PCB News - Several PCB transmission line signal loss measurement methods

1, the preface

Printed circuit board signal integrity is a hot topic in recent years, there have been many domestic research reports on the influence factors of PCB signal integrity analysis, but the status of signal loss testing technology is rarely introduced.

The signal loss of PCB transmission line comes from conductor loss and dielectric 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 acceptance index of CCL and PCB express factory adopts dielectric constant and dielectric loss. The index between PCB express plant and terminal usually adopts impedance and insertion loss.

For 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

Currently, PCB signal loss measurement methods used in the industry are classified by the instruments used, which can be divided into two categories: time-domain or frequency-based. The Time domain test instrument is Time DomainReflectometry (TDR) or Time domain Transmission (TDT). The frequency domain test instrument is Vector Network Analyzer (VNA). In ipC-TM650 test specification, five test methods are recommended for PCB signal loss measurement: frequency domain method, effective bandwidth method, root pulse energy method, short pulse propagation method, single-terminal TDR differential insertion loss method.

2.1 the frequency domain method

Frequency Domain Method mainly uses vector network analyzer to measure S parameter of transmission line, directly reads insertion loss value, and then uses the fitting slope of average insertion loss to measure the pass/fail of plate in a specific Frequency range (e.g., 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 subdivided into SLOT (short-line-open-thru), multi-line TRL (thru-reflect-line) and Ecal (Electronic calibration), etc.

SLOT is usually considered a standard calibration method [5], a total of 12 error calibration model parameters, SLOT way of calibration is determined by calibration parts, high 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 measurements [6]. TRL calibration components are designed and made according to the transmission Line materials and test frequencies used by users. 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.

In order to solve the problem of time-consuming calibration, measurement equipment manufacturers have introduced Ecal electronic calibration method [7]. Ecal is a transmission standard, and calibration is mainly determined by the original calibration parts. Meanwhile, the stability of test cable, repeatability of test fixture and interpolation algorithm of test frequency also have an impact on the test. Generally, the reference surface is first calibrated to the end of the test cable with an electronic calibration piece, and then the cable length of the fixture is compensated by embedding it.

Taking the insertion loss of 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 strictly a qualitative measurement of transmission line loss α. It does not provide a quantitative value of insertion loss, but 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 slope of the rise time after the connection between the TDR instrument and the tested part, which is determined as the loss factor, in MV/s. Rather, it determines a relative total loss factor that can be used to identify changes in loss in the transmission line from surface to surface or layer to layer [8]. Since the slope can be measured directly from the instrument, the effective bandwidth method is often used for mass production tests of printed circuit boards.

2.3 Root pulse energy method

?? The Root ImPulse Energy (RIE) method usually uses TDR instruments to obtain the TDR waveform of the reference loss line and the test transmission line respectively, and then conducts signal processing on the TDR waveform.

2.4 Short pulse propagation method

Short Pulse Propagation (SPP) test principle is to measure two transmission lines of different lengths, such as 30 mm and 100 mm, and extract parameter attenuation coefficient and phase constant by measuring the difference between the two transmission lines. This approach minimizes the impact of connectors, cables, probes, and oscilloscopes. With high-performance TDR instruments and Impulse Forming Network (IFN), the test frequency can be as high as 40 GHz.

2.5 Single-ended TDR differential insertion loss method

Single-ended TDRto Differential Loss method (SET2DIL) is different from the 4-port VNA Differential Loss method. The TDR step response is transmitted to the differential transmission line, and the differential transmission line ends are short-circuited. The typical measurement frequency range of 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 tested parts. The SET2DIL method has the advantage that it does not need to use expensive 4-port VNA and its calibration components, and the length of the transmission line of the tested components is only half of that of VNA method. The calibration components have simple structure and the calibration time is also greatly reduced, so it is very suitable for batch testing of PCB manufacturing.

3. Test equipment and test results

SET2DIL test board, SPP test board and Multi-line TRL test board were fabricated by CCL with dielectric constant 3.8, dielectric loss 0.008 and RTF copper foil 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, endnotes

This paper mainly introduces several measurement methods of PCB transmission line signal loss. Because the test methods used are different, the insertion loss values measured are also different, and the test results cannot be directly compared horizontally. Therefore, it is necessary to choose the appropriate signal loss test technology according to the advantages and limitations of various technical methods and their own requirements.