Author: Greg Ordy - ordy@seed-solutions.com
by Greg Ordy
The ICOM 756PRO
documentation is silent when it comes to describing the CW
filter shape factor, and how it can be selected. ICOM tried to
improve the situation with a brief mention of this topic in its
Advanced Operation Guide
for the 756PRO, but it does not clearly describe how to select the
filter shape. I will try to do a little better on this page. I
would like to thank Rob Peebles, W8LX, for bringing these
capabilities of the radio to my attention.
The 756PRO receive audio,
coming out of a digital signal processing (DSP) intermediate
frequency (IF) stage, is much different in character than other
purely analog receivers. I have captured some of my impressions on
another page.
On CW, the filter response
created by the DSP is extremely sharp. That is, signals outside of
the pass band are substantially attenuated. In the analog domain,
sharp filters can lead to ringing, and other artifacts.
Digital filters can have similar problems. In addition, it has
been claimed that the 756PRO receiver highlights flaws (key
clicks) in the transmitted signal. In other words, it hears the
clicks better than other receivers. These various factors lead to
CW receiver performance which some operators do not like. One
suggestion to improve performance has been to alter the CW filter
shape. Perhaps if the filter was not as sharp, the artifacts would
not be as pronounced. Apparently a
special DSP ROM
(software) is available in Japan, but information on this
modification has been difficult to obtain, and in any case,
changing the DSP software is claimed to be a major project, in
terms of taking apart the radio to access the relevant parts.
It turns out that for filter
widths of 500 Hz and less, it is possible to select one of two
filter shape factors. One is the sharp CW filter, and the other
appears to be the SSB shape factor, which is not as sharp. Calling
one the CW shape factor and the other the SSB shape factor is my
own terminology. Rob Peebles, W8LX, estimated the sharp CW shape
factor to be 1.5, and the SSB shape factor to be 2.0. This page
describes how to select the two different shape factors (in the CW
mode, it cannot be selected in SSB mode).
The top line of the main
display contains a yellow indicator labeled BPF.
This indicator is not mentioned in the documentation (756PRO
user's manual). It appears immediately to the right of the
pass band width indicator (page 10 of my manual). It is my
belief and claim that this indicator shows the IF filter shape
factor state. When the indicator is off, the display is dark, the
SSB shape factor is in use. When the BPF
characters are visible, the CW (sharp) shape factor is being used.
Early 756PRO users
discovered the BPF indicator soon after the radio
was introduced. It was clear that it was tied to the CW filter
width. As best as I can tell, you must be in CW mode in
order to be able to use the sharper filter. When the CW filter
width is set to 500 Hz or less, via the BW (F-1) button, in IF
filter selection mode, the BPF indicator and mode
automatically turns on. Until I talked to Rob Peebles, W8LX, I
assumed that all CW filters 500 Hz or less in width would have the
BPF indicator on. This turns out to be untrue. It
is possible to have CW filters at and under 500 Hz without having
the BPF indicator on. Since the BPF
indicator is not on, you will be using the SSB shape factor, even
if the filter width is reduced to the minimum value of 50 Hz.
The current filter width and
shift (offset) are displayed on the top line of the main display.
The BW field indicates the filter bandwidth, and
the SFT field indicates the filter shift. I
believe that these two fields, along with the BPF
indicator, completely capture the IF filter state of the radio.
There is no more data to view or change.
There are two different ways
to specify the filter width. Until this issue came up, I
assumed that both ways would always arrive at identical results.
This is not true. One way changes the BPF
indicator and the other does not. Again, this discussion applies
to CW mode. In SSB mode, the BPF indicator will
never turn on.
The first way to adjust the
IF filter width is to use the BW [F-1] button in
conjunction with the main VFO knob. You must enter the IF filter
set mode in order to use this way. When this way is used, the
BPF indicator will turn on for all widths of 500
Hz or less.
The second way to adjust the
IF filter width is to use the Twin PBT knobs.
These concentric knobs can be adjusted to narrow the filter width.
Here is the entire point of this web page: When the IF
width is adjusted with the Twin PBT knobs, the BPF indicator will
not come on, even for widths equal to and under 500 Hz.
Obviously you must start with a width greater than 500 Hz so that
the PBT indicator is off to begin with.
My own opinion is that this
is a bug that really is a feature.
My guess is that ICOM engineers wanted the sharp CW filter to be
on for all widths of 500 Hz or less. When using the BW
button, it works like that. But when the Twin PBT
controls are used, the check for crossing the 500 Hz boundary is
not made, and the filter does not change shape.
The best way to evaluate the
two different filter shape factors is to A/B compare them for a
constant filter width. Here's my suggestion for doing that.
-
Pick a band like 40 meters
that usually has a number of CW signals, and some background
noise (especially at night).
-
Set the 756PRO to CW mode,
and enter the filter set mode by pressing the Filter
button for 2 seconds.
-
Select the first filter
and press the DEF button to return the filter
to the factory default width of 1.2 KHz. Since this width is
greater than 500 Hz, the BPF indicator will be
off. Turn the outer Twin PBT knob to the right
10 clicks. Turn the inner Twin PBT
knob to the left 10 clicks. This should result in a 200
Hz filter. Since the filter was narrowed with the Twin
PBT knobs, the BPF indicator should
still be off. You now have a 200 Hz wide filter with the SSB
shape factor.
-
Select the second filter
and press the DEF button to return the filter
to the factory default width of 500 Hz. Since the filter width
is 500 Hz or less, the BPF indicator will be
on. Press the BW [F-1] button and while holding
the button, rotate the main VFO knob to the left until the width
reduces to 200 Hz. You now have a 200 Hz wide filter with the CW
shape factor.
You now have two filters
with identical widths, but with the two different shape factors.
Tune around the band, and
use the Filter button to select the different
filters.
Remember, when evaluating
filters, check the BPF indicator state. When
BPF is on, you are using the sharp CW filter
shape. When the indicator is off, you are using the broader SSB
filter shape.
So, is this a distinction
with a difference? While the difference is not large, my own
subjective opinion is that it significant. The SSB filter shape
sounds much more like traditional filters in an analog IF radio.
The CW signal edges are not as sharp and grating. The sound is a
bit softer. Even the background noise of the band
changes. I could imagine that it would be much less tiring on the
ears to listen to the SSB filter shape for extended periods. In
contest situations, however, with crowded bands, the CW filter
shape will probably be more desirable.
Your mileage may vary.
With appropriate test
equipment it would be possible to make a more accurate estimate of
the shapes of the filters. Although I don't have that equipment, I
do have the program
Digipan,
which is PSK31 software with an integrated audio spectrum
analyzer. Here is a Digipan screen capture showing three filter
choices.
Digipan Display of
Selected Filters
The Digipan screen capture
shows the audio spectrum output of three different filters. The
capture was made on a noisy 80 meter band, monitoring background
noise. My CW center frequency was 400 Hz. Since the program
scrolls from top to bottom, the first filter selected is actually
at the bottom of the captured screen. The bottom filter is a 200
Hz filter with the CW shape factor (BPF is on).
Note the sharp edges, indicated by the straight vertical lines
that separate the black (no output) zone from the yellow zone
(random background noise coming through the filter). Above the 200
Hz BPF filter is the 200 Hz non BPF
(SSB) filter. The edges are no longer as sharp. Additional energy
around the 200 Hz width is coming through the filter. The blue
lines indicate intermediate energy levels (between yellow and
black). Finally, the top and last filter is a 300 Hz BPF
filter.
Larry Benko, W0QE, a 756PRO
user who independently discovered the two CW filter shapes,
informed me of another program,
Spectrogram,
which is a free audio analysis tool. I used Spectrogram to create
spectrum plots of the 200 Hz wide BPF and non
BPF CW filters. These screen captures, made while
listening to the same noisy 80 meter band, reveal the different
filter shapes.
 |
200 HZ BPF filter (CW shape) |
 |
200 Hz non BPF filter (SSB shape) |
The nonBPF
filter shape factor does have a character all its own. In some
circumstances, you may find it useful, and at the least, ICOM
should document it.
The use of a broad-band
uniform RF noise source, such as 80 meters at night, combined with
an audio spectrum analyzer implemented in software on a computer,
is a simple but effective approach for making reasonable
measurements of receiver filter pass band characteristics. More of
these sort of plots can be found on a
web page maintained by Adam,
VA7OJ/AB4OJ. These plots compare the ICOM IC-756PRO against the
ICOM IC-765.
This modification
can also be found on
Greg Ordy
homepage.
|
KB2LJJ
