After reading my article about Bob Pease’s solenoid drivers, Howard Evans wrote me a letter explaining how he drives solenoids with an H-bridge. Its great stuff and we are working on a follow-up article. Evans mentioned some FCC approval things and that got an email thread started between my consultant pal Dave Mathis, who has already weighed in on FCC requirements here and here. Then Howard asked his pal Scott Proffitt to chip in. Scott runs an EMC approval lab and was kind enough to clear the air.
The first lesson is that I have been too cavalier in my terminology. Saying “FCC certification” is different than saying “FCC approval”. Dave Mathis kept calling me on this, because we were talking orthogonally. Dave was thinking about radios and I was thinking about computer equipment. Understand that a radio is an “intentional radiator” and it gets its own section in the hundreds of pages of rules. A computer without a wireless system is an “unintentional radiator”.
Dave would get exasperated with me when I would say something “…needs to get FCC approval”. To him that sounded like I wanted to get a TV station or some other licensed use where you need FCC permission to operate. As Dave keep reminding me, Zigbee and Wi-fi are unlicensed. That means the end user does not need to ask the FCC for approval to use the device. It does not mean there are no rules and you can just build anything you want and sell anything you want. Dave does note that you are allowed to build 5 units for personal use, but prototypes for a salable product are not personal use, so you need to worry about the FCC right from the start. Dave reminded me of the $10,000 per device penalty if you exceed the FCC limits on your Gizmo.
So let’s get Scott Proffitt to clear the air about what FCC things you need with what gizmos:
“It all depends on the type product and category it falls under.”
“FCC ‘Certification’ is intended for all radio transmitters (Intentional radiators) per FCC 47 CFR, Part 15, Subpart C, Section 15.201. Certification is also required for Scanning Receivers, Radar Detectors and Access BPL and is an option for TV interface devices, personal computers, computer peripherals, personal computer mother boards and supplies and all other receivers except TV and radio broadcast receivers per FCC 47 CFR Part 15, Subpart B, Section 15.101.
“FCC ‘DoC’ [Declaration of Conformity] is required for Cable System Terminal Devices and a personal computer employing certified components. A “DoC” is an option for a TV interface device, personal computer, PC peripheral and all receivers except a scanning receiver and broadcast receivers.
“FCC Verification is for everything else, that is not captured above, to include broadcast receivers and all other digital devices.
“The above categories have two classes. Class B is intended for residential environments… including apartments, nursing homes, etc… any living situation. Class A devices are anything that is not Class B. Class A would be office environments, commercial, retail, public areas that are not residential areas.
“Now, given all that… there are some exceptions and exemptions, laboratory equipment for example, as long as it’s not a radio transmitter. These are too many and too complex to list. So at this point we ask what the device is and what it does and see if any of the exemptions apply.”
Now in the context of the solenoid circuits Howard Evans and I were talking about, I asked if you would need FCC anything if you used an Atmel chip and did not bring the oscillator out to any pin. I thought I was being clever and beating the FCC requirement that you have to test and self-certify (using Scott’s lab or equivalent) anything with a clock that runs faster than 9kHz. This is a big deal, the fact that anything with a clock frequency over 9kHz falls into the perview of the FCC rules. Dave parsed the FCC rules and told me that even a clock internal to the IC will still require testing. But Scott Proffitt chimed in with the reminder that testing is only needed for end user equipment:
“Your last point of discussion regarding your and Dave’s pondering on the chip… I think if I understand it correctly, this chip is a component not to be defined as an electronic device requiring FCC approval. The end user device that the chip will be integrated within, will be subject to FCC rules, but not the chip itself. The end user device is where all compliance requirements should be applied. (There is the exception for modular components of a system, such as components for a PC per Part 15, Section 15.101 where those components require authorization.)“
“If the intent of the question may have been regarding what sources should be considered in determining the maximum Radiated Emissions measurement frequency, then you are correct, it is above the threshold and the end device the chip is integrated in, would fall under the FCC requirements for testing and approval.”
So this means that the certainly the chip itself is not tested, but if you make a solenoid driver board that is for sale to companies that integrate into their equipment, then you don’t have to test it either. But both Proffitt and Mathis agree that just because the 32kHz in internal to the chip and does not appear on any pin, if you do sell it as part of an end-use device, you still have to test it. Howard Evans had some advice as well, and note how he too got confused when I used the term “FCC approval” instead of FCC verification”:
“Well as long as you are not an intentional radiator of RF you do not need FCC approval. By law you are still required to meet FCC limits for emissions, but you can test this yourself or go to a compliance lab. If it is later found your emissions are too high, the government could force you to pull your product from the market. In practice, you can get away with radiating too much as long as it doesn’t interfere with enough people to garner the FCC’s attention. That said, you should be a good citizen and not pollute the RF spectrum. Your IC will radiate some, but it is usually the traces from a bad PCB layout and the cables that radiate the most. If you keep your edge rates no faster than necessary and keep your signals well coupled to their return paths (i.e. follow good EMC design practices), you will be well on your way.”
Making your devices FCC compliant is being a good citizen. I note that the engineers who seem to care most about this are also ham radio operators who want to keep the radio spectrum clean of unwanted junk.
And although I have been talking about FCC compliance, getting world-wide approvals under CE (Conformité Européenne) is similar. Howard Evans notes those are even tougher:
“I am by no means a certifications expert and being that my background is in industrial equipment, I can only speak to how we deal with the FCC in relation to our class of equipment which is to say we do not deal with the FCC at all. So I was incorrect in saying that as long as the device is not an intentional radiator, it does not need certification. That is only true for my situation. It is safe to say that unless your device falls into an exempted category, it does indeed need FCC certification for sale in the US. Sorry for the misinformation.”
“We perform emissions testing to CE levels which are stricter than the FCC’s, so we give no mind to passing FCC limits. Yes, when doing emissions, we must operate the device in its intended worst case scenario (highest expected emissions state for standard use). So the cycle rate and duty cycle that you operate the solenoid will have a dramatic effect on your average and quasi-peak readings.
“We test at only one distance, usually 3 or 10meters depending on the size of the test facility’s semi-anechoic chamber or OATS [Open Air Test Site]. I believe the standard for the class of equipment we test to (CISPR 11) allows us a choice of distance as long as you adjust the limits accordingly. I don’t have the standard in front of me so I may be incorrect and that it is a device class standard that allows us to do that. CISPR 11 is a generic standard that applies to industrial equipment in general but there are device class specific standards that override parts of the generic one. You should consult with a competent EMC engineer to determine which standards apply to your product.”
I noted that some engineers dither the clock frequency to spread out the interference. This does not really lower the interference, it just looks lower since the spectrum analyzer is sweeping a narrow bandwidth as it tests so the dithering just gives a lower reading, it doesn’t really make the interference go away. Evans, agreed, pointing out,
“Dithering is certainly frowned upon by most engineers I know because, like you suggest, it doesn’t really lower your emissions. It just spreads them out and takes advantage of the fact that testing limits pertain to average and quasi-peak measurements, not peak measurements. It might lower interference if the victim equipment is susceptible to only a narrow band within the dithering band. Without dithering, the polluting equipment could be radiating continuously in that band causing continuous interference while only some of the time with dithering, but is interfering some of the time really acceptable? It really depends on how well the victim handles the interference, but practically I’d rather just lower the total emissions than play games with the test method. But that said, it is a tool which you can use if needed.”
“With PWM circuits like this and assuming the PCB layout is solid, it really comes down to containing the edge rates on the signals leaving the board. It is hardly the switching frequency that bites me, but the edge rate of all the signals. I always use gate resistors on my MOSFETs to slow the turn-on and turn-off which has worked well for me in the past. It does increase the switching losses some but not too much if the resistance is reasonable. I also usually add some capacitors between the outputs and the rail to divert the high frequencies back in to the driver. “I also often add a common-mode choke (either ferrite or wound) for the common-mode noise which radiates very well from your cables given the miniscule currents involved. Cable design is critical. I always use shielded twisted pair with the shield bonded (360 degree is best) to the metal enclosure of the driver. The shield limits E-field radiation while the twisting helps lower the H-field in the far field. It helps too if the coil is in an enclosure with a decent RF ground.
“Finally, I would encourage you to keep the cable distance as short as possible. There can be some very high voltages (3X the bus voltage or more) that can develop along the cable that can surpass the insulation rating of the cable and/or the magnet wire of the coil. This has been well written about with regard to variable frequency motor drives. This is probably not an issue for you unless you run the bus from rectified line.”
Howard Evan’s comments about cable length reminded me of a problem I had with CE immunity when I was a consultant. Many engineers are finding it is tougher to pass immunity than emissions. Immunity testing is when you bombard your machine and its cables with RF and verify that it does not malfunction. I will tell that story in a future blog post, since Evans also points out: “I’ll comment on immunity in a future email. I just went through a somewhat difficult issue with that.”