Nuts and Volts - July/August 2018

July/August 2018 19

different in what is important.

Once a filter family is selected, the next order of business is to determine how many “sections” of filtering are needed to meet the performance requirement. The number of sections is the filter’s order — the higher the order, the more filtering is applied to the input signal.

Order describes the number of LC or RC pairs in a passive filter, such as in Figure 1A which is a fifth-order filter. Figure 5 shows the effect of cascading three filter sections of Figure 1B’s active RC low-pass filter. You can see the rolloff of the filter increasing as order is increased.

In general, as order increases, performance can more closely approach the ideal filter’s response. However, filter complexity is increased, tuning is more critical, and component variation and losses can have a more pronounced effect on performance. We’ll cover order in more detail next time.

Common Filter Applications

Let’s discuss a few applications for filters. Figure 6 is a block diagram showing where these filters are found in typical ham radio transceivers and accessories. Signal-processing filters are more and more likely to be performed by software (DSP), but there are still many analog filter circuits in radio.

• Audio Filters: Along with the CW band-pass filter mentioned above, audio filters are also used to get rid of hum ( 50 or 60 Hz signals caused by magnetic fields), buzz (caused by 120 Hz rectified AC and ac neutral currents), and high-frequency hiss. FM modulators and noise-reduction circuits also use preemphasis (high-pass) and de-emphasis (low-pass) filters n FIGURE 5. The effect of increasing a simple RC filter’s order from 1 to 3. Note the increasing steepness of the filter’s rolloff at higher orders. (Graphic courtesy of Spinningspark at Wikipedia, CC BY-SA 3.0, https://en.wikipedia.org/w/ index.php?curid=23488139.)

n FIGURE 6. A transceiver block diagram showing typical applications of filters for different functions. Filters are used at frequencies from AC power through the transmitted and received RF signals.