Passive RC Filters Afford Low Cost And High Selectivity

At audio frequencies, LC filters were undesirable because of expensive and bulky inductors. Prior to the advent of solid-state active devices, passive RC filters were an audio design option. During the last 30 years, analog active RC filters and digital filters have taken over most low-frequency filter applications. Occasionally, very simple passive RC filters, with four or fewer circuit elements, are used for instructional purposes in laboratory experiments. But they provide limited selectivity. Recently, however, passive RC filters have been revisited to achieve low cost and enhanced selectivity.

RC filters are frequency-sensitive attenuators that supply large pass-band insertion losses (almost 10 dB, including reflection loss), compared to LC filters. The low-pass filter uses cascaded pi sections, while the high-pass filter employs cascaded tee sections.

The filter design procedure is very simple: All resistors have a value that equals the circuit impedance. All capacitors provide a normalized (to source and loaded impedance) reactance of unity at a design reference frequency. Both filters have a nominal reference frequency of 1.2 kHz. The two filters were constructed on vector boards and installed in die-cast aluminum enclosures with input/output BNC panel jacks. Filter testing required a 600-O signal generator and a high-impedance meter preceded by a 600-O feedthrough termination.

Figure 1 shows an RC low-pass filter schematic. All capacitors are 50-V polypropylene units of 0.22 µF with ±2% tolerance. The resistors are 1/4-W metal film units of 590 O with ±1% tolerance. See Table 1 for measured test data.

Figure 2 illustrates the RC high-pass filter schematic. Capacitors are 50-V polypropylene units of 0.22 µF with ±2% tolerance, and resistors are 1/2-W composition units of 620 O with ±5% tolerance. Measured test data is shown in Table 2.

Passive RC filters are easy to design at audio frequencies, and they don't require external dc excitation. Evaluation of passive RC low- and high-pass filters reveals response shapes that seem to be monotonic. A relative stop-band rejection in excess of 40 dB was obtained for the equal element filters. Cost savings are possible by using a minimum number of different value components.

Passive RC filters are a design option for high-voltage and high-power applications.