Notes on Filtering & DSP

Contributed by George T. Baker, W5YR

There is a widely-held misconception that "a filter is a filter" and"you can't have too many of them." Nothing could be further from the truth. . .

Every conventional crystal lattice or ladder filter in the signal pathcontributes attenuation, varying response within the passband, relativelypoor adjacent-frequency rejection (shape factor), comparatively poorultimate rejection, substantial phase distortion at and near the passband"edge" frequencies, and non-trivial frequency-dependent time delay. Aproperly designed and implemented DSP filter has none of thesedeficiencies.

Some of these shortcomings impair noise-blanker operation. Effective noiseblanking depends uponsensing the fast rise-time of the noise pulse before it propagates intothe main signal channel, and switching off that channel until the pulse haspassed. Every filter in the path deteriorates the pulse shape and degradesthe rise time in addition to adding delay. The result is anoise blanker that doesn't blank well, due to noise pulse distortionattributed to the narrow filter.

Since the advent of DSP both for noise reduction and brick-wall filtergeneration, there has been another ill-conceived notion that conventionalfilters can be as "good" as DSP filters, provided only that enough money isspent. Wrong again, of course. No crystal filter can be constructed from afinite number of crystals that will equal the performance of a properlydesigned and applied DSP filter. The single drawback to DSP filters todayis the inevitable time delay (group delay) which they introduce due to processing time.As processors get faster and cheaper, and that last IF moves to a few hundred kHz, even that minor disadvantage will fade away.

The need for narrow roofing filters vanishes if the system designer canensure that from the antenna terminals to the output of the roofing filter- even at the usual 15 kHz bandwidth - and to the input of the A/Dconverter the operation of all circuits is linear, and that gains arecontrolled and distributed such that no signal or linear combination ofsignals can achieve an amplitude sufficient to overload the A/D converter.1This is a challenging task but neither impossible nor unfeasible. The Ten-TecRX-340, and the Icom IC-756Pro/Pro II, are real-world examples of "almost" meeting these designrequirements. (Rockwell Collins gets even closer.)

Having met those non-trivial conditions, a narrow roofing filter is justanother source of signal distortion.But, if you don't succeed in meeting those conditions, then narrow roofingfilters are the easy way out of a poor design. If you can't prevent signalsfrom distorting and cross-modulating due to non-linearities, then justaccept only that narrow slice of the spectrum that contains the signal youwant and try to reject the rest. Of course, the damage is already done inthat the desired signal is now accompanied by distortion products, etc.,but only those which lie within the narrow filter passband have a chance toproceed further and cause further deterioration, which they will.

As Adam VA7OJ/AB4OJ has said so many times, "provided that you do not overdrive the A/Dconverter," and as I have said so many times, "provided that you operatethe front end in as linear a fashion as possible," then there is nothingthat a narrow roofing filter can do that subsequent DSP filtering cannot domuch better at significantly less cost. Take that back - the roofing filtercan distort the signal in several ways which the DSP filter can not.

Bottom line: nothing today - and this was true back in the 50's when Istudied Network Analysis and Synthesis under Ernie Guillemin at MIT gradschool - in the conventional crystal filter universe can equal thecapabilities of DSP filters. To use narrow crystal filters at all is adirect admission of an inferior front end design. Conversely, as the RX-340 or Pro/Pro IIdemonstrates every time you turn it on, narrow roofing filters are unneededif the front end has been done "right."

We hear that Icom is taking the basic IC-781 design, with itsquality/performance level still aimed primarily at the large and moneyedcommercial and government/military markets, and updating it with what theyhave developed in the PRO series and taking advantage of the Rohde &Schwarz technology that they have licensed. With that combination theywill set a new benchmark in the amateur market that will be hard to equalmuch less exceed. The radio is expected to be expensive - probably inexcess of $5K or even $6K - but should be pack-leader by a country mile in the amateurHF marketplace.

IC-7800 Update: As discussed in theIcom publication "IC-7800Technical News", the IC-7800 utilizes switched 15 kHz and 6 kHz 1st-IFroofing filters. While a 15kHz, or more, roofing filter is used in almost allcurrent amateur transceivers, it is not the “optimal design” for SSB, CW,or AM. The IC-7800 utilizes two 1st IF roofing filters, one for FM operationand the second with a 6kHz bandwidth for SSB, CW, AM, and theData modes. The effect of the 6 kHz filter is to reduce the broadband noiseinput to the ADC by 4 dB, as compared to the 15 kHz filter. This will improveclose-in dynamic range in these modes.

1 "A High-Performance Digital Transceiver Design, Part 1" by James Scarlett KD7O, QEX,July/August 2002

2 "IC-756Pro II Receiver IMD and DSP Filter Performance", by J. Saito JA7SSB, CQ Ham Radio, January 2002. Summary by N. Oba JA7UDE (PDF)

Copyright © 2002, George T. Baker, W5YR
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