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According to the FCC:

The following devices are subject only to the general conditions of operation in §§15.5 and 15.29 and are exempt from the specific technical standards and other requirements contained in this part. The operator of the exempted device shall be required to stop operating the device upon a finding by the Commission or its representative that the device is causing harmful interference. Operation shall not resume until the condition causing the harmful interference has been corrected. Although not mandatory, it is strongly recommended that the manufacturer of an exempted device endeavor to have the device meet the specific technical standards in this part.

And one of the following items is:

(h) Digital devices in which both the highest frequency generated and the highest frequency used are less than 1.705 MHz and which do not operate from the AC power lines or contain provisions for operation while connected to the AC power lines. Digital devices that include, or make provision for the use of, battery eliminators, AC adaptors or battery chargers which permit operation while charging or that connect to the AC power lines indirectly, obtaining their power through another device which is connected to the AC power lines, do not fall under this exemption.

Obviously the hairy part is the "highest frequency generated" issue.

Is there any way to tell if a device can qualify for this exemption in light of this? Is there any tests I can/should preform? Should I just forget it and get a full blown FCC test?

Example:

I have a 32 kHz 5v square wave (at the worst case scenario).

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Highest frequency can be a stickler of you have any signals that has transitions that approach square waves (i.e. have rise and fall times in the nanoseconds range). So any frequency that is not a sine wave at or below ~1.5 MHz could be subject to making your product emitting EMI or RFI above that limit.

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  • \$\begingroup\$ What if I have a 32kHz Square wave? \$\endgroup\$ – DarthRubik May 19 '16 at 23:00
  • \$\begingroup\$ I think youre making an incorrect assumption here. There would be no value describing digital circuit frequencies in the way you imply, the fundamental would be more logical. \$\endgroup\$ – Sean Houlihane May 19 '16 at 23:02
  • \$\begingroup\$ @SeanHoulihane Ok....what if I have a square wave with a fundamental frequency of 32kHz? \$\endgroup\$ – DarthRubik May 19 '16 at 23:06
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    \$\begingroup\$ I have no experience of FCC, but it would make sense for a watch like circuit to be exempted. Wait for a better answer. \$\endgroup\$ – Sean Houlihane May 19 '16 at 23:09
  • \$\begingroup\$ I recall reading a separate exemption for devices whose highest repeating frequency was something like 7KHz, thinking it would have been a lot more convenient if it were about 35,000Hz (32768Hz plus tolerance), and wondering if anyone makes micros with built-in oscillators that would qualify. \$\endgroup\$ – supercat May 20 '16 at 0:00
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If you can measure the rise and fall times of your digital signals (or your logic gates), then you can use the formula BW = 1/pi*t_r (or t_f if it is faster) to calculate the effective (or maximum effective) bandwidth of the signal (i.e. the point where the spectrum starts to roll off at 40dB/decade instead of 20dB/decade).

(P.S. This is from Ott, p.380)

As to whether the FCC will accept this? Well, since square waves go off to infinity in theory, they have to stop looking at them somewhere in order to avoid mooting the exceptions...and this is about as practical a definition of somewhere as I can think of.

(A general rule of interpreting statutes and rules is that "all the words are there for a reason" i.e. Congress and the various agencies don't put words into statute because they like to hear themselves talk :)

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