Understanding the Maximum Speed that can be Transmitted over a Cable

The maximum frequency is mostly related to the frequency-dependent loss characteristics of the cable. Eventually you get to a frequency where you simply don't get enough signal at the other end to use.

• Resistive losses in the conductors (including skin effect)
• Dielectric losses in the insulating materials
• Radiation losses if the cable is not fully shielded

All of these tend to increase with frequency.

This is why we generally switch to other technologies at very high frequencies: waveguides for microwave radio equipment and optical fibers for high-speed data.

You can't just "source" FFC/FPC cables for USB 3.x. These cables (and corresponding connectors) are not qualified for USB 3.x channels. The cables for USB 3.0 have to meet many more requirements than just a wire parameters and not just having certain differential impedance.

To use non-standard (not within USB defined configuration) cables you will need to run all USB cable qualification tests on your own, ensure limits on insertion loss, NEXT/FEXT crosstalk, differential impedance across mated connectors, etc. etc., if you want your product to work with any reasonable degree of reliability.

To run your own qualification you will need at least a 8-16GHz oscilloscope and a 20-GHz TDR (Time-domain reflectometer) instrument, plus make a dedicated break-out fixture to access signals in correct way. The list of electrical requirements for USB 3.0 transmission lines is given in the following USB-IF document. Although the document is mostly for qualification of standard cables and mating connectors, appendix to the document shows the general electrical requirements to meet.

The frequency that can be used inside a wire is highly dependant on the skin effect. Simply put, the bigger the wire, the lower the frequency it can carry without having signal loss caused by an increase of its impedance.

At low frequency, the signal will be equally distributed through most of the wire, with a higher frequency, the signal will be predominantly distributed around the perimeter of the wire (the ''skin'').

The wires that allow the best characteristics will always be really small and with multiple conductors to reduce the skin effect. Despite this, the higher you go, the more loss you will get. Then the protocol steps in and will increase the voltage, use twisted differential pairs and push the boundaries to the maximum until you need to switch to a different transfer technology altogether.