Electronic – Are specific impedances for high-speed signals maintained beyond the PCB connectors

Summarizing comment trail as an answer:

The requirement is for a PCB layout for a pass-through between USB2.0 A and B connectors on a PCB. The rest of the circuit on the PCB does not interact with the USB signal path.

Suggested solution:

By changing the physical arrangement of the two sockets to be close together rather than at opposite sides of the board as originally envisaged, the signal trace length and transmission effect concerns are alleviated.

Further, by setting the two connectors at right angles to each other, at one corner of the board area, the need to leave space between them to allow cables to be plugged in, is addressed: The cables would be connected along different edges of the board and would not touch each other.

This allows simplification of routing as well:

• The recommendation for equal length signal paths is inherently addressed
• The arrangement does not interfere with rest of PCB layout, as it is off in a corner
• With the indicated small trace length, transmission line and antenna effects are negligible for USB 2.0 High-Speed transmissions

(as posted by OP).

Concerns that may need addressing:

• Physical robustness of PCB to cope with stresses of repeated cable insertions - A mounting bolt at the corner between the connectors should address this.
• Effective total length of USB cable, adding up the A-side and B-side cables, may exceed USB maximum cable length. The very short PCB section would act merely as an extension of the cable.
• Creative solutions needed for suitably boxing the board with connectors at the corner.

Length matching is about timing so if you want to know how tightly you have to length match you have to understand the timing budget for your interface. Signals will leave your source, and arrive at your destination with some timing relationship. Your receiver requires a certain timing relationship between clock and data in order to be guaranteed to function correctly. This is usually defined as setup and hold time, or how long before the clock edge your data needs to be valid, and how long after it must stay valid.

There are several things in a system which eat into this budget, one of which will be your routing. Sometimes a manufacturer will tell you this information, other times you must derive it from the input and output timing data of your transmitter and receiver. Of course it's easy to just say well I must match exactly, because then you don't have to think about it :)

But let's think about it for a minute. You have a 170Mhz signal? That's a period of 5.882ns. What would happen if you routed all your data to within one inch of clock. What would be the worst case timing difference. Propagation time for a top layer trace, a microstrip is about 150ps/in. So a 1 inch difference will skew a data signal from clock +/- 150ps. That's really not bad at all considering your 5.882ns clock period. In fact 170Mhz is really not that fast.

If you understood the output skew from your transmitter, and your setup and hold times for your receiver you could come up with a number for the acceptable routing delay. Of course there are other factors, clock jitter, ISI, etc, but this should give you a good idea of what you can do.