Frequency (GHz)

-30

-25

-20

-15

-10

-5

0

0

2

4

6

8

10

12

14

16

18

20

D100

20

TUSB1002

SLLSEU4A – MAY 2016 – REVISED MAY 2016

www.ti.com

Product Folder Links: TUSB1002

Submit Documentation Feedback

Copyright © 2016, Texas Instruments Incorporated

In this particular example, the channel A-B has a trace length of 8 inches with a 4 mil trace width. This particular

channel has about 0.83 dB per inch of insertion loss at 5 GHz. This equates to approximately 6.7 dB of loss for

the entire 8 inches of trace. An additional 1.5 dB of loss is added due to package of the USB3.1 Host,

TUSB1002, and the A/C coupling capacitor. This brings the entire channel loss at 5 GHz to 6.7 dB + 1.5 dB = 8.2

dB. A typical USB 3.1 host/device will have around 3dB of transmitter de-emphasis. Transmitter de-emphasis

pre-compensates for the loss of the output channel. With 3 dB of de-emphasis, the total equalization required by

the TUSB1002 is in the 5.2dB (8.2dB - 3dB) range. The channel A-B for this example is connected to

TUSB1002's RX1P/N input and therefore CH1_EQ[2:1] pins are used for adjusting TUSB1002 RX1P/N

equalization settings. The CH1_EQ[2:1] pins should be set such that TUSB1002 equalization is between 5dB

and 8dB.

The channel C-D has a trace length of 4 inches with a 4mil trace width. Assuming 0.83 dB per inch of insertion

loss, the 4 inch trace will equate to about 3.32 dB of loss at 5 GHz. An additional 2dB of loss needs to be added

due to package, A/C coupling capacitor, and the USB 3.1 receptacle. The total loss is around 5.32 dB. Because

channel C-D includes a USB 3.1 receptacle, the actual total loss could be much greater than 5.32dB due to the

fact that devices plugged into the receptacle will also have loss. The device plugged into receptacle will have

either a short or long channel. USB3.1 standard defines total loss limit of 23dB that is distributed as 8.5dB for

Host, 8.5dB for device, and 6.0dB for cable. With variable channel of devices plugged into the USB3.1

receptacle, configuring TUSB1002's RX2P/N equalization settings is not as straight forward as Channel A-B.

Engineer can not set TUSB1002's CH2_EQ[2:1] pins to the largest equalization setting to accommodate the

largest allowed USB3.1 device/cable loss of 14.5dB. Doing so will result in TUSB1002 operating outside its linear

range when a device with short channel is plugged into the receptacle. For this reason, it is recommended to

configure TUSB1002's CH2_EQ[2:1] pins to equalize a shorter device channel. This will result in requiring

USB3.1 host to compensate for remaining channel loss for the worse case USB3.1 channel of 14.5dB. The

definition of a short device channel is not specified in USB 3.1 specification. Therefore, an engineer must make

their own loss estimate of what constitutes a short device channel. For particular example, we will assume the

short channel is around 3 to 5 dB. The device's channel loss will need to be added to estimated Channel C-D

loss minus the typical 3db of de-emphasis. This means CH2_EQ[2:1] pins should be configured to handle a loss

of 5 to 7 db.

8.2.3 Application Curves

Freq = 5 GHz

dB(SDD21) = –6.666

Figure 19. Insertion Loss for 8inch 4 mil FR4 Trace