In order to solve the problem of time-consuming calibration, measurement equipment manufacturers have introduced Ecal electronic calibration method , which is a transmission standard. The calibration accuracy is mainly determined by the original calibration parts. Meanwhile, the stability of the test cable, the repeatability of the test fixture device and the interpolation algorithm of test frequency also have an impact on the test accuracy. In general, the reference surface is calibrated to the end of the test cable with an electronic calibrator, and then the cable length of the fixture is compensated by unembedding. In the process of PCB printed circuit board design and production, signal loss of transmission line is an important parameter of plate application performance. Signal loss test is one of the important means to characterize the signal integrity of printed circuit board. I would like to briefly share with you the principles and related applications of the signal loss measurement methods of several PCB transmission lines currently used in the industry, and analyze their advantages and limitations.
Printed circuit board (PCB) signal integrity is a hot topic in recent years. There have been a lot of domestic research reports to analyze the influencing factors of PCB signal integrity -, but the current situation of signal loss testing technology is rarely introduced.
PCB transmission line signal loss comes from conductor loss and medium loss of materials, and is also affected by copper foil resistance, copper foil roughness, radiation loss, impedance mismatch, crosstalk and other factors. In the supply chain, the permittivity and dielectric loss are adopted for the acceptance indexes of CCL manufacturer and PCB express factory. The index between PCB express factory and terminal is usually impedance and insertion loss
For the design and use 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, the simulation and debugging, and the control of production process.
Current status of PCB insertion loss testing technology
Currently, PCB signal loss testing methods used in the industry can be classified into two categories: time-domain or frequency-domain. Time DomainReflectometry (TDR) or Time domain Transmission (TDT) were used. 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 test: frequency domain method, effective bandwidth method, root pulse energy method, short pulse propagation method, single-end TDR differential insertion loss method.
2.1 the frequency domain method
The Frequency Domain Method mainly USES the vector network analyzer to measure the S parameter of transmission line, directly reads the insertion loss value, and then measures the plate qualified/unqualified with the fitting slope of the average insertion loss in a specific Frequency range (e.g. 1 GHz ~ 5 GHz).
The difference of measurement accuracy in frequency domain mainly comes from the calibration method. According to different calibration methods, SLOT(short-line-open-thru), multi-line TRL(thru-reflection-line) and Ecal(Electronic calibration) can be used.
SLOT is usually considered a standard calibration method , a total of 12 error calibration model parameters, the calibration precision of SLOT way is determined by calibration parts, high precision calibration is provided by measuring equipment manufacturer, but the calibration is expensive, and generally applies only to coaxial environment, the calibration time consuming and with the increase of the number and geometric growth.
Multi-line TRL is mainly used for non-coaxial calibration measurement . According to the materials of transmission Line used by users and the test frequency, TRL calibration parts are designed and made, as shown in figure 2. Although multi-line TRL is easier to design and manufacture than SLOT, the calibration time of multi-line TRL also increases geometrically as the number of measuring ends increases.
Taking the insertion loss of differential transmission line as an example, the comparison of three calibration methods is shown in table 1.
Root pulse energy method
2.2 Root ImPulse Energy (RIE) usually USES TDR instrument to obtain the TDR waveforms of the reference loss line and test transmission line respectively, and then carries out signal processing on the TDR waveforms.
2.3 effective bandwidth method
Effective Bandwidth (EBW) is strictly a qualitative measurement of transmission line loss alpha and cannot provide a quantitative insertion loss value, but provides a parameter called EBW. Effective bandwidth method is through the TDR signal emission will rise in a particular time step to the transmission line, TDR measurement instrument and measured a link after the rise time of the maximum slope, identified as loss factor, unit/s MV. More specifically, it sure is a relative to the total loss factor, can be used to identify the loss in surface and the surface or the change of the transmission line between layer and layer . Due to the maximum slope can be obtained directly from the instrument, the effective bandwidth is often used in mass production of printed circuit board test.
2.4 short pulse propagation method
In the Short Pulse Propagation (SPP) testing principle, two transmission lines of different lengths, such as 30 mm and 100 mm, are measured to extract parameter attenuation coefficient and phase constant by measuring the difference between the two transmission lines, as shown in FIG. 6. This approach minimizes the impact on the accuracy of connectors, cables, probes, and oscilloscopes. With the use of high-performance TDR instruments and IFN(Impulse Forming Network), the test frequency can reach up to 40 GHz.
2.5 single-end TDR differential insertion loss method
The single-ended TDRto Differential Insertion Loss (SET2DIL) method differs from Differential Insertion Loss test using 4-port VNA, because it USES a two-port TDR instrument to transmit the TDR step response to Differential transmission lines, which are terminated in short segments, as shown in figure 7. The typical measurement frequency range of SET2DIL method is 2 GHz ~ 12 GHz, and the measurement accuracy is mainly affected by the time delay inconsistency of the test cable and impedance mismatch of the tested piece. The advantage of SET2DIL method is that it does not need to use expensive 4-port VNA and its calibration parts. The length of transmission line of the tested part is only half of that of the VNA method. The structure of calibration parts is simple and the calibration time is greatly reduced.
Test equipment and test results
SET2DIL test board, SPP test board and multi-line TRL test board were respectively made by CCL of dielectric constant 3.8, dielectric loss 0.008 and RTF copper foil. ,
This is the printed circuit board (PCB) currently used in the industry several PCB transmission line signal loss measurement methods. Due to different test methods, the measured values of insertion loss are different, and the test results cannot be directly compared horizontally. Therefore, the appropriate signal loss test technology should be selected for testing according to the advantages and limitations of various technical methods and in combination with its own requirements.