How Circuit Impedance Affects Filter Performance
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.
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.
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.
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.