Application of Sequid TDR in PCB impedance testing in IoT devices

　　In modern electronic circuit design, more and more flexible printed circuit boards (FPC) and high-speed printed circuit boards (PCB) are applied to the communication and complex electronic devices of the Internet of Things, such as WIFI devices, Bluetooth and related wireless terminal devices, etc. For these devices, signal integrity becomes very important, especially the impedance of the circuit board plays a vital role in signal integrity.

　　As a professional circuit board impedance measurement tool, SequidTDR has superior performance, a differential step signal source with jitter Jrms <500fs, and a physical rise time of <65ps. Sequid's unique simulation algorithm can calculate the impedance difference of different rise times through the algorithm, and the minimum algorithm rise time can reach 25ps, which can fully test high-speed lines with very short distances. Since the error of the shorter the line test, the greater it is, it is very necessary to compensate the line through accurate calibration. Sequid's TDR has a precise compensation function. Through the calibration of open, short and load, the connection position and line loss are well compensated. As shown in the comparison between Figure 1 and Figure 2, it can be clearly seen that there is a significant difference between the compensated and uncompensated tests. Because the relevant compensation is done in the calibrated test, the test impedance curve has a significant change compared with Figure 1, and the measured value is closer to the true value.

　　Figure 1: Uncalibrated Figure 2: Calibrated

　　Sequid has many different types of probes, which can meet the requirements of differential, single-ended adjustable and non-adjustable spacing. In order to let everyone know more clearly about the measurement characteristics of DTDR-65, we used Sequid18GSDTP-P adjustable differential probe to analyze the impedance and other related parameters of the 2cm long FPC line (as shown in Figure 3) used in IoT devices, and adjusted the SDTP-P spacing to 0.1mm to measure the red line.

　　Figure 3: 2cm long FPC

　　Two measurement results are measured respectively. From the measurement results in Figure 4, we can see that Sequid's TDR has a real-time waveform similar to the sampling oscilloscope function, which allows R&D customers to understand the changes in the time domain reflection signal of the contact part or whether the probe is in good contact in real time. At the same time, in the result measurement waveform, the start (time or distance can be selected, and the accuracy of the distance is related to the relevant parameter settings) and the impedance tolerance range can be set according to the product specifications. It is obvious that the difference in the length of the two FPCs (about 0.3cm) and the impedance value, the software will also provide the maximum, minimum and average values ​​of each test sample.

　　Figure 4: FPC measurement results

　　The impedance test template can automatically generate a custom template of impedance change over time based on the characteristics of the line on the PCB, as shown in Figure 5. This can accurately judge and detect the quality of special PCB routing, such as short circuit and open circuit of the coil circuit on the PCB board.

　　Figure 5: Custom non-rule template

　　Sequid's TDR includes single-ended STDR65 and differential DTDR65. In addition to the above impedance test related functions, you can choose the frequency-based dielectric constant test function for PCBTestCoupons dielectric constant test, or you can choose S11 and VSWR test functions for antenna testing in IoT devices. The introduction of these tests will be introduced one by one in future application articles.