IF DSP Performance Example
There are now a number of rigs that offer true IF DSP processing as a replacement for virtually all internal filtering, including crystal filters. In the past, these expensive crystal filters were necessary to provide sharp bandpass filtering for working in crowded band conditions.
By placing these filters at the receiver IF stage, only the desired signals are passed to the final stage of the receiver for detection. Filtering before the AGC (automatic gain control) minimizes front-end overload or 'desense' from these nearby signals. There are radios and outboard devices that offer bandpass and other filtering at the audio stage and although this can help ease listener fatigue, any desense caused by a strong adjacent signal will have already occurred.
Icom offers a DSP IF filtering test any IC-756Pro/Pro2/Pro3* owner can perform to show the value and performance of these IF DSP systems. This test has the virtue of being easily reproduced without any special equipment. I recorded the video and audio output of an IC-7000 and went through the various modes and filters. I tuned WWV, started with AM and cycled through the default filters, then SSB and finally CW. I made the narrowest CW filter only 50hz wide.
With the 50hz filter at 15.000 MHz, all you hear is the WWV carrier. As the radio is tuned up only 40hz away, the carrier is gone off the S-meter and only filter ringing is audible. At 100Hz above the carrier (15.000.100 on the display), the data subcarrier is clearly audible. You can hear the second ticks and the data bursts between. As the filter is widened, the subcarrier audio is swallowed almost entirely by the carrier and other WWV audio.
I didn't complete the rest of the test on video because not all the tones are available each minute and I didn't want a huge video file. It's clear from this short clip how effective IF DSP can be. The filter ringing is a function of width and shape factor. This filter is very narrow but has the 'soft' shape chosen to reduce ringing a bit. Even this shape factor offers incredible selectivity.
The IC-7000 uses low power DSP processors to make it more compatible with portable or mobile use. These low power processors don't offer the same performance as those found in the IC-756Pro, IC-746Pro and IC-7800 family. As a result, although the IC-7000 filters are sharper than crystal filters, they are not as sharp as those found in the larger desktop models. These larger units offer even better isolation!
Click here to play a video example of these filters .