EMI suppresion - Page 1
Forum section: electronics / projects

author: brownt
author_rank: Regular Contributor
timestamp: December 31, 2017, 11:43:04 pm
content: Hi,I am using some Murata ferrite beads for EMI suppression above 30MHz.These Murata ferrite beads in any particular model are available in various impedances @100MHz. For example 10 - 1000 ohms. I don't understand why anything other than the maximum impedance type would be chosen. Why are there so many impedance types?There are so many models available that I am really not sure what to get. The attached picture shows six graphs for six devices. The DC resistance and current capability is about the same for all of them, and will suffice for my application.There are three different physical sizes, 15, 18 & 21, and any of those size are OK for me. I need one bead on the VCC line of the 64MHz MCU, and perhaps also on the 4MHz SPI line, for the clock, data in and also data out, I guess.It seems to me that if I want to limit radiated emissions at 30MHz+ then the best choice is BLM21RK102N1, because its resistive area starts lower than any of the others, at about 7MHz (where X crosses R). But if it is that simple then I don't know why there are so many choices and why anyone would ever choose anything else.It seems that a device with a resistive curve starting above 30MHz is no good and so none of the devices on the right are satisfactory. But again with everything commercial needing EMI certification for radiating emissions above 30MHz, I don't know why there are so many options above 30MHz.appreciate any information. thanks.

author: snarkysparky
author_rank: Frequent Contributor
timestamp: January 01, 2018, 01:32:58 am
content: Excellent question. I can only contribute my .02$.The main devil in the details is how the various impedance choices affect your circuit operation. For VCC line the more is better probably works but the SPI line will get complicated. Placing one of these beads there introduces an inductance into a rapidly changing signal. The effects on the signal propagation will have to be studied closely. Since SPI is usually confined to the same board I would question whether beads are needed there.The reason for the multitude of options is that a bead or inductor has several independent attributes. The ones I can think of now.DC resistance.self resonant frequency.current saturation level. ( where the core saturates with flux and no further inductance is present)quality factor. ( determines how strong the peak is in transfer characteristic. sometimes lossy is good )I apologize in advance if you already know an consider these things, just running through the basics.good luck

author: cdev
author_rank: Super Contributor
timestamp: January 01, 2018, 01:43:36 am
content: What kind of RF is present on the line you want to cure and is it a DC power line, or is there some signal you need to preserve?Could you give us more information? How do you know there is RFI there?There is a good PDF that I have, its file name is "RFI-Ham-RS" from Jim Brown K9YC:http://www.audiosystemsgroup.com/RFI-Ham.pdfThat may be enough to answer your questions.. See also his other pubshttp://www.audiosystemsgroup.com/publish.htmQuote from: brownt on December 31, 2017, 11:43:04 pm

author: brownt
author_rank: Regular Contributor
timestamp: January 01, 2018, 02:32:02 am
content: I don't know if RF is present on the line. But I need to make sure there is nothing there before EMI compliance testing. It is a costly process and I would like to have my bases covered. Standards require limiting radiating emissions above 30MHz. I assume my possible emissions will come on the DC VCC line from the MCU with a clock of 64MHz and its harmonics. There is also another IC running at 16MHz, so I guess there may be some emissions from there too.SPI is running at 4MHz. I don't know if that will generate harmonics above 30MHz.

author: snarkysparky
author_rank: Frequent Contributor
timestamp: January 01, 2018, 02:49:39 am
content: brownt,You have set yourself a very difficult task. To be sure the goal is to have no failures at test time but frequently the best designers fail. It's called a dark art for a reason.""SPI is running at 4MHz. I don't know if that will generate harmonics above 30MHz.""no offense intended but this is something you should know if you are designing for EMC compliance.I suspect you need a bit more experience to attack this challenge. Not trying to be difficult but it is something you should consider.We will help in the forum but there are limits.

author: brownt
author_rank: Regular Contributor
timestamp: January 01, 2018, 03:00:30 am
content: I see.Assuming I was going to make a PI filter using one of these beads, how does one know what cap values to use, when the ferrite beads don't have inductance values?

author: cdev
author_rank: Super Contributor
timestamp: January 01, 2018, 03:00:40 am
content: An RTLSDR is cheap and I think is much better than a scope for getting a quick idea of how much RFI is present from an unintentional radiator. Cant beat their price either. You can modify one to receive HF also if your soldering skills are good. You can use a short length of coax with a tiny loop on the end - possibly with an attenuator (also put some ferrite beads around the coax to block common mode noise) Use that as an RFI sniffer.This is not at the compliance stage its at the early stage to help you save money and time.

author: brownt
author_rank: Regular Contributor
timestamp: January 01, 2018, 03:13:09 am
content: Difficult, yes, but I'll get there :-)I think I have most things in hand, but I want to be sure about a few things. The harmonics are multiples of the fundamental, so 4MHz, 8, 16, 32 etc. I am thinking that the power level above 30MHz is going to be insignificant. In fact, I don't think I need any suppression on this 4MHz line. But as I said, I want all bases covered, and will play with the configurations during testing.I require more experience, definitely, but the situation is as it is.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 01, 2018, 03:14:10 am
content: Two reasons:1. Sometimes you only need a little impedance. And sometimes you need a lot.2. Saturation current.Very few manufacturers talk about ferrite bead saturation. That should raise your hair a bit. Laird's catalog is the only complete listing including DC bias I know of. Some manufacturers provide that in the datasheet, or hide it away on a web application, but even then, they are spotty at best.In short: use current compensating (common mode) chokes on DC power lines, so that DC bias in the core is balanced. (Take careful note of galvanic grounding, if you have multiple grounding points AND +/- DC power supplied in the cables. RF grounding is preferable, i.e., using capacitors to RF-ground a given board, while allowing DC power to flow in the cable.) Use ferrite beads on low power and signal lines (typically under 200mA).I can guarantee you SPI lines will radiate, and running SPI at 4MHz over an open line will fail radiated emissions, period.* You must use, at the very least, the shortest length possible, with as many grounds as possible. You may get away with a short ribbon cable, where every other line is ground. Otherwise, you'll need shielded cable. Which, at that point, it's a good opportunity to add proper line transmitters and receivers, anyway; and then, you might simply go with RS-422 signaling instead, for which you can get away with unshielded twisted pair (though shielded is still better, especially if you need to deal with more than 3 V/m immunity).*There are always exceptions. A heroic RF filter could cut all frequencies just over 8MHz, achieving better than 40dB attenuation by 30MHz. This would be a high order LC filter, with dampening on the line-facing side. Both tx and rx sides are filtered this way, and a very reliable receiver is needed (such as a 74HC7014 precision schmitt trigger inverter, or a proper comparator with hysteresis -- which includes using an LVDS receiver as a comparator, a nice alternative use!). You may still need to perform delay matching, error detection, clock recovery or such to absolutely** ensure reliable communications. In which case, SPI may not be so great, but async serial over RS-422/485 or any other self-clocking signal might prove more fruitful.**There are always exceptions; "absolute" in this case being a good enough bit error rate (BER), you'll never care. You can always dig deeper on these things***, there's always some chance left, or some detail left to investigate. It's when you see the BER down below the 10^-9's, or the details in the ppms or less, when you know you've investigated far enough.***Just kidding.But, er, yeah, that would be just that: "heroic". This just to show an example of, "well you could, BUT". Clearly, there are better ways, and if your biggest hurdle right now is getting a slam-dunk through EMI testing, shielded or balanced data is the way to go.Good luck,Tim

author: brownt
author_rank: Regular Contributor
timestamp: January 01, 2018, 03:32:49 am
content: I see.SPI does not extend outside of the enclosure. There is an 80mm SPI clock line on the PCB.Current draw is 75mA, so beads seem fine for the DC lines.

author: CopperCone
author_rank: Super Contributor
timestamp: January 01, 2018, 04:50:51 am
content: What you could do ,that gets rid of guess work (ferrite beads give me anxiety) is use a RLC filter instead, choosing an inductor that has a high self resonant frequency.Useful to know if the show must go on.

author: David Hess
author_rank: Super Contributor
timestamp: January 01, 2018, 05:31:45 am
content: The DC resistance and saturation current vary between beads of the same type that have a different impedance.

author: brownt
author_rank: Regular Contributor
timestamp: January 01, 2018, 07:26:27 am
content: I see. I think I am beginning to see why so many types would suit my needs. Because my current draw is only 75mA, and because even 1ohm DC resistance is OK for my 5V non-critical design, and because I know the beginning cut off freq is no more than 30MHz, there are many that would suit. That just leaves me with the higher frequency cut off. Some types extend into GHZ and others just a few hundred MHz.Assuming a 64MHz MCU clock, how high could the harmonics go and still be significant?

author: Tomorokoshi
author_rank: Super Contributor
timestamp: January 01, 2018, 08:02:36 am
content: Quote from: brownt on January 01, 2018, 07:26:27 amI've seen failures in the 5x to 10x range relatively easily. For instance, perhaps the track want to resonate at 5x the fundamental for 320MHz.How much current can the clock output drive? How much current input do all the peripheral devices need? A series resistance with your clock at the source can help with edges. This can be tuned a bit with an oscilloscope. Start without a resistance. If there is ringing, increase until the ringing goes away while the system still works. How high can you go before the clock signal degrades too much?From a previous post:"The harmonics are multiples of the fundamental, so 4MHz, 8, 16, 32 etc."Note that they will be at 4MHz, 8, 12, 16, 20, 24 etc.If your system can handle it with the main clock, some processors have a "frequency modulation" mode which spreads the radiated energy over a band. I got a couple dB with that once. Sometimes a couple dB is all you need!

author: jbb
author_rank: Super Contributor
timestamp: January 01, 2018, 08:14:00 am
content: I believe that one generally goes to 7th harmonic, so 64 MHz * 7 = 448 MHz.If your SPI bus is on a ground plane on a single PCB you may well be OK. Ground planes (without big slots in them!) are your friend. If you have trouble, some series resistors (say 47 Ohm) at thesourceend of the signal can be very effective.FYI, if you ask the EMC lab nicely, you might be able to do a ‘debug’ test. It won’t come with a proper certificate, but could save you some time and money by being right there.Also, it really pays to have a good test plan in mind. Have you got the software ready (if appropriate), the right loads, leads, cables etc. And bring along those different ferrite beads to swap in and out.

author: brownt
author_rank: Regular Contributor
timestamp: January 01, 2018, 10:58:13 am
content: The MCU is using an internal 64MHz clock rather than an outside RC or crystal oscillator. From an EMI view, is that a good thing?

author: Tomorokoshi
author_rank: Super Contributor
timestamp: January 01, 2018, 04:58:35 pm
content: Quote from: brownt on January 01, 2018, 10:58:13 amIn general, it's probably good because the loop areas of any circuits are necessarily as minimum as possible. It may be that they optimized the rise times to reduce outside EMI. Look at the layout guidelines. There is usually a fairly detailed example for ground planes, bypass capacitors, etc. for the clock circuit.By "64MHz clock", is that a 64MHz crystal input to the clock circuit, or a 64MHz clock output from a PLL circuit? If it's a PLL, what is the input crystal frequency?

author: nctnico
author_rank: Super Contributor
timestamp: January 01, 2018, 05:37:23 pm
content: Quote from: brownt on December 31, 2017, 11:43:04 pmWhen selecting ferrite beads you really need to look at the graphs. The impedance @100MHz says next to nothing about the suitability if a ferrite bead for a given situation. If you want to use them in signal lines like USB or ethernet then you don't want attenuation at certain frequencies. When used in power lines you may want to match the maximum impedance to the frequency range where you have most of the noise.

author: brownt
author_rank: Regular Contributor
timestamp: January 01, 2018, 11:32:27 pm
content: An internal 64MHz oscillator is used as the clock. PIC18F26K40.

author: brownt
author_rank: Regular Contributor
timestamp: January 02, 2018, 02:49:29 am
content: I see.chip ferrite beads can peak up to 1500 ohms, whilst the axial type ones peak at about 60 ohms. That seems like a lot of difference. How can I workout how much impedance is required at a particular frequency to reduce it as needed.

author: brownt
author_rank: Regular Contributor
timestamp: January 02, 2018, 02:52:42 am
content: When it comes to putting a cap on either side to make a pi filter. How can the cap value be calculated when the ferrite beads don't have inductance values?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 02, 2018, 06:36:54 am
content: Quote from: brownt on January 02, 2018, 02:52:42 amHow would you do it with a resistor inbetween?When you do it with an inductor, what are you really doing -- namely, what inductive reactance are you looking for at the cutoff frequency?What does both of these tell you, with respect to an arbitrary component whose impedance curve is given?Roughly speaking, low frequency inductance of a ferrite bead is around 10 times the value you'd calculate from its rated impedance. But this isn't a very useful figure, as both the 10x and the impedance curve show.Tim

author: cdev
author_rank: Super Contributor
timestamp: January 02, 2018, 11:04:39 pm
content: Ferrite beads have some capacitance too. They will increase capacitance over a plain conductor, in some cases a lot.Quote from: brownt on January 02, 2018, 02:52:42 am

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 02, 2018, 11:38:59 pm
content: Based on my models, the terminal-to-terminal capacitance of an average ferrite bead is in the low single digits pF. A 1206 size, "390 ohm" part shows 1.6pF (relevant above 200MHz or so).If you need serious attenuation at high RF frequencies, consider air core RF inductors and shielding. That's also the domain where layout is as critical as the components you're using.Tim

author: brownt
author_rank: Regular Contributor
timestamp: January 04, 2018, 12:40:03 am
content: ok., thanks for that. parts are ordered. I'll see how it turns out.