A DSP IF filtering test any IC-756Pro/Pro2/Pro3* owner can perform
by Rob Peebles, W8LX,Dublin, OhioMay 4, 2002
" [The 756Pro's 15 kHz roofing filters] are not “narrow filters” by classical analog-radiostandards. Although their primary purpose is image rejection, they areadequately narrow to ultimately prevent overload of the DSP A/Dconverter(s), and that is all that is required." (From George, W5YR's "Noteson roofing filters")
Try this test to convince yourself that the above is true.
- Take your IC-756PRO (or 756Pro II or 756Pro III) and set it up in the CW mode, with a bandwidth of 50 Hz (BPF indicator illuminated).
- Tune in WWV at a time when the signal is strong (currently 20over S9 @ 15 MHz). When you are zero beat perfectly (i.e. sidetone pitchmatches beat note from the WWV carrier) all you hear is the carrier (nomodulation, voice etc.).
- Now tune up 100 Hz (due to calibration errors, itmight be easier for you to just move up with the RIT instead of the main VFOafter you find the true zero beat) and listen to the BCD time code. Noticehow well you can hear the bit pattern of the time code (and how you don'thear much of anything else). Set the AGC to fast and watch the digital Smeter, paying particular attention at the top of the minute when there is no100 Hz tone transmitted. Pretty impressive for being 100 Hz away from thatstrong carrier!
- Now go up to 150 Hz above the WWV carrier....don't hear a thing. See howlow 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 tothe standard audio frequencies. Pay attention to the ramp of the tonecoming and going in conjunction with the ticks (during the period of thetick the 500 or 600 Hz tone is briefly muted).
- 1000 Hz is the fundamental frequency of the ticks. At that offset you seethe ticks very nicely but nothing else - well unless it is an even minutethen you hear the harmonic of the 500 Hz tone being transmitted!
- By tuning 10 MHz, I can barely hear WWVH in the background. WWVH's top ofthe minute tone is 1200 Hz, and I can isolate it in the passband too (it is soweak it doesn't move the S meter). I hear it much better on 15 MHz (where WWVHis stronger), and on 20 MHz (where I can't hear WWVH) it's gone.
When you tune through WWV in such a manner and see how you can pick apartand separate the various components of their transmission without overloadfrom the main carrier, you get a pretty good feel for what kind of "close-in"ultimate selectivity and noise floor can be achieved with DSPfiltering. What we have here is, in effect, the aural equivalent of a spectrum analyzer set to a narrow resolution bandwidth.
The above test procedure should enable you to try this test in your own shack.
I've just re-convinced myself Idon't want to go back to life before DSP.
*Editor's Note: This test is also applicable to the IC-7800, IC-746Pro, IC-7400 and IC-7000.
- View a video of the test, as performed by Scott N7SS.
Contents copyright © 2002 Rob Peebles W8LX. All rights reserved.
Last revised: 09/25/2019
A. Farson VA7OJ/AB4OJ edited W8LX's original text, and created this page, with his kind permission.
