How much change is allowed after a device has been FCC certified?

Question

We are trying to decide between two different DACs in the same family that are pin compatible. One DAC is about 3x more expensive, but has higher resolution and will definitely work. The second is less expensive, but has lower resolution. We will have to do more extensive testing to verify that the lower resolution DAC works for our applications. We are a startup and would like to sell our product out as soon as possible. If we pass the FCC test with the higher resolution DAC, and switch to the lower resolution DAC later would we have to retest?

The rules I found on the FCC site said that if the part is electrically equivalent we would not necessarily need to re-test, but I am not sure if changing DAC resolution is considered "electrically equivalent".

Our device uses RFID, so it is an intentional radiator, but the DAC is not related to the RFID circuit.

Answer 1

Since your device is a intentional radiator, you need to be very conservative. Common sense may say some changes don't matter, but that's a judgement call the paper pushers get to make and it's very expensive to argue with them. To be fair, some little seemingly inconsequetial changes can effect emissions in unexpected ways.

To be really safe, test your final product and then ship only that.

Get cracking on prototyping with the low resolution DAC so you know which one you want as soon as possible.

Answer 2

I'm a bit surprised that the FCC allows any changes at all. What's "electrically equivalent"? Even the same schematic on a different PCB layout may cause big changes in radiation. So two identical schematics aren't necessarily electrically equivalent. And, like Olin says, don't assume your change is only a minor one which won't matter much.

To put this in perspective: When I was a fresh design engineer I had to make a few minor changes in a timing lookup table. Of course I was sure the product didn't need retesting! (wasn't FCC.) Until my boss asked me if I was prepared to personally pay the 100 000 euros it would cost to fix it if it turned out to be a problem after RFP (Release For Production). Next thing I did was starting the testing procedure.

You never know how big a small change might be.

I agree with Olin: design for one solution and test that one. Regard the other variant as a different product, and expect new testing if you ever decide to replace your original product with that one.