[ICOM] 751A and DB from Heaven

[John Geiger]

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

----- Original Message -----
From: "Adam Farson" <farson at shaw.ca>
To: "'ICOM Reflector'" <icom at mailman.qth.net>
Sent: Wednesday, September 21, 2005 6:06 PM
Subject: RE: [ICOM] 751A and DB from Heaven


> Hi John,
>
> Glad you enjoyed the article. George W5YR (SK), Andrew ZS1AN and I
> collaborated on it. The title was inspired by a comment by our EE
professor
> at the University of Cape Town many years ago, in which he described the
> theoretical 9 db S/N advantage of SSB over AM as "Kilowatts from Heaven".
>
> My only two serious "beefs" with the original 756 were the lack of a
manual
> IF notch filter inside the AGC loop, and the relatively poor
spectrum-scope
> sensitivity. The IC-756Pro II Technical Report, Sec. 5-11, shows the Pro2
> scope vertical sensitivity as 18 dB better than that of the 756.
>
> The Technical Report is on http://www.qsl.net/icom/manuals.html (the
mirror
> site is faster.)
>
> 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.
>
> The IC-7800 probably has the best NR/NB combination; both NB and NR are
> implemented in the DSP. I have found both NR and NB on the Pro3 to be
> slightly more effective than on the Pro2.
>
> Cheers for now, 73,
> Adam VA7OJ/AB4OJ
>
>
>
>
> -----Original Message-----
> From: icom-bounces at mailman.qth.net [mailto:icom-bounces at mailman.qth.net]
On
> Behalf Of John Geiger (NE0P)
> Sent: 21 September 2005 07:09
> To: icom at mailman.qth.net
> Subject: [ICOM] 751A and DB from Heaven
>
> Hi Adam and list members
>
> Thanks for the link to the DB from Heaven article.  I have read that
before,
> but never tried it out yet until this AM.  Using my Icom 756 I was
listening
> to and then working JA1NUT on 40 CW.  Noise level here without the CW
> filters in line was S5.  Adding 6db of ATT lowered the noise level to
S2-go
> much for the 6db per S unit idea.  I think switched antenna ports, lowered
> the RF gain, and then switched to the main antenna again.  Yes, the noise
> was reduced, and JA1NUT was a little easier to copy.  Thanks for the tip.
I
> just worry about the stations I might be missing because they are in the
> noise, and am wondering which rig would have the best NB and NR controls
for
> taking out that noise at the beginning.
>
> I just looked at the ARRL expanded report for the Icom 756, the MDS for 80
> meters, preamp off is -134.9dbm and for 40 meters, preamp off, is -134.2.
> That seems overly sensitive on the low bands, as I am sure that all of us
> get more band noise than that.  So adding the 6db of attenuation probably
> brings the MDS to a more reasonable level for 80 and 40. The expanded
report
> show the MDS for the FT1000MP to be around -127dbm on 40 and 80 which is
> probably a more reasonable level.  One nice thing about the 756 series is
> the incremental attenuator.  Many radios just have 20db, which is probably
> too much in many cases.  I like being able to add 6 or 12db as needed.
Now
> if the 756 original just had a manual IF notch....
>
> 73s John NE0P
>
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