Interpreting USB3.0 test items

Editor's note ：USB It is the best in all interface development , It has been nearly... Since its launch 30 Years of time . at present , almost 90% The above mobile phones are still used USB Charging line , Some of them use wireless or other charging methods . This article still comes from Mr. Wang Bing , The main introduction is USB3.0 Cable electrical performance test .

because USB3.0 There are two new sets of difference lines （SS） And compatible with USB2.0 A set of differential line pairs （D+/D-）. We've introduced USB2.0 To avoid repetition , Here, I only provide you with two sets of high-speed line pairs, one of which is used as a test example for you to interpret .

The fixture and test connection used in this interpretation are shown in the figure below ：

The tested cable assembly is a cable with a length of 3.07 rice TYPE A To TYPE B Of USB3.0 Cable assembly .

The measured results are given in this paper , combination USB3.0 Physical layer index requirements of cables , Share and interpret each specific indicator with you .

First, let's look at the indicators tested in the frequency domain .

Loss test . In order to facilitate you to understand the specific requirements of loss more intuitively , I opened it on purpose ”Limit Line” Edit window for , In this way, the requirements for loss at different frequency points can be clearly understood . Refer to the figure on the next page for the test results ：

The next indicator is Scd21. The physical meaning of this parameter refers to USB3.0 Differential excitation signal at one end of the cable , A common mode output signal is generated at the other end of the cable . That is to say, if this indicator is poor , It was supposed to output a differential signal at the other end . As a result, a common mode signal with equal amplitude and phase is output . In this way, the differential signal passes through this cable , It is easy to generate interference signals to interfere with other circuits in the equipment . Specification requirements scd21 Less than -20dB.

Next , Let's take a look at some parameters tested in the time domain ：

First of all Differential impedance , Specifications are divided into USB The impedance at the cable joint and the impedance of the cable part . The test conditions and requirements are different .

First look at the impedance at the joint , Because there is solder at the cable and joint , There will be a sudden change in impedance . So the impedance requirements in this area will be relaxed . The specific test conditions are 20-80%,50ps The rise time of . The impedance requirements are 75 o -115 Between Europe . From the top left of the figure below Tdd11 and Tdd22 Test results （ Respectively the TYPE A and TYPE B） It can be seen that , The test result is passed .

Let's look at the impedance of the cable part . The test condition is 10-90%, 200ps The rise time of . The impedance requirements are 83 o -97 Between Europe . In the upper right of the figure below Tdd11 and Tdd22 Represent the TYPE A and TYPEB The impedance of the cable part corresponding to both sides . The result is also passed .

Next is Transmission delay Test results . The general requirement is less than 5.2ns/ rice . The measured result is 13.438ns. Because the cable length exceeds 3 rice . So it is up to standard .

Next is Near end crosstalk Test results , It should be noted that general cables are tested in the frequency domain . and USB3.0 The requirement is to test the index in time domain . Let's first interpret the requirements of this indicator . According to different joints , The specifications of indicators required by the association are different . The following table is USB3.0 Requirements for near string between high-speed line pairs .

The graphic test on the next page is TYPE A This way NEXT, Test result peak - The peak is 2.6394mV. Less than 3.6mV, Yes. .

and USB3.0 High speed pairs and USB2.0 The requirements for near string and far string between pairs are different , The following table .

Test results of the graph on the next page （ High speed line pair SS and D+/D- Near string of ） by 9.5782mV. Greater than 8mV. It means that the standard is not met . The remote connection is just different , There is no test result here .

Next is Internal time delay Test of . The required indicators are in 10-90%,200ps Under the condition of rising time . Less than 100ps. The bottom left of the figure below is intraskew Test results . The measured results are 29.26ps. adopt .