#emi analysis

Why a software subscription may be your best option

Microsoft, Intuit, Adobe, and hundreds of other large and small companies are offering subscription licenses for their software.

Subscription-based software licensing is on the rise

These are several reasons why a lease makes sense.

Lower upfront cost

Pass cost along to customers 

Free upgrades 

Built-in elasticity

Tax savings

Flexible payment options

According to Jeff Yoders, author at Redshift, “For small businesses, there are good reasons to rent software rather than buy perpetual licenses, especially for small, agile architecture and engineering firms that move quickly from one project to the next.”

For large companies, leasing offers tax benefits, growth flexibility, and infrastructure savings.

Low Upfront Cost

Renting software requires little or no upfront cash outlay. Lease rates are highest for short-term usage, month-to-month for example, but drop significantly as lease term increases.

In the case of cloud-based software as a service (SaaS), there is no software to install or maintain, and no high-end computer hardware to purchase, power, and cool. All of this makes for lower cost of ownership.

Pass cost along to customers or projects

Software rental offers the opportunity to pass software-access costs to clients by including leasing costs in project estimates. By renting software, the cost becomes an operating expense instead of a capital expense and is easier to pass along to the project.

Case in point: Engineering analysis costs more upfront but saves on testing and redesign later on. EMI Analyst™ software is available for monthly or yearly lease terms and can be easily justified by its overall cost savings.

Upgrades included

No maintenance contract required. Maintenance contracts are a standard, ongoing expense for purchased perpetual software licenses. Leased software usually includes upgrades at no additional cost.

Growth flexibility

With a purchased license, the number of users is set. If you need to add seats, you have to purchase more. Leased software, on the hand, is more flexible. As your usage grows or shrinks, adding seats or removing seats can often be done on the fly.

For example, EMI Analyst™ on Demand uses a “fee plus usage” model. The license is infinitely elastic. Multiple users can use the software simultaneously, and when no one is using it the only cost is the nominal monthly fee.

Tax savings

Computer software can be considered a capital expense or an operating expense. If you buy a software license, it is a capital expense. It may not be fully tax-deductible in the year purchased. However, if you opt to rent the software on a monthly or yearly basis, the software becomes an operating expense and can be fully deducted.

Flexible payment options

Many companies budget capital expenses. If the budget for the software you want isn’t available now, it may have to planned into next year’s budget, and you have to wait until next year to buy it. Leased software can be billed monthly or yearly and can be invoiced or put on a credit card.

Do the Math

In the end, the decision whether to purchase a perpetual license or a software subscription comes down to the cost of ownership. Before you decide whether to own or subscribe, ask yourself,

Will I be using the software long-term or just for my next project?

Do I have the funds to purchase the software?

Do I need to buy a new computer to run the software?

What is the break-even point for leasing versus buying?

EMI Filter Performance – Actual versus Published

Bode plots are great. When it comes to EMI filters, Bode plots provide an intuitive way to compare the performance of one filter to another. However, when comparing filters based on the manufacturer’s published Bode plots, you have to be careful.

The performance of the EMI filter you select may be dramatically different than the data sheet when you install it on your system. The data sheet Bode plot may be correct, but only for specific controlled conditions. The difference between published and actual filter performance lies in the way EMI filters are measured.

Bode Plot Basics

In the most general terms, a Bode plot is a graph of system frequency response. For electromagnetic interference purposes, Bode plots are used to graph EMI filter attenuation. They are a convenient way to display filter performance versus frequency, offering a high-level summary at a glance.

Bode plots can also be used to graph filter phase shift. However, for EMI control purposes, filter phase shift is not usually important. Most often when selecting a filter, we are concerned with how well a noisy signal is attenuated as it passes through the filter and whether that attenuation is sufficient to control EMI.

Filter insertion loss

Insertion loss is the figure of merit that defines EMI filter performance. Insertion loss is the amount of loss a signal experiences when the filter is inserted in the signal path. The signal in this instance is unwanted noise voltage or noise current superimposed on power, control, status, or data lines.

Insertion loss is the ratio of the input signal to the output signal. More specifically, it is the radio frequency (RF) voltage, current, or power amplitude at the filter input terminals divided by RF voltage, current, or power amplitude at the filter output terminals, respectively.

Most often, filter insertion loss is specified in decibels as,

An EMI filter Bode plot graphs insertion loss versus frequency, with insertion loss on the y-axis and frequency on the x-axis. The chart below shows a typical Bode plot. Since the frequency range of interest for most EMI filters covers several decades, the x-axis is usually a logarithmic scale. When insertion loss units are decibels, the y-axis is a linear scale; when expressed as a ratio, it is logarithmic.

In the Bode plot above, the ratio of filter input signal to filter output signal is on the left-hand y-axis, and the equivalent value in decibels is on the right-hand y-axis. Below 10 kHz the filter provides no attenuation. Above 30 kHz the attenuation increases steadily in proportion to increasing frequency. The corner frequency of the filter is the frequency at which the filter reduces the input signal to one-half its value, i.e. the filter insertion loss is 6 dB.

Measured Filter Insertion Loss

When selecting circuitry, measured data is often more desirable than calculated data. However, when it comes to EMI filters, measurement data is only helpful as a starting point because the way a filter is measured and the way it is used are often very different.

Filter insertion loss measurements are made using a 50 ohm measurement system. Why? RF test equipment, such as signal generators, spectrum analyzers, and network analyzers have standardized 50 ohm source and load impedances. When filter insertion loss is measured, the signal source connected to the filter input, and the measurement instrument connected to the filter output, are both 50 ohm devices.

Insertion loss in a 50 ohm system

Most EMI filters are measuring using methods defined in MIL-STD-220. With the EMI filter removed from the test fixture, a baseline reading is made. Then a second reading is made with the filter installed in the test fixture. The ratio of the two measurements, usually converted to decibels, is the filter insertion loss.

Measuring the filter in a 50-ohm system tells you is how well the EMI filter works when connected to circuits that have 50 ohm impedance at all frequencies, but unfortunately it does not tell you how well the filter works when connected to circuits that are not 50 ohm. In practice, circuits rarely have 50 ohm impedance.

Calculated Filter Insertion Loss

To understand how the filter performs in your circuit, you either have to build prototype hardware and make measurements, or you have to simulate the filter and circuit in software. The software approach is usually faster and more cost-effective and accurate enough for making design decisions.

EMI Analyst software makes quick work of filter simulations. It provides a convenient means for calculating filter insertion loss, not only for 50 ohm systems but circuitry having any impedance.

To illustrate the importance of simulating filter performance with the correct source and load impedances, consider the following three three source and load impedance scenarios:

Standard 50-ohm source and 50-ohm load

High-Z source and a 50-ohm load

High-Z source with 50-ohm LISNs and a 2-meter cable

Standard impedance insertion loss

Let’s take a look at a hypothetical T-filter consisting of two 1 uH inductors and one 1 uF capacitor, as shown in the figure below. This filter configuration is a common low-pass topology, commonly used in situations where the source and load impedances are relatively small compared to the impedance of the filter inductors.

The signal source an ideal voltage source in series with a 50 ohm resistor. The load, a measurement instrument, is simulated as a 50 ohm resistor. The EMI filter is a ladder network containing the three filter elements. For simplicity, the inductors and capacitor are modeled as ideal elements. In a more rigorous analysis, parasitic elements would be included to simulate filter behavior more accurately over the frequency range.

The calculated Bode plot for the T-Filter in the figure above is shown in the graph below. This is the Bode plot for the T-Filter in a 50 ohm measurement system.

Non-standard impedance insertion loss

In real applications, the source and load impedance are almost never 50 ohm. What happens if we need to filter a switching power supply, which is more like a current source than a voltage source? And has source impedance much greater than 50 ohms?

Keeping all other settings from the previous example the same, but changing the source to be a current source with 1 M impedance, the insertion loss for the same filter changes significantly, as shown in the Bode plot below. At low frequencies, the filter provides slightly more insertion loss than before, but at high frequencies, it provides substantially less.

In this configuration, the filter output is 0.30 (-10.3 dB) of the input at 10 kHz and 2.5 x 10-6 (-112 dB) of the input at 100 MHz. Contrast that to 0.54 (-5.3 dB) and 4.0 x 10-7 (-128 dB), respectively in the 50-ohm system. At 10 kHz, the filter is 5 dB better, but at 100 MHz is it 16 dB worse. The filter attenuates differently when source or load impedance changes.

Installed insertion loss

Now let’s look at a circuit that is closer to something you might encounter in practice. Use the same source and filter as in the previous example, but replace the 50 ohm load with 2-meters of wiring and a power source that consists of two Line Impedance Stabilization Networks (LISNs) and a dc power supply, a standard configuration for EMI testing of input power circuits.

The insertion loss of the filter for this real-world circuit configuration is dramatically different than its insertion loss in the 50-ohm system. As shown in the Bode plot below, the filter provides much more high-frequency insertion loss. At 10 MHz this configuration attenuates to 1.5 x 10-6 (-116 dB) compared with 2.4 x 10-4 (-72 dB) for the 50-ohm system. However, the Bode plot also shows that the filter amplifies noise between about 4 kHz and 20 kHz. Greatest amplification occurs at 15 kHz. Instead of being attenuated, RF noise emitted from the source at 15 kHz is 10 times bigger after passing through the filter.

The insertion loss curve from the 50-ohm system shows no such amplification. If the decision to use this filter were made solely on the 50 ohm datasheet curves, you might be unpleasantly surprised when the circuit has EMI problems near 15 kHz in the final installation.

Final Thoughts

Bode plots are a convenient way to display filter performance, but because insertion loss measurements are made using a standardized 50 ohm test setup, the filter attenuation may be very different in your circuit than it is on the data sheet.

A quick way to see how the filter is going to perform in your circuit is to model the filter using EMI Analyst. Use the same topology and component values, and then compare insertion loss for the standard 50-ohm setup with insertion loss when used in your circuit.

With EMI Analyst, you are not limited to modeling the just EMI filters. You can easily and efficiently model complete circuits to get a clear picture of their electromagnetic interference performance.

Coupled inductor characteristics directly affect conducted emissions and radiated emissions on power and signal lines.

EMI filters frequently employ coupled inductors to control common mode noise. But how do coupled inductors work? And what do they actually accomplish?

The most common type of coupled inductor is the common mode choke, a passive component that provides significant inductance for common mode signals while providing minimal inductance for differential mode signals. Common mode signals get blocked, while differential mode signals pass through unimpeded.

Less common are differential mode coupled inductors. For low-current applications these devices can provide the same inductance as two separate, uncoupled inductors, but in a smaller volume.

In this article we will focus primarily on common mode chokes, but the principles presented apply equally to differential mode coupled inductors.

Coupled versus Uncoupled

Coupled inductors differ from standard inductors in several ways. A standard inductor is a two-terminal device that has a single conductor wound into a coil, usually around a magnetically permeable core. A standard inductor is shown schematically on the left in the figure below. In contrast, a coupled inductor has two or more conductors wound on a single core. Schematically, common mode and differential mode inductors are as follow.

Standard Inductor

Common Mode Choke

Differential Mode Inductor

A coupled inductor is most often a four-terminal device, but common mode chokes may have six terminals for 3-phase applications, or more for multi-conductor applications.

Coupled inductors provide high inductance in a small volume. Common mode chokes obtain high inductance by using a high permeability core. Their inductance is proportional to the number of turns squared, on each winding. Differential mode inductors achieve high inductance because their inductance is proportional to the square all winding turns on the core.

Electrical characteristics of coupled inductors vary significantly over the frequencies of interest for EMI filtering. The useful frequency range of most power chokes is from a few kilohertz to a few tens of megahertz. Signal line chokes work up to somewhat higher frequencies, topping out around 100 MHz.

Ideal Model versus Non-Ideal Model

A common mode choke may be adequately modeled as a sub-circuit comprised of several passive lumped circuit elements.

Common Mode Choke Model

Not only does this model provide for the common mode inductance of the choke, but it also takes into account the effects three important parasitic elements:

a. Winding resistance

b. Interwinding capacitance

c. Leakage inductance

The paragraphs below discuss each in more detail.

Inductance

Inductance is a function of magnetic core permeability and the number of wire turns on the core. Inductance is proportional to the number of turns squared. Core permeability varies with material, temperature, dc bias, and frequency. Acurate inductor modeling requires consideration of each of these properties. However, for many applications, leakage inductance, interwinding capacitance and winding resistance are the dominant properties and are sufficient for predicting EMI filter performance.

Coupling Factor

The coupling factor of a common mode choke is a measure of how completely the magnetic flux from one winding couples to the other winding(s). For example, a coupling factor of 0.95 means 95% of the magnetic flux is coupled and 5% is not.

Magnetic flux that couples all windings on the core equates to mutual inductance between the windings. The flux that is not coupled, directly relates to leakage inductance. Leakage inductance is a parasitic element present in all physical inductors that can have a profound effect on filter performance.

To illustrate, consider a common mode choke that has a hypothetical coupling factor of 100%. Both windings are perfectly coupled. Current through either winding induces an equal current in the other winding. If the current through both windings is equal, the magnetic flux in the core is zero. All of the core flux generated by current in one lead cancels the core flux generated by current in the other lead.

However, if the coupling factor is less than 100%, say 0.95, then 5% of the magnetic flux is not coupled. Current in one winding does not induce equal current in the other winding, so the magnetic flux in the core is not zero, and some of the magnetic flux is outside the common mode choke.

The uncoupled flux has three effects:

If the magnetic flux in the core is strong enough, the core will begin to saturate. Partial saturation reduces the inductance of the device, making the choke less effective.

Flux that is not in the core is leakage flux. For a common mode choke, leakage flux introduces differential mode inductance. (Likewise, for a differential mode coupled inductor, leakage flux introduces common mode inductance.) This differential inductance interacts with other filter components, and while it does help provide additional differential mode filtering, it also introduces new resonances that have the potential to amplify circuit noise at the newly created resonant frequencies.

Stray magnetic fields may couple to nearby circuits and may be emitted from the device containing the common mode choke. For applications containing sensitive magnetic components, these stray magnetic fields may be problematic.

Interwinding Capacitance

Each turn of wire on a magnetic core has a small amount of capacitance to adjacent turns and to the wire and the core. This distributed capacitance for common mode chokes is typically between 10 and 50 pF, depending on how many turns of wire are used and construction and physical size of the inductor.

Although the inter-winding capacitance is distributed across the windings of the inductor, a lumped element capacitance from input terminal to output terminal is usually sufficiently accurate for most filter analyses.

The inter-winding capacitance, along with the inductance, sets the inductor self-resonant frequency (SRF), as shown in the red line in the graph below. At frequencies below the SRF the common mode choke behaves as an inductor, with impedance increasing in proportion to increasing frequency. Above the SRF the choke behaves as a series capacitor, and impedance decreases in proportion to increasing frequency.

A second SRF is present in real common mode chokes. It is caused by the parallel resonance of the leakage inductance and the interwinding capacitance. This secondary resonance is graphed above by the green line.

Winding Resistance

Resistance of the inductor windings has a subtle, but important effect on filter performance. Winding resistance is beneficial because it adds to the series impedance of the inductor and it provides some damping for the filter, especially at high frequencies. Since the inductor will resonant with filter capacitance and capacitive elements connected to the filter through cabling or other circuits, the damping effect of the winding resistance is beneficial. Winding resistance reduces the amplitude of signals amplified by filter component resonances.

The downside of winding resistance is thermal. Current flowing through the wires produces heat. Because the length of wire used to construct the inductor may several meters, and the power dissipation is concentrated in a relative small area, temperature rise may be significant.

Winding resistance also causes voltage drop between the input and output sides of the inductor, which may affect circuit operation if excessive.

Ac resistance of the windings is important. As frequency increases, resistance of the windings increases due to skin effect. For example, an inductor winding that has 25 mohm resistance at dc may have more than 100 mohm resistance at 1 MHz. The increase in resistance for a given increase in frequency is a function of wire gauge. Relative to dc resistance, ac resistance increases more for larger wire gauges.

Winding Method

The figure below, courtesy of Wurth Electronik shows two different ways a coupled inductor can be wound. Schematically the inductors are the same, but their high frequency characteristic differ somewhat.

Two-in-Hand

The inductor on the left is wound using a method called two-in-hand. Both wires are wound together through the core. This method provides very closely matched inductance for both wires. For a common mode choke this equates to better circuit balance and lower leakage inductance. Two-in-hand also provides lower inter-winding capacitance because the distance between successive turns is greater than for bank-wound inductors, discussed below.

Winding a coupled inductor two-in-hand, provides superior performance but at higher cost, since the inductor must be wound by hand. Due to its inherently higher cost, two-in-hand inductors are usually used in applications where price is less important than performance.

Three phase common mode chokes can also be wound this way by wrapping the three phase leads three-in-hand through the core as a triplet.

Bank Wound

The inductor on the right in the figure above is bank wound. The wires are wrapped an equal number of turns on opposite sides of the core. Bank wound inductors may be fabricated by machine, and are therefore less expensive than inductors wound two-in-hand.

Electrically, bank wound inductors have higher leakage inductance and higher inter-winding capacitance than inductors wound two-in-hand. However, for many applications bank wound inductors are adequate.

Because of their lower cost and mechanized production, bank wound coupled inductors are more readily available. Off-the-shelf common mode inductors are almost always bank wound.