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@emianalyst-blog
CE Analyst™ - Conducted Emissions Analysis software calculates interference induced on cables by active circuitry in electronic systems.
CE Analyst™ - Conducted Emissions Analysis software calculates interference induced on cables by active circuitry in electronic systems.
Calculate the inductance of 11 different conductor configurations with our free inductance calculators. Provides formulas and instant, accurate results.
Calculate the inductance of 11 different conductor configurations with our free inductance calculators. Each calculator provides formulas and instant, accurate results that you can use whenever you need to compute inductance.
Circuitry misbehaves when subjected to high frequency interference. Learn how to model diodes for high-frequency susceptibility analysis.
Circuitry misbehaves when subjected to high frequency interference. Learn how to model diodes for high-frequency susceptibility analysis.
Learn the basics to control conducted emissions. Make your electronics more robust and less expensive to develop.
A conducted emission, abbreviated “CE,” is one of four phenomena in the field of electromagnetic interference.
A robust design applies right combination of electromagnetic shielding and EMI filtering. Provide radiated susceptibility protection to electronic equipment
All electronic equipment is vulnerable to electromagnetic interference. Electromagnetic fields are everywhere. Without safeguards, circuitry may malfunction or may even be damaged if exposed to strong enough field levels.
Quickly and accurately calculate the resistance of a wire pair with wire pair resistance calculator. Instant, accurate results that you can use every day.
Quickly and accurately calculate the resistance of a wire pair with our wire pair resistance calculator
Predict conducted susceptibility. Ensure immunity to noise conducted on signal cables, power lines by calculating and providing optimum amount of protection
Is your circuitry susceptible to interference? Ensure immunity to noise conducted on signal cables and power lines by calculating and providing the optimum amount of protection.
There are so many design elements that affect cable radiation, the only meaningful way to predict radiated emissions and field levels is with good software.
Ensure your design complies with radiated emissions limits by calculating cable radiation from power and signal lines ahead of time.
Improve EMC testing results by modeling EMI filters correctly. A computer model that includes parasitic elements is essential for predicting how filters will …
Improve EMC testing results by modeling EMI filters correctly. A computer model that includes parasitic elements is essential for predicting how filters will perform in your circuit.
Learn how to extract parasitic values from from component datasheets, using a capacitor as an example.
Improve EMC Testing Results with Realistic EMI Filter Modeling
EMI filters are a rudimentary tool for controlling electromagnetic interference. EMC testing success often hinges on EMI filtering.
Selecting or designing a suitable filter seems like a straight-forward task. However, because EMC testing covers a broad range of frequencies, coming up with a good filter is not always easy. Filter components have parasitic elements that fundamentally affect performance, differently at different frequencies. The effect of parasitics can be dramatic.
Ideal versus Real The two graphs below show predicted performance for the same EMI filter. The one on the left models the filter using ideal components, the one on the right uses realistic components.
At very low frequencies, the ideal model is adequate. However, over most of the frequency range using ideal components to model the filter produces performance predictions that are very different than the real filter.
A good computer model that includes parasitic elements is essential when synthesizing a custom filter design or predicting how an off the shelf filter will perform in your circuit.
Ideal Components
In order to understand the importance of parasitics, let’s first look at ideal components. Ideal components are simplified models of real components that include only the core element, but ignore the parasitics.
Most EMI filters are constructed using passive components: capacitors, inductors, and resistors; all are linear components. Their impedance changes linearly as frequency changes. When graphed on a log-log scale, which is convenient when evaluating frequencies that span several decades, their impedance is a straight line.
Ideal Capacitor
The impedance of an ideal capacitor decreases in proportion to increasing frequency.
Ideal Inductor
The impedance of an ideal inductor increases in proportion to increasing frequency.
Ideal Resistor
The impedance of an ideal resistor is constant with increasing frequency.
Real Components
Models of real components are constructed using ideal component elements in various combinations. Parasitic elements may be in series with or parallel to the core element. The addition of the parasitic elements allows the component to be more accurately modeled over range of frequencies to better simulate real component behavior.
Real Capacitors
Real capacitors have inductance and resistance in series with the capacitance, and leakage resistance in parallel. Values for the equivalent series inductance (ESL), equivalent series resistance (ESR), and leakage resistance vary by capacitor type and value.
When capacitive reactance and inductive reactance are equal, the capacitor self-resonates. At frequencies below its self-resonant frequency, capacitance dominates and the impedance of a real capacitor decreases with increasing frequency. Above the self-resonant frequency, the ESL dominates and the impedance increases. At the self-resonant frequency, the impedance equals the ESR.
Real Inductors
Real inductors have resistance in series with the inductance, and stray capacitance in parallel. Values for these parasitics vary with wire gauge and inductor construction.
At the frequency where the inductive reactance and capacitive reactance are equal, the inductor self-resonates. Below the self-resonant frequency, inductance dominates and impedance increases with increasing frequency. Above the self-resonant frequency, capacitance dominates and impedance decreases.
Real Resistors
Real resistors have inductance in series with the resistance, and stray capacitance in parallel. Values for these parasitics vary with resistor type and size.
Except for very small value resistors, resistor ESL is usually negligible. At low frequencies, resistance dominates, but at high frequencies the impedance of the resistor rolls off due to the stray capacitance. The frequency at which the capacitive reactance equals the resistance is the corner frequency. At the corner frequency the resistor impedance is one-half its resistance value.
Filter Insertion Loss
Insertion loss is a measure of how well an EMI filter attenuates a signal as it passes through the filter. Normally expressed in decibels, filter insertion loss is the ratio of the input signal to the output signal.
EMI filters are measured by connecting a signal source across the filter input terminals and then measuring the signal amplitude across the output terminals. Normally source and load impedance is 50 ohm. Differential mode insertion loss and common mode insertion loss are measured separately. The figure below shows the measurement setup for differential mode insertion loss.
Filter With Ideal Components
Insertion loss for a low-pass filter modeled with ideal components steadily increases as frequency increases.
To illustrate, the simple L-C filter below has a 500 kHz corner frequency and two-pole roll-off of 40 dB per decade.
Using ideal components yields an unrealistic prediction of filter performance. At high frequencies, predicted attenuation may be orders of magnitude greater than for the actual filter.
Filter With Real Components
Using component models that include parasitic elements results in a more useful prediction of filter performance. Consider the following adjustments to make the components more realistic.The wire used to construct the 100 microH inductor has 25 m ohm resistance, and when wound on the inductor core there is 20 pF stray capacitance from one end of the winding to the other.
The 1 nF capacitor has 10 nH ESL, which includes internal, lead, and trace inductance. Its ESR is 5 m ohm and its leakage resistance is 10 M ohm.
Insertion loss predicted using realistic models for the components provide a more realistic prediction of how the filter will behave over the frequency range we need it to operate.
The first dip is due to the self-resonance of the inductor. The second dip is due to capacitor self-resonance. At high frequencies, the filter becomes increasingly less effective, providing little attenuation above 100 MHz.
Get Real
It takes a little longer to dig up the information needed to model a filter using realistic components, but the difference is worth the effort. Compare the realistic filter prediction to the ideal filter prediction. At 100 MHz the real filter model predicts the filter has just 20 dB attenuation, whereas the ideal filter model predicts 90 dB. That is a 70 dB error if ideal components are used.
One of the easiest ways to improve EMC testing results is to select or design appropriate filtering for end circuits and power lines. Use real component models when building filter simulation models and your designs will more closely match your actual circuits.
For more information, check out these other helpful links:
EMI Filter Insertion Loss: How Circuit Impedance Affects EMI Filter Performance EMC Analysis: How to Calculate Filter Insertion Loss
Filter insertion loss is a measure of how well an EMI filter attenuates a signal as it passes through the filter. Let's find out how to calculate it.
EMI Filter Insertion Loss - How Circuit Impedance Affects Filter Performance
Insertion loss for a circuit that has a 1 k ohm source and a 1 k ohm load.
EMI filters are used extensively in electronic equipment to control electromagnetic interference. For electromagnetic interference control, passive low-pass filters are the most common type.
When selecting or designing a filter, it is important to understand that filter performance is different when the filter installed in the circuit than when measured outside the circuit. Why? End-use circuit impedance is almost always different than measurement system impedance. Radio frequency test equipment most often has 50 ohm input and output impedance. Digital and Analog circuit impedance in electronic systems varies significantly with frequency and is rarely 50 ohm.
Attenuation
Insertion loss is a measure of how much the filter attenuates a signal at a given frequency. Numerically, the insertion loss of a filter is the ratio of the signal level at the input to the filter to the signal level at the output of the filter. The ability of the filter to remove unwanted noise from a circuit is called its insertion loss. Filter insertion loss if frequency-dependent, often changing several orders of magnitude over the filter’s useful frequency range, so it is convenient to express insertion loss in units of decibels (dB).
A low-pass filter has low insertion loss at low frequencies and high insertion loss at high frequencies. The frequency at which the signal amplitude passing through the filter attenuates to one-half its unfiltered amplitude is called the corner frequency.
Bode Plot
Bodes plots are a convenient way to illustrate filter insertion loss. They provide an intuitive graph of filter attenuation over a range of frequencies and allow direct comparison of the performance of different filters. The insertion loss of two filters is only comparable if the same source and load impedances are used.
Filter Topology
Passive low-pass filters are constructed using capacitors, inductors, and resistors. Simple filter topologies include those below. Practical EMI filters often multiple stages, damping networks, and magnetically coupled inductors.
Measured Insertion Loss
Most RF test equipment has 50 ohm output impedance and 50 ohm input impedance. When filter insertion loss is measured, the results are said to be for a 50 ohm system.
The graph below shows insertion loss for five filters topologies in a 50 ohm system. For each filter, all capacitor values are 100 nF, and inductor values are 1 micro H.
As expected, above their respective corner frequencies, the C-section filter provides one-pole roll off (20 dB per decade), the LC-section and CL-section filters provide two-pole roll off (40 dB per decade), and the Pi-section and T-section filters provide three-pole roll off (60 dB per decade).
In-Circuit Insertion Loss
Circuit source and load impedances are rarely 50 ohm, and usually vary significantly over frequency. As a result, when a filter is installed in the circuit its insertion loss can be very different than when measured in a 50 ohm system.
As illustrated in the following graphs, the in-circuit effectiveness of a filter is a function of both circuit impedance and filter topology. Insertion loss is greatest when the impedance of the filter component nearest the input terminals is a mismatch for the source impedance, and the impedance of the filter component nearest the output terminals is a mismatch for the load impedance. When filter component impedance is close to circuit impedance, the filter component has little effect.
The same five filters analyzed for the 50 ohm system graph above are again analyzed below for three different combinations of source and load impedance. For all analyses, which were performed using EMI Analyst software, capacitor values are again 100 nF, and inductor values are 1 micro H.
Low Impedance Source, High Impedance Load
Insertion loss for a circuit that has a 10 ohm source and 10 k ohm load:
Low Impedance Source, Low Impedance Load
Insertion loss for a circuit that has a 10 ohm load and a 10 ohm source:
High Impedance Source, High Impedance Load
Insertion loss for a circuit that has a 1 k ohm source and a 1 k ohm load:
What Have We Learned?
Because the impedance of circuits on both sides of the filter affects the ability of the filter to remove unwanted noise, it is naïve to select a filter from a catalog and expect that it to provide the same level of filtering as its datasheet specifies. It could be more, but it could be less.
To do the job right, you need to model the filter using the topology and component values used in the filter. Do the best you can to estimate parasitic element values. Then simulate the filter in-circuit.
Since EMI filters are frequently located at the point where power and signal cables connect to the equipment, the cable conductors and circuitry at the other end of the cable must be simulated if you want meaningful simulation results.
That is what EMI Analyst software does. Next time you need to design an EMI filter, run it through EMI Analyst to see how it performs.
For more information, check out EMI Analyst at https://www.emisoftware.com.
Effect of Cable Twisting on Radiated Emissions
It is common knowledge in electrical and electronic cable design that twisting wire pairs together is usually a good thing. However, the benefits of twisting and the degree to which twisting is valuable is less well understood.
This article illustrates the effects that twisting can have on cable radiation.
Get More Background
In an earlier article we discussed in conceptual terms, how cable twisting improves electromagnetic compatibility (EMC) and reduces electromagnetic interference (EMI). In this article, using EMI Analyst™ software, we explore tangible examples that quantify the EMC/EMC benefits of wire twisting. The previous article, How Cable Twisting Improves EMI can be found in the download area of the EMI Software website.
Which Wires to Twist Together?
Twisting can reduce cable radiation when two adjacent wires are carrying signal and return current in opposite directions. The benefit is greatest when the opposing current has equal magnitude and is 180-degrees out of phase. Fields emitted by adjacent half-twists tend to cancel. Even if the wires carry unequal current or if the current is not completely out of phase, partial field cancellation is still beneficial.
Likewise, when a wire pair having a voltage differential is twisted together the electric field lines from adjacent half-twists are in opposite directions and tend to cancel one another. Over the cable length, net electric field emitted from the wires is lower.
We Are Talking Differential Signals Here
Twisting reduces radiation caused by differential mode signals. Twisting does nothing for common mode signals.
Differential mode signals are time-varying voltage and current waveforms on the wires. Signals in this context may carry information or may be an unintended byproduct of circuit operation. Differential mode current flows one direction on one wire and the opposite direction on the other wire. Differential mode voltage is the potential difference between the wires.
Common mode signals, also time-varying, exist between the wire pair and another conductor, usually ground, chassis, or some other reference. Common mode current flows in the same direction on both wires and returns on the third conductor. Common mode voltage is the potential between both wires and the third conductor.
Let’s See an Example
The five graphs below show how radiated field levels decrease as the number of turns increase. In general, more twists beget lower radiation.
The circuit for this example is a digital driver that produces a 3-volt, 1 MHz trapezoidal waveform. The output of the driver is connected to a digital receiver through a 1-meter long unshielded wire pair cable in free space. The wires are #22 AWG with 1 mm spacing between them. There is no common mode signal component. Twisting is varied from 0 to 100 uniformly spaced twists per meter. The limit line shown on the graphs is for reference only.
0 Twists Per Meter
5 Twists Per Meter
10 Twists Per Meter
40 Twists Per Meter
100 Twists Per Meter
The Effect of More Twists
Clearly, more twists equate to lower radiation. However, there is diminishing return. For this example, the first five twists provide about 25 dB improvement. It takes another 95 twists, 100 twists total, to get another 25 dB. For any wire pair, there is a physical limitation to the maximum number of twists per meter.
About the Calculations
Radiated emissions calculated in the examples above were done with the RE Analyst™ application module of the EMI Analyst™ software suite. For more information, visit https://www.emisoftware.com/.
MIL-STD-461 specifies maximum frequency step size for susceptibility tests. The intent is to subject the equipment under test to enough test frequencies that any design vulnerabilities are exposed. Better to find out...
CS114 is a conducted susceptibility test method imposed on nearly all military and space electronics. EMI Analyst manages its conditional injection levels effortlessly.
Signal integrity becomes more important in electronic design as circuit speeds increase. Have you ever noticed? Let's discuss here.