eeworldonline.com | designworldonline.com 4 • 2020 DESIGN WORLD — EE NETWORK 23
2.0, and potentially also DisplayPort and PCIe. It might be natural
to assume that if the link runs correctly at 20 Gbps, then for sure it
would run at 10 Gbps and the slower rates. So why bother testing
the lower rates if testing at 20 Gbps passed?
The reason is each of these speed rates takes place under a
different set of conditions and experience a different channel loss.
So, though a bit rate may be slower, the cable used will be much
longer and lossier. There are numerous instances where a link will
test fine at 20 Gbps and yet fail at 10 Gbps when tested with a
longer cable model.
An understanding of the entire link’s loss budget is critical
to designing, testing, and implementing a low-BER system.
Comparing USB4 IL to the USB 3.2 IL spec, the loss budget for the
link partners has shrunk from 8.5 dB to 5.5 dB at the 10-G rate.
So the USB3.2 link implementations may not work with the much
tighter USB4 IL budget.
The good news is the cable loss increases from 6 to 12 dB at
10 Gbps. The negative to this relaxed cable loss is that although
USB4 10 G runs at the same rate as USB3.2 10 G, it must work with
a 12-dB cable and not a 6-dB cable. Thus it’s important to have a
thorough understanding of the insertion loss budget.
The next step is understanding where and how the compliance
test points are defined. There are no specific rules for naming
test-points, so TP0, TP1, TP3’, TP3EQ will mean different things in
different specifications.
For USB4 Tx testing, TP2 is the near-end or short channel test
point at the Type-C connector. TP3 is the far-end or long channel
use case test point - note the definition of TP3 includes the
receiver equalization.
For Rx testing, TP3’ would be the short channel test point. TP2
would be the long channel use case. It’s important to know the
test points precisely to accurately set up the tests and perform the
compliance measurements.
There are significant channel losses
with the passive cable use case, so both
Tx and Rx equalization are required in
the implementation and when testing.
When performing Tx testing, it is critical
to find the optimal continuous time linear
equalization (CTLE) and decision feedback
equalization (DFE) setting that provides
the largest eye opening. CTLE is a linear
filter applied at the receiver that attenuates
low-frequency signal components, amplifies
components around the Nyquist frequency,
and filters out higher frequencies. DFE is
a filter that feeds back a sum of detected
symbols to the symbol decoder for thePresent
numberPre-shoot
(dB)De-emphasis
(dB) C-1 C 0 C 10 0 0 0 1 0
1 0 -1.9 0 0.90 -0.10
2 0 -3.6 0 0.83 -0.17
3 0 -5.0 0 0.78 -0.22
4 0 -8.4 0 0.69 -.31
5 0.9 0 -0.05 0.95 0
6 1.1 -1.9 -0.05 0.86 -0.09
7 1.4 -3.8 -0.05 0.79 -0.16
8 1.7 -5.8 -0.05 0.73 -0.22
9 2.1 -8.0 -0.05 0.68 -0.27
10 1.7 0 -0.09 0.91 0
11 2.2 -2.2 -0.09 0.82 -0.09
12 2.5 -3.6 -0.09 0.77 -0.14
13 3.4 -6.7 -0.09 0.69 -0.22
14 3.8 -3.8 -0.13 0.74 -0.13
15 1.7 -1.7 -0.05 0.55 -0.05Informative filter coefficientsTransmitter Equalization Pre-Shoot and De-Emphasis
Symbol Description Min Max Units CommentsTJ Total jitter -- 0.60 UI pp See note 2, note 3.UJSum of uncorrelated DJ and RJ
components (all jitter components
except for DDJ)-- 0.31 UI pp See note 2.UDJDeterministic jitter that is
uncorrelated to the transmitted data-- O.17 UI ppX1 TX eye horizontal deviation -- 0.23 UIMeasured for 1E6
UI. See Note 2,
Note 4, and figure
3-15.Y1TX eye inner height (one-sided voltage
opening of the differential signal49 mVMeasured for 1E6
UI. See Note 1, Note
2, and figure 3-15.Y2TX eye outer height (one-sided voltage
opening of the differential signal-- 650 mVMeasured for 1E6
UI. See Note 1, Note
2, and figure 3-15.Long Channel Transmitter Specifications
Long channel refers to the situation
where the USB device connects to the
controller through a 3-m-long cable.TESTING USB4