EMI Solutions provides a variety of custom filter solutions to meet our customers’ needs including unique layout and designs with various filtering, grounding, and transient suppression. Our standard products can also be reconfigured to accommodate your needs or we can design a custom solution to meet most any EMI filtering challenge.
EMI Solutions regularly designs and builds custom filters for many types of connectors including RF (BNC, BMA, N, SMA, etc.), Telecom/Ethernet (RJ45), and filtered pin headers.
Types of EMI Filtering Options
There are a multitude of configurations designed to mitigate EMI problems contingent on specific system requirements, though all reside within a single subset of electrical engineering. Filter products added to a system to help eliminate noise are considered “Low Pass” filters. In other words, these items allow the passage of data in the lower frequencies while blocking or eliminating higher frequency signals. Low pass filters come in a few implementations as follows:
- C filters - pure capacitive filter
- Chip Capacitor
- Discoidal/feed thru
- Planar Array
- CL/LC filters - combined inductance and capacitance filtering
- Pi filters – Capacitive and Inductive filtering, named due to the shape resembling the pi symbol, π
It should be noted that grounding is the most important aspect of any good electrical system, excellent ground conductivity is required for filters and filtered connectors to work properly. These designs assume that the enclosure/bulkhead to which they are mounted is a “Good Ground” which provides a low impedance connection path to the system ground.
“C” filters are the simplest, most straight forward solution. They are built with a single decoupling capacitor from the pin or signal line to Ground.
“CL” or “LC” filters provide additional filtering as they are built with a single capacitor to ground and are coupled with an inductor or “choke”. This enhances both the filtering effectiveness of the decoupling capacitor as well as the filtering associated with the impedance of the inductor.
“LC” filter circuits are most effective when the source impedance is less than the load impedance.
“CL” filters are best suited in circuits when the load impedance is less than the source impedance.
Pi filters, built with two decoupling capacitors separated by an inductor, effectively trap the target “noise” and provide superior higher frequency filtering performance.
Filters must be selected based on the frequency of the noise in a system in relation to the targeted frequency of the data signals being transmitted through the interface.
The capacitance of the filter must be selected so that it does not interfere with or “clip” the edges of the data signals. Additionally, the type of filter selected must meet the frequency spectrum of the noise being addressed. In other words, does the filter only need to eliminate a narrow range in the frequency band where the noise exists, or does it need to operate more like a broadband filter blocking a wide frequency range up into the GHz range?
Selecting the Right EMI Filter for your Application
Selecting the capacitance for the filter depends on calculating the -3dB Cutoff Frequency of the filter, which indicates the frequency where the response of the filter is 3dB down in amplitude from the level of the passband.
Where fc is the cutoff frequency of the filter as shown in the performance versus frequency plot in Figure 2 and R (Resistance) and C (Capacitance) of the equivalent circuit, as seen in Figure 4:
This information can be difficult to determine as systems continue to compound and grow for electrical engineers, let alone other specialists working in associated fields. Thankfully, most reputable filter manufacturers provide insertion-loss plots which clearly show the -3dB Cutoff Frequency associated with the various filters they offer. This data is typically presented in tabular form and is normalized for a standard 50 Ohm load. Consequently, the selection of filters is made easier by reviewing the published insertion loss or filter performance data at the various frequencies for the differing capacitances.
Note: Chip capacitor filters behave in a less than perfect mode, they act more like a “notch” filter rather than a high pass filter due to the self-resonance of the chip capacitors. While this behavior is negligible in most applications, these “notches” of filtering capacity are of utmost importance, and must be understood to effectively reduce high-frequency noise in an electrical system. These chip capacitor filters have limited high frequency performance versus the predicted filter performance of an “Ideal Capacitor” as shown in Figure 5.
Additionally, the mechanical packaging and circuit layout of filters using chip capacitors greatly impacts the performance of the filtering. As seen in Figure 6 below, the equivalent series inductance directly impacts the filter performance. Design teams should take great care in the layout and conductivity of the traces used when connecting these devices to the signals AND to the grounds to minimize this impact, allowing the best filtering performance possible from the chip capacitor.
These low pass filtering devices are available in many physical forms but also come with varying performance levels. Selection of these devices is dependent upon the frequency or frequencies of the noise issues as well as the magnitude of the problem. Figure 2 above also shows the relative filter performance for the various filter types as well as for the simple chip capacitor filters.
From this data, you can see that chip caps act like “notch” filters, whereas C, CL, LC and Pi filters made with planar arrays, discoidal capacitors or ceramic tubes provide better broad frequency and higher overall levels of filter performance.
EMI Filters in Application
Armed with this basic understanding of the field and the associated components, the next step is to look at some real-time applications and analyze the filter selection process from the perspective of a specialist in EMI.
Below in Figure 7, notice the measured noise in a system versus the allowable limits as shown by the solid red line:
Consider the above plot to be a customer’s noise output measured with no EMI filters - this performance resulted in a costly production stoppage. It is important to note that the frequencies between 30MHz and 70MHz exceed the allowable limit, meaning the design could not go to market regardless of the actual product’s success. The above area, even though this customer had a shielded enclosure, requires either an expensive design overhaul or additional filtering to reduce this noise to acceptable levels.
The largest outage for this customer was at 39MHz, shown in Figure 7 with a green hash mark.
Based on this observation, it is necessary to select a filtered connector that will allow the “Low Frequency” data, in this case, below 1.0MHz, to be transmitted without compromise or degradation, while filtering out the high frequency noise in the system.
Armed with this this information, coupled with reviewing the filter manufacturer’s performance data shown above in Figure 8, it can be determined that filtering in the range of 10,000pF to 30,000pF is most likely the best solution for this application.
Next, it is necessary to determine if a chip capacitor filter would be able to block the frequencies of concern, due to the low cost and high reliability of that type of filtering systems. With this in mind, the customer moves forward to test with two general system solutions, a 10,000pF capacitor filter insert and a 22,000pF capacitor filter insert.
This customer started the process by trying a 10,000pF Chip Cap press in filter. The results in Figure 9 below indicate that this device was acceptable in the upper frequencies but there was not sufficient filtering in the lower frequencies between 10MHz and 60MHz to bring the noise down below the allowable test limit.
Next the customer tried a 22,000pF chip cap filter, attempting to adjust the maximum filter performance into the lower frequency area where the noise persisted. As seen below in Figure 10, performance improved, with only small spikes at or near the noise limit in the lower frequencies, but this filter caused “ballooning” of energy into the higher frequencies around 105MHz, exceeding the allowable limit.
These test results helped the customer conclude that filtering was still required, but that the generic chip capacitor filter inserts did not provide sufficient filtering and would not fix the system without a more specialized approach. Next, the customer tested the EMI Solutions FlexFilter insert, built with maximum attention to the reduction of the equivalent series inductance. This is achieved by mounting the chip capacitor across an isolation channel separating the pad contact area from the signal pin and the solid ground plane. This design delivers maximum shielding effectiveness and minimizes the equivalent series inductance, providing the best possible performance available from the chip capacitor while capitalizing on the shielding already present within the system. The customer, with the specialized 22nF or 22,000pF filter, achieved the results shown in Figure 11 below, meeting EMC regulations with a cost effective long term solution .
After considering the results of these tests, the customer decided to go with the better broadband filter (Figure 11 above, the specialized filter) as there was not a lot of margin in the higher frequencies where the chip capacitors provide minimal filtering. This specialized 20,000pF discoidal filtered connector, a C Filter built with discoidal capacitors which provide higher levels of filter performance as well as broader frequency performance. This design improves on the other filters above and even the one in the same capacitance by utilizing the shielding already present rather than attempting to filter the system as a “magic bullet” solution. It supplemented the parts of the system that were successful, enhancing the overall performance of the entire system, instead of treating the system as a separate entity.
Custom filter connectors are our specialty at EMI Solutions so please contact us and one of our engineers will help you with your most challenging EMI design problems.