Remember taking math exams in school? If you left out or misinterpreted the unit of measurement, you could lose valuable marks, often reaching a wildly different solution than what was intended. In the electromagnetic compatibility (EMC) world, there's more at stake than your grade point average. Here are some terms and units of measurement that are sure to come up on your EMC journey.
Radiated Emissions
Electric field emissions are generally measured in decibel-microvolts (dBµV). This is a voltage level in decibels (dB) referenced to 1 microvolt (µV) across 50 ohms (Ω) impedance—which is the impedance of the testing instrument after accounting for antenna factors, cable loss, amp gain, distance, measurement bandwidth and filtering.
Magnetic field emissions Magnetic fields are often calculated from the electric field results and can be expressed in microamps per meter (µA/m).
Near-field probes can measure local emissions and often quote their correction factors as decibel microamps per microvolt meter (dBµA/µVm).
Magnetic loop sensors can measure magnetic field emissions at lower frequencies. The ratio of field probe output voltage to magnetic field input is commonly expressed as decibel pico Teslas (pT) per microvolt (dBpT/µV), which is the probe's transducer factor unit.
Conducted Emissions
As with radiated emissions, conducted emissions are typically cited as dBuV referenced to the 50 Ω impedance of the testing instrument.
However, there are often specific criteria around the product layout, electrical connection, bandwidth and filtering settings of the measurement device. Note that measuring product emissions with a peak, average, or quasi-peak filter setting can yield different results, depending on the nature of the emission source.
Radiated Immunity
The field strength of the electromagnetic waves that heat the food in your microwave is measured in volts-per-meter (V/m) and dB variants of that. To use another common example, think of your 12 V car battery with the negative terminal 6 inches away from the positive. A ratio of 12V/6” gives you a static electric field of 78.74V/m, that simple.
Conducted Immunity
This test mimics the effect of interference coupling onto the product at lower frequencies. The unit of disturbance is root-mean-square voltage (VRMS). Testing sensors are in direct contact with the product or connected to a known load impedance, which in turn couples onto product cables.
Bulk Current Injection
Milliamps (mA) are a natural unit for measuring injection current by means of an injection probe. In automotive EMC testing, bulk current injection introduces a common mode disturbance current to the product cabling harness and increases the current incrementally to test conducted immunity. Bulk refers to the initial high-current charging stage of a rechargeable battery.
There is a second section to this test for frequencies above 400MHz. With the introduction of a tubular wave coupling device comes another unit of measurement: Power relative to 1 milliwatt (mW), typically presented in decibel milliwatts (dBm).
Transients
The dangerous electrical impulses known as transients are denoted by their highest voltages.
Electrostatic discharge (ESD) is expressed as a peak voltage in kilovolts (kV). This value must then be confirmed through the measurement of current in a low impedance load.
Electrical fast transients (EFTs) are also measured in kV, then verified into a 50 ohm load.
Surge is specified in kV, verified as an open-circuit voltage and short-circuit current waveforms.
Common mode impedance measures the total opposition−both resistance and reactance−of the entire bundle of cables plugged into your product, including positive and negative supply feeds, the differential communications channel, and the odd single-ended signal line. It is expressed in ohms (Ω).
Common mode impedance represents the disturbance energy the product under test and its cabling will absorb and react to. But while it might be essential for conducted immunity testing, it's not as common as the name suggests; you'll rarely hear it mentioned outside the realm of EMC.
Permeability of free space is a physical constant expressed in henries per meter (μ0 or H/m) or newtons per amp squared (N/A2). It measures the amount of magnetization of a material in response to a magnetic field.
Permittivity of free space ε0, also known as the electric constant, is expressed as farad per meter (F/m). Permittivity is a measure of the electric polarizability of a dielectric material: The more polarizable, the more energy can be stored, and thus the higher the capacitance.
Relative permeability and permittivity are important in EMC test setups, as you don’t want the bench or tabletop to unduly affect a product's electromagnetic behavior. These relative values can be calculated using the permeability and permittivity of each material, respectively.
Understanding permeability and permittivity helps control variables that ensure test repeatability and reproducibility. In EMC, one must carefully manage the magnetic and electric properties of the unit under test, measurement equipment, coupling devices, and any other materials present.
Plane linearly polarized wave. CC via Wikimedia Commons
The magnetic wave proponent is driven by the permeability of the medium it’s traveling through. The electric proponent is driven by the permittivity of the medium (though of perpendicular transverse nature) to ultimately give the ratio of the electric field to magnetic field levels. This important parameter is the wave impedance of 377Ω, the characteristic impedance of free space—in other words, the impedance an electromagnetic wave would face in a vacuum.
It has gotten intense in this one. We covered a lot of EMC terms and units of measurement in very close detail. It could be useful to bookmark this blog for future reference, especially if you're new to the world of EMC.
Thank you for reading the EMCGuy blog series—today, this year, and hopefully for many merry returns! In the new year, we'll explore how a product's clock circuitry can radiate unintentionally, leading to failed emissions at the far field antenna. Join us again in 2025... this electromagnetic wave's journey might just surprise you!
