Signal loss test technology of PCB
Signal integrity of printed circuit board (PCB) is a hot topic in recent years. There are a lot of research reports in China to analyze the influencing factors of PCB signal integrity  - , but the current situation of signal loss testing technology is rarely introduced.
The signal loss of PCB transmission line comes from the conductor loss and dielectric loss of the material, and it is also affected by the copper resistance, copper roughness, radiation loss, impedance mismatch, crosstalk and other factors. In the supply chain, dielectric constant and dielectric loss are used as the acceptance indexes between CCL manufacturer and PCB express factory, while impedance and insertion loss are usually used as the indexes between PCB express factory and terminal.
Aiming at the design and application of high-speed PCB, how to measure the signal loss of PCB transmission line quickly and effectively is of great significance to the setting of PCB design parameters, simulation debugging and production process control.
2. Current situation of PCB insertion loss testing technology
At present, PCB signal loss testing methods used in the industry can be classified into two categories: Based on time domain or based on frequency domain. Time domain reflectometry (TDR) or time domain transmission (TDT) is used as the time domain testing instrument, and vector network analyzer (VNA) is used as the frequency domain testing instrument. In the ipc-tm650 test specification, five test methods are recommended for PCB signal loss test: frequency domain method, effective bandwidth method, root pulse energy method, short pulse propagation method, single terminal TDR differential insertion loss method.
2.1 frequency domain method
Frequency domain method mainly uses vector network analyzer to measure the S parameter of transmission line, directly reads the insertion loss value, and then uses the fitting slope of average insertion loss to measure the qualified / unqualified plate in a specific frequency range (such as 1 GHz ~ 5 GHz).
The difference of measurement accuracy of frequency domain method mainly comes from the calibration method. According to different calibration methods, it can be divided into slot (short line open thru), multi line TRL (thru reflect line) and electronic calibration.
Slot is generally considered as a standard calibration method . There are 12 error parameters in the calibration model. The calibration accuracy of slot is determined by the calibration parts. The high-precision calibration parts are provided by the measurement equipment manufacturer, but the price of the calibration parts is expensive, and they are generally only applicable to the same axle environment. The calibration time-consuming and geometric growth with the increase of the number of measurement ends.
Multi line TRL mode is mainly used for non coaxial calibration measurement , and TRL calibrators are designed and manufactured according to the materials of transmission lines used by users and test frequency. Although multi line TRL is easier to design and manufacture than slot, the calibration time of multi line TRL also increases geometrically with the increase of the number of measuring ends.
In order to solve the problem of time-consuming calibration, the measurement equipment manufacturers have introduced the ECAL electronic calibration method . ECAL is a transfer standard, and the calibration accuracy is mainly determined by the original calibrator. Meanwhile, the stability of the test cable, the repeatability of the test fixture device and the interpolation algorithm of the test frequency also have an impact on the test accuracy. Generally, the reference surface is calibrated to the end of the test cable with the electronic calibrator, and then the cable length of the clamp is compensated by the way of embedding.
Taking 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 strict sense. It can not provide quantitative insertion loss value, but it provides a parameter called EBW. The effective bandwidth method is to transmit the step signal of a specific rise time to the transmission line through TDR, and measure the maximum slope of the rise time after the TDR instrument and the DUT are connected, which is determined as the loss factor, in MV / s. more precisely, it determines a relative total loss factor, which can be used to identify the change of the loss in the transmission line between the surface or between the layers  The effective bandwidth method is often used in the mass production test of PCB.
2.3 pulse energy method
Root pulse energy (RIE) usually uses TDR instrument to obtain the TDR waveforms of reference loss line and test transmission line, and then processes the TDR waveforms.
2.4 short pulse propagation method
The principle of short pulse propagation (SPP) is to measure two transmission lines with different lengths, such as 30 mm and 100 mm, and extract the parameter attenuation coefficient and phase constant by measuring the difference between the two transmission lines. With this method, the influence of connector, cable, probe and oscilloscope accuracy can be minimized. If high performance TDR instrument and IFN (impulse forming network) are used, the test frequency can be up to 40 GHz
2.5 single ended TDR differential insertion loss method
The single ended TDR to differential insertion loss (set2dil) method is different from the 4-port VNA differential insertion loss test. This method uses two port TDR instrument to transmit the TDR step response to the differential transmission line, and the end of the differential transmission line is short circuited. The typical measurement frequency range of set2dil method is from 2 GHz to 12 GHz. The measurement accuracy is mainly affected by the time delay inconsistency of the test cable and the impedance mismatch of the tested part. Set2dil method has the advantage that it does not need to use expensive 4-port VNA and its calibrator. The length of the transmission line of the tested part is only half of that of VNA method. The structure of the calibrator is simple and the calibration time is greatly reduced. It is very suitable for batch testing of PCB manufacturing.
3. Test equipment and test results
Using CCL of dielectric constant 3.8, dielectric loss 0.008 and RTF copper foil to make set2dil test board, spp test board and multi line TRL test board respectively; the test equipment is dsa8300 sampling oscilloscope and e5071c vector network analyzer; the test results of differential insertion loss of each method are shown in Table 2. 4. Conclusion
This paper mainly introduces several methods of measuring the signal loss of PCB transmission line used in the industry. Because of the different test methods and different insertion loss values, the test results can not be directly compared horizontally, so we should choose the appropriate signal loss test technology according to the advantages and limitations of various technical methods, and combined with our own needs.