High-Speed Digital Design (Part 3)

Michael Barger, Director of Electrical Engineering at SIGMADESIGN

By Michael Barger, Director of Electrical Engineering

This is Part 3 of a multi-part series. You can revisit High-Speed Digital Design Part 1 and Part 2 on our blog.

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One of the major challenges in the design of high-speed digital electronics is that of electromagnetic compatibility. As mentioned in the beginning of Part 1 (in the section titled Electromagnetic Coupling and Interference), electromagnetic fields are generated by current flow through a conductor. Whenever current changes, some electromagnetic energy breaks loose and radiates out indefinitely into free space, where it’s called electromagnetic radiation.

Electromagnetic Radiation

Current flow fluctuates in all electronic devices; therefore, all electronic devices emit some amount of electromagnetic radiation. Since digital electronic devices utilize periodic square-wave signals, where the voltage (and therefore current) changes regularly at a set frequency, they are particularly prone to radiation. Devices where current changes more often (higher frequency) radiate more.

Electromagnetic radiation can be utilized purposefully (intentional emission) to transmit information wirelessly over long distances. We refer to such devices as radio transmitters; they are designed such that maximal radiation is emitted through a specialized transducer called an antenna. For the majority of devices that do not attempt to purposefully utilize electromagnetic radiation for communication purposes, emission is considered to be unintentional.

Electromagnetic radiation pervades the world around us, whether it’s from radios, power transmission lines, electric motors, a microwave oven, or even light! The electromagnetic energy emitted from various sources add up with each other: for example, if we have two different sources of electromagnetic radiation, each radiating energy of a different frequency, then a sample of the electromagnetic energy at a point nearby would represent the sum of both of those signals. By analyzing the electromagnetic energy in the frequency domain (see Part 2), we can more easily differentiate between and isolate the two disparate frequencies. (See Figure 1.)

Figure 1: Two sine waves of differing frequency, followed by the combined (summed) wave.

Frequency is the main parameter by which different channels of information can differentiate themselves and share the same airspace. We call frequencies ranging from 3 thousand to 300 billion Hertz (3 kHz – 300 GHz) radio frequencies (RF for short), and everything within that spectrum is carefully allocated and licensed out for various specific uses. (See Figure 2.) Light is even higher in frequency than RF: roughly 400 to 700 trillion Hz (THz). Our eyes are receivers designed to sense electromagnetic radiation in the 400 to 700 THz band!

Figure 2: The official US RF frequency allocation table

Electromagnetic Compatibility (EMC)

Why would it matter if an electronic device we design emits electromagnetic radiation? That radiation could interfere with other electronic devices (electromagnetic interference, or EMI), causing them to function improperly; in more extreme cases, it could permanently damage equipment or even inflict bodily harm.

Every developed country in the world maintains a regulating body that allocates, assigns, and monitors the electromagnetic spectrum. In the US, this is the Federal Communications Commission (FCC). By licensing out bands of the electromagnetic spectrum for specific uses, the FCC works to control EMI, ensuring that our cell phones don’t interfere with TV or radio stations, and that emergency services are able to freely communicate wirelessly with each other without interference.

EMI falls under the umbrella of electromagnetic compatibility (EMC), which is a term we use to describe a device’s ability to coexist with the invisible electromagnetic world around it: does it interfere with other devices; can other devices interfere with its own intended operation; is it safe for humans to be around? Worldwide, EMC regulations help ensure that everyone’s electronic devices work well with each other.

Intentional emitters such as radios must work with the FCC to certify their devices by proving that they operate strictly according to the rules. This is especially important when the radio is meant to be used close to a person’s body, such as a cell phone!

Unintentional emitters (electronic devices that weren’t designed to be radios) don’t need to be certified by the FCC, but under Title 47 CFR Part 15, anyone who markets such a device must have had a representative unit tested in an anechoic chamber at an approved lab (a process called “EMC verification”, as opposed to “certification”) and be able to furnish the passing test report upon demand. The testing and reports provided by these labs are an additional expense that product development companies must bear; failure to go through the process carries severe civil and criminal penalties. A sample passing electromagnetic emissions report can be seen in Figure 3 below; the spectrum of detected radiation must remain entirely below the horizontal bar, which represents the limits.

image of emission report
Figure 3: Sample passing EMC verification report

Some countries have regulating bodies with very similar standards to the United States’ FCC, such as Canada’s Industry Canada (IC). Other countries have even stricter standards, such as the European Union’s regulating body Conformité Européenne (CE), which also mandates tests for electromagnetic susceptibility (your device’s resilience against radiation from other devices) and electrostatic discharge (static shock) immunity. Unfortunately, there is no truly “global” EMC regulatory process, although the standards set out by the FCC and CE are the most widely imitated. Care is required to ensure that the necessary processes have been followed for each and every country the device is marketed in.

One of the two primary challenges in designing high-speed digital electronics is the increased difficulty in passing electromagnetic compatibility testing: it is very common for an otherwise functioning product to fail EMC testing, potentially triggering expensive redesign. High-speed electronics are inherently prone to excessive radiation unless carefully designed by a team who truly understands the nature of high-frequency (AC) electricity. There are many reasons why high-speed digital electronics behave differently and tend to radiate more than low-speed digital or analog electronics; we will discuss just one example.

Antennas

Intentional emitters like radios utilize an antenna as a transducer to radiate as much energy as possible. Most antennas are designed on the phenomenon of resonance: the length is carefully matched to create constructive interference at a specific frequency to maximize radiation.

Radio signals are different than digital signals in that their waveform is sinusoidal instead of square. Sine waves’ frequency content are tightly constrained (there are no higher harmonic frequencies to be concerned with), so antennas can be optimized for a narrow band of frequencies. We’ll need to keep in mind that digital signals, being square waves, carry higher-frequency harmonic content in addition to their fundamental clock frequency.

Monopole antennas (such as the whip antenna for an FM radio) resonate best when the length of the antenna is around is around a quarter of the wavelength. In the US, FM radio ranges from 87.5 to 108 MHz, which translates to a quarter-wavelength of 69.4 to 85.6 cm; this means that your car’s radio antenna should be roughly that long in order to receive the signal optimally. It’s also common to use monopole antennas for Bluetooth and Wi-fi, even though the antenna may look quite different. Bluetooth and Wi-fi often utilize the 2.4 GHz band, which has a quarter-wavelength of only 31.2 mm. Since the quarter-wavelength is so short, it’s common for Wi-fi and Bluetooth to utilize PCB antennas, which are nothing more than conductive traces carefully designed into the PCB to radiate maximally. (See Figure 4.)

Figure 4: FM radio whip antenna (left) and Bluetooth PCB trace antenna (right). Bluetooth PCB trace antenna photo credit: https://www.allaboutcircuits.com/uploads/articles/alex_udanis_bluetooth_speaker_-_1_(14).jpg

As the frequency of a signal increases, its wavelength decreases proportionally, greatly increasing the odds that its conductor will form an effective antenna, thus increasing the amount of radiation. Furthermore, it’s important to remember that digital signals, being similar to square waves, have strong 3rd and 5th (and higher) harmonic components. Since harmonics are multiplicative factors of the fundamental frequency, their respective quarter-wavelengths can become quite short, increasing the odds of a conductor becoming an unintentional antenna.

Below, you’ll find Table 1, which shows the relationship between the clock (fundamental) frequency of various digital interfaces and the respective quarter-wavelengths (λ/4) of their first, third, and fifth harmonics.

Table 1: Common digital interfaces and quarter-wavelengths

Interface Clock Frequency 1st Harmonic λ/4 3rd Harmonic λ/4 5th Harmonic λ/4
I2C 400 kHz 187 m 63 m 38 m
SPI 100 MHz 75 cm 25 cm 15 cm
1000BASE-T 312.5 MHz 24 cm 8 cm 5 cm
USB 3.0 SS 5 GHz 15 mm 5 mm 3 mm
10GBASE-R 10 GHz 7.5 mm 2.5 mm 1.5 mm

One can observe from this table that the quarter-wavelengths of the harmonics of higher-speed digital signals become very short. Therefore, PCB traces or other small conductive features may unintentionally serve as efficient antennas, increasing the amount of unwanted radiated energy! For example, when routing 5 GHz USB 3.0 SuperSpeed signals on a PCB, trace lengths of 15, 5, or 3 mm are all physically ideal for radiating as much energy as possible, which is the opposite of what we want unless we are intentionally designing an antenna. While there is little that we can do to avoid this reality, we should always keep in mind that the laws of physics are stacked against us when working with high-frequency electricity. We must, then, seek to manipulate everything that we can control to ensure that radiation does not exceed the limits.

Hopefully, this helps you understand why electronics where high frequencies are present are more at risk for radiation problems. Please join us next time as we dive into signal integrity concerns with high-speed digital electronics!

https://www.edn.com/design/sensors/4342455/2/Understanding-electromagnetic-fields-and-antenna-radiation-takes-almost-no-math