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EMI and RFI are suppressed by twisted pair and low pass filter

Release time:

2013-11-08 00:00

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introduction

"The Twist" refers to the twisted pair, which was patented by Alexander Graham Bell in 1881. And the technology is still used today because of the convenience it provides. In addition, with the increasingly powerful processing capabilities of Field Programmable Gate Array (FPGA) devices, combined with circuit simulation and filter design software, the application of twisted pair cables in the field of data communication is becoming more and more common.

FPGA provides design engineers with powerful and flexible control capabilities, especially those small-volume design projects that cannot obtain application-specific integrated circuits (ASICs), can use FPGA to realize the design; many mass-produced products also use FPGA in the early stage of project design Prototype and test new features before customizing silicon. The power of FPGA lies in the complex digital processing function, while some analog signals will be limited by the interference of digital noise. It is necessary to provide analog amplification externally, as well as offset, filter and signal processing to ensure that the FPGA meets the overall requirements of the system.

This article discusses how to combine twisted pair cables with low-pass filters to suppress radio frequency interference (RFI) and electromagnetic interference (EMI). We also showed how to design a custom differential amplifier using high-precision resistor banks to eliminate signal interference and improve the performance of FPGA systems. When we choose the frequency response characteristics, we use high-precision resistors to set the gain and common-mode rejection ratio.

The importance of twisted pair

Twisted pair cables are of great significance to data communication, and can greatly reduce crosstalk, RFI and EMI.

The popularity of the Internet and computers has led to the popularization of twisted pair applications. Many people mistakenly think that twisted pair is a new invention, which is not the case. Figure 1 shows a copy of a patent filed by Alexander Graham Bell as early as 1881, which describes the interaction between multiple pairs of twisted wires.


     Figure 1. Alexander Graham Bell received a US patent in 1881

Mr. Bell pointed out that multiple circuits are connected by two wires—a through wire and a return wire—to form a conductive loop of metal wire. When the metal wire conductive loop is placed near other circuits, if the peripheral circuit senses different signals on the two lines, the telephone and other electrical equipment connected to the metal wire will induce interference signals; obviously, if the direct line and the return line have the same effect on the wires, the current produced by one of the wires will cancel the current produced by the other wire. Interference can be avoided if the two wires have the same inductive relationship to the interfering current, or if the two wires are placed at the same distance as in the above circuit (making sure other conditions are identical).

These 125-year-old truths laid the foundation for modern differential signaling principles. As shown in Figure 2, the magnetic field generated by the current in wire A will generate an undesired current in wire B.


Figure 2. Crosstalk between wires: The magnetic field generated by the current in wire A produces an undesired current in wire B

The capacitance between the wires in the figure represents the stray distributed capacitance. When the frequency of the crosstalk signal is increased, the capacitive coupling will be more obvious. In Figure 3, we observe the "offsetting" effect proposed by Mr. Bell. When equal interfering signals are applied on both sides of the twisted pair, the interfering signals will be canceled out. In an RF environment, stray capacitance can couple energy between wires. In the same way, since the interference of the twisted pair is equal and opposite, RFI tends to cancel. Receiving the twisted-pair signal differentially will enhance the "cancellation" effect.


Figure 3. Crosstalk between wires is canceled out when equal interfering signals are applied to both sides of the twisted pair.

The twisted pair can also be wrapped with a shielded conductor to play the role of electrostatic shielding. Shielding increases stray capacitance and acts as a low-pass filter, further attenuating RF interference. The resistance and inductance of the wire are series elements, and the scattered capacitance forms a low-pass filter to the ground. This feature helps improve transmission when the communication link transmits only low frequency signals, such as telephone audio or other narrowband signals.

Reducing RFI with a Low-Pass Filter

For example, the speed of temperature measurement may be limited by the physical mass of the object being measured. A home heater may only need to measure the temperature every minute or two. Due to the relatively large mass of air, walls, floor and ceiling, the temperature changes very slowly. So, measuring temperature millions of times per second is meaningless for heater temperature measurement or temperature control.

We turn outside, and RFI generated outside can enter the room. As an example, an office is approximately 1 mile from a 50,000W AM radio station. Unfortunately, the phone line picked up the station's 1.37MHz signal. The signal is detected in the phone to recover the radio's audio signal. It is unbearable to hear this interfering signal every time, which seriously affects the modem of the phone. The radio broadcasting room is adjacent to the transmitter and antenna, and the system maintenance is relatively convenient. It stands to reason that engineers are better at removing 1.37MHz signals from the audio and phone systems, so we made a repair request over a "noisy" phone call and asked what low pass filter they were using.


Figure 4. Low-pass filter

Using the very simple filter in Figure 4 can achieve good results, why? The reason is physics: what do we want to keep on the line, what to suppress? In this example, our normal telephone signal is 300Hz to 3kHz, and the signal to be suppressed is 1.37MHz, a frequency difference of 450 times. Using Nuhertz's FilterFree software, we made a Butterworth response filter and plotted its response characteristics (Figure 5). The filter is essentially flat below 3kHz and attenuates over 135dB at 1.37MHz. 135dB is equivalent to an attenuation of 5.6 million times. After the radio station uses a filter, this problem is effectively solved, and the telephone line is no longer disturbed.


Figure 5. Telephone audio passes the line with a low-pass filter, while radio RFI is suppressed.

Can a simple filter circuit solve the problem? The software tool Solve Elec is a circuit simulator with design files for a low-pass filter, which is a simple RC filter. Using this RC filter, change the parameter value to obtain 3dB attenuation at 8kHz, and the frequency response characteristics are shown in Figure 6.


Figure 6. Shown is the response of a simple RC filter to RFI in a telephone line.

How to receive signal through twisted pair

For audio signals, the attenuation is less than 0.5dB at 3kHz, while RFI interference to radio stations is attenuated by 44dB, or 150 times. In fact, we also used the resistance and inductance series elements of the telephone line, but added a small ground capacitance to further attenuate the radio's RFI.

Now, let's rethink the temperature measurement system in the factory, where the wires are hundreds of feet long and act as a radio antenna, so the chances of being affected by RFI are very high. If the temperature measurement data remains consistent within a specified time period, a low-pass filter can be connected in series with the detection line to eliminate RFI. So, how to receive signal through twisted pair? Of course, use differential signals to ensure that interfering signals cancel each other out. Figure 7 shows such a circuit.


Figure 7. Using the MAX5426 high-precision resistor network to form a differential amplifier, the amplifier parameters can be set flexibly

The circuit configuration shown in Figure 7 is also known as an instrumentation amplifier. There are several fully integrated solutions available on the market. The MAX5426 high-precision resistor network provides designers with the convenience of controlling the amplifier parameters. High precision resistors allow digital selection of differential gain: 1, 2, 4, or 8, with selectable 0.5% to 0.025% accuracy. Precise matching of the resistors ensures a common-mode rejection specification of over 79dB. Circuit designers can easily select operational amplifiers, tailor frequency response characteristics to specific applications, and improve front-end filtering.

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