A twist. "The Twist1." Twisted pair.
Alexander Graham Bell patented twisted-pair wires in 1881. Remarkably, we still use them because they work so well. In addition, by harnessing incredible computer power in field-programmable gate arrays (FPGAs), circuit simulators, and filter design programs, we can make a twisted-pair wire even more useful in data communication.
FPGAs place considerable power and flexibility in a design engineer’s control. Relatively low-volume projects that would never be feasible in custom application-specific integrated circuits (ASICs) become extremely practical. Many high-volume projects use FPGAs to prototype and try new functions before committing to custom silicon. The strength of an FPGA is in its complex digital processing and, as a result, some analog signal processing is limited by the digital noise. External analog gain, offset, filtering, and processing can help the FPGA better serve an application.
This article discusses how to combine a twisted-pair wire and lowpass filters to produce spectacular rejection of radio frequency interference (RFI) and electromagnetic interference (EMI). We also illustrate use of a precision resistor array to produce a customizable differential amplifier which helps pull the signal out of the interfering noise and improves FPGA performance. The precision resistors set the gain and common-mode rejection ratios while we choose the frequency response.
The Importance of a Twist
Amazingly a “twist” has become important in data communications. The seemingly simple twist of a pair, or pairs, of wires can reduce crosstalk, RFI and EMI.
With the explosive growth of the Internet and computers, we might think that twisted-wire pairs are a recent invention. We would be wrong. Figure 1 is a copy of Alexander Graham Bell’s 1881 patent. He describes the interaction between many twisted pairs.
Figure 1. US Patent 244,426, granted in 1881 to Alexander Graham Bell.2
In Mr. Bell’s words:
"The several circuits are composed each of two wires--a direct and a return wire--forming a metallic circuit. Inductive disturbance in the telephone and in other electrical instruments connected with a metallic circuit when the later is placed in the neighborhood of other electrical circuits arises from the unequal inductive effect of the later upon the two wires, for it is obvious that if the direct and return wire were affected equally the current generated in one would neutralize and destroy that created in the other. The disturbance can be avoided by placing the two wires in the same inductive relation to the disturbing currents, or, other conditions being the same, by placing them at equal distance from said circuits.3”
In these sage words more than 125 years old we have the modern principle of differential signaling.4 Figure 2 demonstrates how a magnetic field generated by current flowing in wire A causes an unwanted current to flow in wire B.
Figure 2. Crosstalk between wires: a magnetic field generated by current flowing in wire A causes an unwanted current to flow in wire B.
Capacitors are drawn between the wires to indicate stray distributed capacitance. As we increase the frequency of the interfering crosstalk, the capacitive coupling becomes more dominant. In Figure 3 we see the canceling effect described by Mr. Bell. When an interfering signal is applied equally to both sides of the twisted pair, the interfering signal is neutralized and destroyed. At radio frequencies stray capacitance will couple energy between the wires. Again, because the interference is equal and opposite in the twisted pair, the RFI tends to cancel out. Receiving the twisted-pair signal with a differential enhances the cancelation effect.
Figure 3. Crosstalk between wires is cancelled when an interfering signal is applied equally to both sides of a twisted-pair wire.
Twisted-pair wiring can also be wrapped with a shielding conductor sheath, which acts as an electrostatic shield. As radio frequency (RF) signals are further attenuated, the shield increases the stray capacitance and acts like a lowpass filter. The resistance and inductance of the wires are a series component; the stray capacitance to ground forms the lowpass filter. This is a good thing when the communications link is only using low frequencies such as telephone audio or other narrowband signals.
Using a Lowpass Filter to Reduce RFI
The speed of a temperature measurement may be limited by the physical mass of the object measured. A home heating unit, for example, may only need to measure temperatures every minute or two. Because the mass of the air, walls, floor, and ceiling is large, the temperature changes slowly. Consequently, measuring millions of times a second will not improve either the heater’s temperature measurement or temperature control.
We move outside. RFI can be generated on outside wires into a dwelling. The example is a home. My home is approximately a mile from a 50,000W AM radio station. Unfortunately, the phone wires picked up the station at 1.37MHz.The signal was rectified in the phones, reproducing the station’s audio on the phone line. This was quite annoying to hear and made it impossible to use telephone modems. The station’s studio is next to the transmitter and antenna, so the fix was simple. By definition, the engineers are experts at removing 1.37MHz from their audio and telephone systems. So we called them on the "noisy" phone and asked what lowpass filter they used.
Figure 4. A lowpass filter schematic.
Why does such a simple filter as Figure 4 work so well? The secret is in the physics: What do we want to keep and what do we want to reject in the lines? In this case our wanted telephone audio was 300Hz to 3kHz and the unwanted signal was 1.37MHz. The difference between the frequencies is a ratio of more than 450. Using FilterFree by Nuhertz5 we made a Butterworth response filter and graphed its response (Figure 5). The filter is essentially flat to 3kHz and > 135dB down at 1.37MHz. The 135dB is an attenuation of 5.6 million times. When the radio station plugged in the filter, they solved the issue and are not interfering with the phone line any more.