[Icom] Wider SSB bandwidth on IC-756

Adam Farson [email protected]
Sun, 12 May 2002 11:29:40 -0700


Bill,

In the IC-76, the 9 MHz and 455 kHz receiver IF filters are not in the
transmit signal path at any time. The DSP generates an SSB signal by
mathematically modelling a phasing-type SSB exciter; the DSP algorithm
determines the transmitted occupied bandwidth.

The N/W switch is connected to a hardware scan point on the CPU, and starts
a routine to change a variable in the DSP modulation process. As far as I am
aware, the maximum SSB Tx occupied bandwidth achievable with the IC-756 is
2.9 kHz at -6 dB.

It should be interesting to run a 2-tone test on a spectrum analyser for the
N and W settings.

Another point: The centre frequency of the FL-102 is 9010 kHz, as opposed to
9011.5 kHz for the FL-80 and other Icom SSB filters. This mean that the
virtual carrier will be within the filter passband, rather than 20 dB down
the skirt as intended for correct operation of an SSB filter. This will
seriously degrade carrier- and opposite-sideband suppression at the
receiver. This observation also applies to the internal ceramic AM filters.

The FL-103 (2.8 kHz)/FL-257 (3.3 kHz) pair is the correct filter selection
for slightly wider receiver SSB bandwidth.

Refer to the Icom filter and IF tables on http://www.icomclassic.com/

Best 73,
Adam, VA7OJ/AB4OJ
North Vancouver, BC, Canada
http://www.qsl.net/ab4oj/
Note new e-mail address:
mailto:[email protected]


-----Original Message-----
From: [email protected] [mailto:[email protected]]On
Behalf Of Bill Gode, Sr.
Sent: Sunday, May 12, 2002 10:09
To: [email protected]
Subject: Re: [Icom] Wider SSB bandwidth on IC-756


Gene,

Yes, the switch is present on later versions, but changing it from N to
W seems to increase the chances for unwanted distortion, in addition to
widening the transmit signal by about 200 Hz.  I measured additional
response in both lows and highs of about 100 Hz at either end, but
returned my switch to N after testing.

This is a TRANSMIT mod and has nothing to do with the receive filters
(stock or optional).  My guess is that the switch bypasses the 2.8 kHz
filter, which is probably used as a roofing filter for the transmit
DSP.  This switch appeared after the major IC-756 mods came out at about
s/n 1900 to clean up the transmitter.  It is not mentioned in the
Service Manual and Icom-America was unable to provide any information.
(Surprise!)

See: <http://www.mods.dk/select.php3?id=2326&radio=Icom&model=IC-756>

If you want really wide SSB receive response, plug an FL-102 9 MHz "AM
Filter" in the optional 9 MHz filter slot and "fool" the IC-756 into
thinking it has the narrow ssb filter installed.  If you configure the
filter menu properly, you can run FL-102 along with the 9 kHz filter at
455 kHz IF in the ssb "narrow" mode and recover audio out to beyond 4
kHz before the DSP rolls off the response.

The FL-257 will get you from 2.8 kHz out to 3.3 kHz wide - RECEIVE ONLY.

73,

Bill, W9NHQ

"Gene A. Williamson" wrote:
>
>         In a review comparing IC-756 and 756Pro/II, W8WWV states the
following:
>
> NOTE: In January of 2002, I learned that some amateurs had discovered that
> models of the 756 past a certain serial number were built with a
> normal/wide transmit audio bandwidth switch. The serial number of the
> change appears to be around 3,000. The switch is totally undocumented, and
> is located within the radio. The switch, S801, is on the bottom of the
> radio. I do not know where it is located. The swiched is marked N (normal)
> and W (wide). By default, it should be in the N position. If you move the
> switch to the other position, your transmit audio bandwidth will become
wider.
>
>         Can anyone here confirm that? And comment on how the "Wide"
setting would
> compare to using ICOM's FL-257 3.3kHz filter? I don't work SSB, but
> thinking about doing so, and am intrigued with the "Hi-Fi SSB" concept.
>
> 73 Gene N7YW
>