avr_fred:
Serious question: Do you think Sparkfun and Adafruit have every new processor board they design certified for FCC compliance?
I would imagine their intentional transmitters would have to be. But all of their products fall under engineering tools/ development tools so they should be exempt.
I could possibly in a round about way twist the marketing of my device to call it an engineering/development tool, but thats not really the market Im going for.
ron_sutherland:
I guess think of free space as a transmission line, but its impedance changes depending on how much is in contact with the circuit node. Those 500 Hz PWM signals have edges, so they can have some high-frequency content.The real question is who do you want for a customer? If you want Centrylink or Comcast then they will need the FCC certification for your blinky. Perhaps Walmart can offer it, but I'm not sure on that. If you want to offer it the way Sparkfun and Adafruit do (as development boards) then there is no need to do FCC at least until someone tells you to get it certified or they won't buy it (maybe Centrylink does want it after all).
As far as keeping those high-frequency edges from radiating EMI all you need to do is to slow them down e.g. perhaps drive the LED with a constant 15mA current source so that the long wires (which have capacitance) running to the LED are not slammed with a quick current flow that ends up radiating a lot of EM.
Yeah, I would be distrubiting through my own means so I wouldnt need to worry about the FCC cert just to get it on shelves so thats good.
Honestly theres a .01% chance this would ever be an issue, but theres always murphy's law.
I may look into rounding the edges slightly with passive components, but adding a constant current source will add quite a bit to the parts cost (its hard to beat a 3 cent mosfet) so I dont really want to do that either.
I may just place the micro as close to the fet/leds as possible to reduce surface area of the pwm trace.
hansibull:
I wouldn't worry too much about the microcontroller itself...Two weeks ago I spent some days at a certification company to test a few products for the company I work for. These products was analog audio equipment controlled by AVR microcontrollers running at 8 MHz. The PCBs are four-layer boards with ground plane on top and bottom
We tested for CE approval and for maritime use. CE approval way quite simple to pass, as it does not require to measure the noise on the input power (conductive noise). The radiated emmision (radiated out from the product) was measured 15m and 3m away IIRC.
Here's a few design rules that will make you probably pass a test like this:
- Use decoupling capacitors for all ICs on the board. 100n should work fine.
- Place the decoupling capacitors as close to the power pin as possible and use a lot of ground vias to reduce the trace inductance.
- Keep your ground plane(s) whole to reduce the impedance
- Switched regulators are the worst, so be careful when designing with these. We use the L5973D in most product, and the switching frequency + harmonics were dominating, even when properly decoupled see attachment). Add an inductor in series to the Vin pin if you're going to measure conductive noise (noise on the power input). this reduced the noise on one of our products by 20dBuV (dB microvolt)
Thanks, this is what i was looking for.
My board will only be a 2 layer board, and it will be using power from a usb source so I don't have to worry about any switched regulators.
Is there a specific type of capacitor that is best for IC decoupling?