Several Methods of Measuring PCB Transmission Line Signal Loss

1 Introduction

Printed circuit board (PCB) signal integrity has been a hot topic in recent years. There have been many domestic research reports on the analysis of factors affecting PCB signal integrity [1]-[4], but the signal loss test Introduction to the current state of the technology is relatively rare.

PCB transmission line signal loss sources are the conductor loss and dielectric loss of the material, and are also affected by copper foil resistance, copper foil roughness, radiation loss, impedance mismatch, crosstalk and other factors. In the supply chain, the acceptance indicators of copper clad laminate (CCL) manufacturers and PCB express manufacturers use dielectric constant and dielectric loss; while the indicators between PCB express manufacturers and terminals usually use impedance and insertion loss.

For high-speed PCB design and use, how to quickly and effectively measure the signal loss of the PCB transmission line is of great significance for the setting of PCB design parameters, simulation debugging and control of the production process.

2. Current status of PCB insertion loss testing technology

The PCB signal loss test methods currently used in the industry are classified from the instruments used, and can be divided into two categories: based on the time domain or based on the frequency domain. The time domain test instrument is Time Domain Reflectometry (TDR) or Time Domain Transmission (TDT); the frequency domain test instrument is Vector Network Analyzer (VNA). In the IPC-TM650 test specification, five test methods are recommended for PCB signal loss testing: frequency domain method, effective bandwidth method, root pulse energy method, short pulse propagation method, single-ended TDR differential insertion loss method.

2.1 Frequency domain method

Frequency Domain Method (Frequency Domain Method) mainly uses a vector network analyzer to measure the S parameters of the transmission line, directly read the insertion loss value, and then use the fitting slope of the average insertion loss in a specific frequency range (such as 1 GHz ~ 5 GHz) Measure the pass/fail of the board.

The difference in the measurement accuracy of the frequency domain method mainly comes from the calibration method. According to different calibration methods, it can be subdivided into SLOT (Short-Line-Open-Thru), Multi-Line TRL (Thru-Reflect-Line) and Ecal (Electronic calibration) electronic calibration methods.

SLOT is generally regarded as the standard calibration method [5]. The calibration model has 12 error parameters. The SLOT method of calibration is determined by the calibration parts. The high calibration parts are provided by the measuring equipment manufacturers, but the calibration parts are expensive and Generally only suitable for coaxial environment, calibration is time-consuming and increases geometrically as the number of measurement terminals increases.

Multi-Line TRL method is mainly used for non-coaxial calibration measurement [6], according to the user's transmission line material and test frequency to design and produce TRL calibration parts. Although Multi-Line TRL is easier to design and manufacture than SLOT, the calibration time of Multi-Line TRL method also increases geometrically with the increase of the number of measurement terminals.

In order to solve the problem of time-consuming calibration, measurement equipment manufacturers introduced the Ecal electronic calibration method [7], Ecal is a transmission standard, calibration is mainly determined by the original calibration parts, while testing the stability of the cable, the repeatability of the test fixture device And the interpolation algorithm of the test frequency also has an influence on the test. Generally, use the electronic calibration kit to calibrate the reference surface to the end of the test cable, and then use the de-embedding method to compensate the cable length of the fixture.

To obtain the insertion loss of the differential transmission line as an example, the comparison of the three calibration methods is shown in Table 1.

2.2 Effective bandwidth method

?? Effective Bandwidth (EBW) is a qualitative measurement of transmission line loss α in a strict sense. It cannot provide a quantitative value of insertion loss, but it provides a parameter called EBW. The effective bandwidth method is to transmit a step signal with a specific rise time to the transmission line through TDR, measure the slope of the rise time after the TDR instrument and the DUT are connected, and determine it as the loss factor in MV/s. More precisely, it What is determined is a relative total loss factor, which can be used to identify the change of the transmission line loss between the surface or the layer [8]. Since the slope can be measured directly from the instrument, the effective bandwidth method is often used for printed circuit boards. Mass production testing.

2.3 Root pulse energy method

?? Root ImPulse Energy (RIE) usually uses a TDR instrument to obtain the TDR waveforms of the reference loss line and the test transmission line, and then perform signal processing on the TDR waveform.

2.4 Short pulse propagation method

The short pulse propagation method (Short Pulse Propagation, referred to as SPP) test principle is to use the measurement of two transmission lines of different lengths, such as 30 mm and 100 mm, and extract the parameter attenuation coefficient and phase by measuring the difference between the two transmission line lengths constant. Using this method can reduce the influence of connectors, cables, probes and oscilloscopes to. If high-performance TDR instruments and IFN (Impulse Forming Network) are used, the test frequency can be as high as 40 GHz.

2.5 Single-ended TDR differential insertion loss method

Single-Ended TDR to Differential Insertion Loss (SET2DIL) is different from the differential insertion loss test using a 4-port VNA. This method uses a two-port TDR instrument to transmit the TDR step response to the differential transmission line , The end of the differential transmission line is shorted. The typical measurement frequency range of the SET2DIL method is 2 GHz ~ 12 GHz, and the measurement accuracy is mainly affected by the inconsistent delay of the test cable and the impedance mismatch of the DUT. The advantage of the SET2DIL method is that there is no need to use an expensive 4-port VNA and its calibration components. The length of the transmission line of the tested component is only half of the VNA method. The calibration component has a simple structure and the calibration time is greatly reduced. It is very suitable for PCB manufacturing. Batch testing.

3. Test equipment and test results

SET2DIL test board, SPP test board and Multi-Line TRL test board are made using CCL with dielectric constant of 3.8, dielectric loss of 0.008, and RTF copper foil; test equipment is DSA8300 sampling oscilloscope and E5071C vector network analyzer; differential insertion loss of each method The test results are shown in Table 2.

4. Conclusion

This article mainly introduces several PCB transmission line signal loss measurement methods currently used in the industry. Due to the different test methods used, the measured insertion loss values are different, and the test results cannot be directly compared horizontally. Therefore, the appropriate signal loss test technology should be selected according to the advantages and limitations of various technical methods, and combined with their own needs.