[ICOM] 751A and DB from Heaven

[Adam Farson]

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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