Figure 5. Using a lowpass filter, the telephone audio passes through the lines while the radio station's RFI is rejected.
Would a simpler filter work as well? A shareware tool, Solve Elec6 is a circuit simulator with "Low pass filter.eln," a single RC filter. Using that RC filter, and changing the values to provide an 8kHz -3dB point gives us the response of Figure 6.
Figure 6 A simple RC filter response for the RFI on the phone lines in the example here.
The telephone audio at 3kHz dropped less than 0.5dB, but the radio station's RFI dropped approximately 44dB, or a ratio of more than 150 times attenuation. In fact, we could also use the resistance and inductance of the phone line as the series element and just add a small capacitor to ground to reduce the radio station RFI even further.
Consider now a factory temperature-measurement system where the wires are hundreds of feet long and can act as a radio antenna. There is more opportunity for RFI here. If the speed of the needed temperature measurement remains consistent over a defined period of time, then adding a lowpass filter in series with the sense wires will remove RFI. How should we receive the signal on the twisted pair? Differentially, of course, to make sure that the unwanted signals really cancel each other. Figure 7 shows such a circuit.
Figure 7. A differential amplifier using the MAX5426 precision resistor network gives the designer choices with amplifier characteristics.
The circuit configuration of Figure 7 is also called an instrumentation amplifier. While many completely integrated circuits are available with the op amps in the package, the precision resistor array used gives a designer unique control over the amplifier characteristics. The precision resistors allow digital selection of differential gains of 1, 2, 4, or 8 with a choice of accuracy from 0.5% to 0.025 %. Precision resistor matching allows excellent common-mode rejection of more than 79dB. The op amp is selected by the circuit designer and allows the frequency response to be tailored to the application. Thus the front-end filtering can be augmented.
We can still listen to Chubby Checker and "The Twist" through the Internet or over the airwaves. And although Alexander Graham Bell long ago understood the principles of twisted pairs, he would marvel at what modern science can do with those twisted-pair wires, circuit design, simulation tools, and FPGAs. With just the right twist of a wire and a lowpass filter, you can reduce EMI and RFI and make data communication more robust.
1 Chubby Checker’s 1960 pop music hit, “The Twist,” has been rerecorded in many forms. According to his web site "The Twist" is the only song to be #1 twice and it is also the #1 song of all time (Billboard Magazine September, 2008).
2 For the patent information, go to Patent. For general information on the invention, see Alexander Graham Bell.
3 Alexander Graham Bell at United States Patent and Trademark Office; US Patent 244,426.
4 The term metallic circuit was used to avoid confusion with earlier telephone circuits. Telephones were originally connected with a single wire and used a ground return path to save copper wire. As electric power and trolley cars wires were placed near the phone wires, interference occurred.
5 For more information, go to Nuhertz.
6 For more information, go to Physicsbox.
Note: Nuhertz is a registered trademark of Nuhertz Technologies.
About the Author
Bill Laumeister is an engineer in strategic applications with the Precision Control Group at Maxim Integrated Products. He works with customers who use DACs, digital potentiometers, and voltage references. He has more than 30 years of experience and holds several patents.