Posted by Eric Huang on December 6, 2017
From USB 2.0 to USB 3.2, USB cables became shorter.
When USB replaced keyboards and mice, USB’s signaling rates of 1.5 Mbps and 12 Mbps. Transmitted more than enough data. If you know what a floppy disk is, those stored data in the Megabytes.
When USB 2.0 launched in 1999 it maintained the 5 meter cable length.
With USB 3.0 the cable length dropped to about 2-3 meters for 5 Gbps.
And with USB 3.1 it dropped to 1 meter for 10 Gbps.
USB 3.2 cables can be 1 meter because it uses 2 lanes of 10 Gbps.
The PHY / electrical signaling for USB 3.1/3.2 is exactly the same so cables can stay the same length.
The specifications don’t actually specific cable length. They specify the amount of signal loss through a “cable” that is allowable. Signal loss is measured in decibels or dB.
For USB 3.1/USB 3.2 the loss budget (allowable signal loss) 23 dB. This means the budget loss is 8.5 dB for the host, 6 dB for the cable assembly, and 8.5 for the device (peripheral) must be less than the total 23 dB.
The USB-IF tests for logo certification of cables specifically to see if they are within the allowable loss. In this case the copper inside needs to have a wire gauge of 20-28 AWG for power and ground. For signals the wire can be a little thinner from 26-34 AWG. See the table for details.
As long as the cable builds to the “The USB 3.1 Legacy Connector and Cable Specification” specification the supported cable length for 10 Gbps or 20 Gbps is 1m. This is in the 10 Gbps signaling mode.
For a 10 Gbps transfer using a 5 Gbps transfer with 2 lanes supports 2 lanes.
If the loss at the host or device is less, the cable can be longer.
If the cable maker can reduce the loss to something less than 6 dB. The cable can be longer.
The resistance of the cable can be reduced by having thicker cables (smaller gauges) or using materials with lower resistance. Most cable makers will minimize cost by using the thinnest copper wire they can and still make specification for the length. So a thick gold cable would be much more expensive, but it could be longer.
So as I’ve written about, it’s actually possible for a really good USB PHY to clean up a signal on the receiving side. This means the transmitting PHY and cable can be less than awesome. The benefit to Synopsys customers is if they use our PHY, more products should interoperate correctly. If you had built a game console, and your end-customer (consumer) plugs in a USB SSD, it has a higher chance of working. The consumer has a good experience and is less likely to return the game console.
All the USB specifications are directed and making passive cables as cheap as possible. A passive cable does not have any chips them. An active cable has a small semiconductor in it which makes it possible to have a longer cable. In theory a cable of almost any length with a repeater chip inside.
The USB 3.1 Legacy Connector and Cable Specification
USB 3.1 ECH CTLE
USB 3.1 ECH PHY 010 for Loss Budget Clarification