[Icom] FW: A DSP IF filtering test any IC-756Pro/Pro2 owner can perform

[Adam Farson]

I am posting this test procedure to the reflector on behalf of, and by kind
permission of, Rob Peebles, W8LX. The test procedure can also be found on my
Icom page:

http://www.qsl.net/ab4oj/icom/w8lx.html

Best 73,
Adam, VA7OJ/AB4OJ
North Vancouver, BC, Canada
http://www.qsl.net/ab4oj/
Note new e-mail address:
mailto:farson@shaw.ca


Good evening gentlemen.

"Perhaps these are not what you would call "narrow filters" but they are
adequately narrow to ultimately prevent overload of the DSP A/D
converter(s) and that is all that is required." (Excerpt from George, W5YR's
posting of 5/1/2002)

Try this test to convince yourself the above is true.

Take your PRO and set up in the CW mode with a bandwidth of 50 Hz (with BPF
illuminated).  Tune in WWV at a time when the signal is strong (currently 20
over S9 @ 15 MHz).  When you are zero beat perfectly (i.e. sidetone pitch
matches beat note from the WWV carrier) all you hear is the carrier (no
modulation, voice etc.).  Now tune up 100 Hz (due to calibration errors it
might be easier for you to just move up with the RIT instead of the main VFO
after you find the true zero beat) and listen to the BCD time code. Notice
how well you can hear the bit pattern of the time code (and how you don't
hear much of anything else).  Set the AGC to fast and watch the digital S
meter paying particular attention at the top of the minute when there is no
100 Hz tone transmitted.  Pretty impressive for being 100 Hz away from that
strong carrier!

Now go up to 150 Hz above the WWV carrier....don't hear a thing.  See how
low the noise floor is?

At 200 Hz above the WWV carrier  we see the harmonic of the 100 Hz BCD.

At 250 Hz above it is spooky how much of the signal you don't hear!

Now, go up to 500 Hz (even minutes) or 600 Hz (odd minutes) and listen to
the standard audio frequencies.  Pay attention to the ramp of the tone
coming and going in conjunction with the ticks (during the period of the
tick the 500 or 600 Hz tone is briefly muted).

1000 Hz is the fundamental frequency of the ticks.  At that offset you see
the ticks very nicely but nothing else - well unless it is an even minute
then you hear the harmonic of the 500 Hz tone being transmitted!

When you tune through WWV in such a manner and see how you can pick apart
and separate the various components of their transmission without overload
from the main carrier you get a pretty good feel for what kind of "close-in"
ultimate selectivity and noise floor that can be achieved with DSP
filtering.

Hopefully you can make enough sense out of my ramblings to try this at home
(I'm tired and just about to go to bed).  I've just re-convinced myself I
don't want to go back to life before DSP.

73,

Rob Peebles, W8LX
Dublin, Ohio

Postscript:
Tuning 10 MHz now I can barely hear WWVH in the background.  WWVH's top of the
minute tone
is 1200 Hz, and yes I can isolate it in the passband too (it is so weak it
doesn't move the s meter).  I hear it much better on 15 MHz (where WWVH is
stronger), and on 20 MHz (where I can't hear WWVH) it's gone.