According to the FCC:
The following devices are subject only to the general conditions of operation in ยงยง15.5 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.
(emphasis added)
My question is why 1.705 MHz? That seems like just a random number to me. Where did they get it? (ie. What maths if any did they do to come up with that number?)
Best Answer
Here's a pretty chart of all the radio spectrum allocations in the USA: https://www.nasa.gov/sites/default/files/spectrum_wall_chart_aug2011.jpg They've been built up over many years in a fairly ad-hoc way, there's no maths behind it. Judging from that, I'd say they want to protect the radiolocation and mobile systems operating above 1.705MHz from interference, and don't care too much about the AM radio system below it.