does lack in sensitivity, but it beats not having a scope at
all, or a pretty useless non-real-time one like in the 706/746/TS2000.  I
found that the increased sensitivity for the scope wasn't all it was cracked
up to be in the 756PROII because it also increased the "choppy seas" noise
picture at the bottom covered up much of the weak signals.  You could turn
on the scope attenuator, but then it was back to lacking sensitivity.  Maybe
the correct level was in the 756PRO?

73s John NE0P




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