Part Number: TUSB4041I
in TUSB4041I and TUSB4020B datasheet, it mentioned in PCB layout that Eight inches is the maximum recommended trace length for USB 2.0 differential pair signals , is it correct?
in USB2 spec, All bits, including the first and last bit of a packet, must meet the eye pattern requirements for timing and amplitude.
Above figure defines four test planes. TP1 and TP4 are the points where the transceiver IC pins are soldered to the hub and device circuit boards, respectively. TP2 is at the mated pins of the A connector, and TP3 is at the mated pins of the B connector.
we use template 5 for near end test for HS eye diagram measurement for hub downstream ports and upstream port.
For near end HS eye diagram measurement, picture 1 is the measurement with TUSB4041IEVM and test fixture only.
picture 2 is the measurement with TUSB4041IEVM +test fixture + 24in PCB trace
from above measurement, 24 in is the max trace length for TUSB4041I downstream and upstream to pass HS eye diagram
since PCB performance varies based on different PCB material ( dielectric), trace width, PCB layout ( vias), the trace length may vary as well.
for this measurement, PCB material is FR4 with dielectric 4.9, PCB trace width is 6mils,
below is the differential pair insertion loss measurement for PCB, at 240Mhz ( USB2 HS signal), the loss for 24in ( 2 ft) is 1.047db, which means you need to measure sure total loss from hub downstream port to USB connector is 1db, which include trace length loss+ PCB layout loss ( vias, stub)+ any cable loss ( like flux cable) + USB2 MUX loss( if have)
By the way, 24 in trace length here is just example . You need to calculate max trace length based on your own PCB design to make sure total system loss is below 1db
