Nuts and Volts - July/August 2018

20 July/August 2018

for good speech reproduction. • Noise Reduction: The ability of DSP to make decisions “on the fly” about what signals are noise makes it possible to create “smart speakers” and noise reduction (NR) algorithms that remove undesired portions of the audio output of a receiver. Although they introduce some audible “artifacts,” the result is much cleaner and easier to understand.

• Notch Filters: A type of band-stop filter, notch filters are designed to remove a very narrow range of signals — often just a single-frequency tone caused by a spurious signal or an over-the-air carrier. Analog notch filters require manual adjustment but DSP notch filters can automatically sense the presence of the interfering signal and “notch it out” — even for several signals at once by using current technology.

• Anti-aliasing: When digitizing analog signals, it is necessary for the signals to contain frequency components no higher than one-half the sampling rate.

Higher frequency signals will appear in the digitized data as low frequency spurious signals through a process known as aliasing. An anti-aliasing filter is a low-pass filter that removes the undesired high frequency signals. Similarly, when a digital signal is reconstituted as an analog signal, the digital “stairstep” created in the output waveform contains high frequency signals that are removed by another low-pass smoothing filter.

• Preselector: Tunable analog LC band-pass filters at the antenna input that attenuate strong “out-of-band” signals that might overload a receiver, causing it to produce spurious signals.

Preselectors are most often used on the HF amateur bands which are shared with or adjacent to powerful shortwave and military stations.

• Broadcast Reject: Instead of band-pass, “BC” reject filters usually have a notch or band-stop response. They are used in the vicinity of strong fixed-frequency AM, FM, or TV stations. A similar filter can be used to attenuate commercial paging or repeater signals.

• Harmonic-rejection Filter: FCC rules require the spurious outputs from amateur transmitters to be at least 43 dB (20,000x) less powerful than the intended transmitter output. These low-pass filters are placed between the output of an RF amplifier and the antenna connection. The cutoff frequency of the filter is selected according to the operating band of the transceiver. External low-pass filters are also used to provide additional attenuation of harmonics and other spurious signals that might interfere with broadcast TV reception. (With TV stations having moved to UHF channels for DTV signals, TV interference is much less of a problem these days.)

• AC Line Filter: Low-pass filters that attenuate RF signals on the hot and neutral lines of an appliance’s AC power connection. Line filters reduce both on RF signals generated inside the appliance that shouldn’t get out to be radiated and on RF signals picked up on the power wiring that might disrupt the appliance. These line filters usually do not filter the equipment ground (a.k.a., the third-wire or green-wire ground) conductor so additional filtering — such as from a ferrite core — may be necessary. (See Ham’s Wireless Workbench columns from the July 2015 and September 2015 issues on ferrite and RFI for more information about this kind of filtering.)

Do Filters Have to Be Circuits?

Analog filters with Ls, Cs, and Rs are very common but they aren’t the only electronics that can perform filtering functions. Indeed, there are no perfect circuits for which frequency doesn’t matter, so technically, every circuit is a filter! However, there are a few notable non-LCR filters you should be aware of:

• Crystal and Mechanical Filters: Once the best available passive filters for receiving RF signals, these filters used quartz crystals and precisely machined mechanical resonators. A crystal roofing filter is still used in many receiver designs to improve strong signals from interfering in crowded conditions.

• Stubs: Resonant sections of feed line can act as band-pass and band-reject filters. Books on transmission

Filter Design Resources

Many books have “canned” filter designs for various common applications. If you are careful to use the same type of component and circuit layout, you’ll probably get acceptable results. Beginners should start with pre-designed filters. As you progress, you’ll be presented with the need to “roll your own” filter. Along with the Tonne Software programs we’ll be using in the next column, there are some tried-and-true resources for the experimenter. For serious readers, the various editions of Arthur Williams’ Filter Design Handbook are widely available as used copies.

If you’re looking for more of a “plug and chug” solution, the venerable Active Filter Cookbook by Don Lancaster in its various printings will provide design formulas and examples for filters in the audio range. Finally, for RF filters, the ARRL Handbook’s chapter on Analog and Digital Filtering provides theory and projects through the microwave range with a long list of references and additional supplemental information online.