The exemption section of the FCC rules on unintentional radiators (Title 47, chapter I, Subchapter A, Part 15, Subpart B, Section 15.103) has the following item :
(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.
I found a a few questions on the site about this (Does anyone know of a microcontroller/DSP with no clock speeds or any oscillators in excess of 1.705 MHz? in 2011, "FCC Low Oscilation" in 2016, and "How do I tell if my device qualifies for FCC exemption" in 2016 as well) and, to me, they all kind of point toward the conclusion :
"Even if your product runs under 1.705 MHz, you still need to get it evaluated and probably tested by a lab."
Well, I've actually done the first part recently, and the compliance engineer I met with refused to discuss the 1.705 MHz exemption. Now, there might be good reasons for this (it was only a short first meeting), but it did peak my interest.
Does anybody have any experience with this "sub 1.705 MHz" exemption? What is it for? How does it apply?
Who could confirm that it applies to a specific product? Is a compliance engineer the right person to ask?
It would be amazing to know that that exemption does "work" for some cases and the guidelines that apply. A lot of electronics can be done under 1.705 MHz.
