Impact of Printed Circuit Board Reference Plane on High Speed Signal Transmission
High-speed serial signal transmission in excess of 1 Gbps, such as PCI-Express and serial ATA, has come into general use. Transmission paths for differential signals on a printed circuit board are often coupled two wires with a pair of +/-, and a solid plane is often used as a reference plane.
It is ideal for this reference plane to be consistent from the viewpoint of signal transmission quality and EMC, but in reality, with the increase in the types of power supplies required for board operation, slits are generated in the solid plane, and differential wiring is formed on this plane may intersect.
Characteristic analysis of signal transmission path
To identify the cause of signal transmission problems and create models for waveform simulation, we use a broadband network analyzer to measure the transmission loss, characteristic impedance, and crosstalk of printed circuit boards.
- Identify impedance mismatch points by TDR measurement and present improvement guidelines.
- Acquires S-parameters of connectors and cables, verifies the validity of transmission quality by waveform simulation, and even proposes component changes as necessary.
- Evaluate transmission characteristics in compliance with transmission standards such as SDI. In the case of non-compliance, we identify the cause, perform electromagnetic field analysis as necessary, and present a design change plan.
This time, with the aim of establishing a method for predicting the effect of a slit on the reference plane on attenuation in a microstrip differential pair line, we created a printed circuit board with varying degrees of differential wiring coupling, and measured these TDRs and TDTs. We calculated the attenuation (S dd21) due to the presence of the slit from the measurement results, and reported the results of comparison with the measured values.
The differential wiring is a common FR-4 microstrip with a length of 300mm and a dielectric thickness of 0.12mm. The line width and gap were adjusted so that the differential impedance (Z diff ) was 100Ω and the common mode impedance (Z com ) was 25, 30, and 40Ω.
The reference plane of the microstrip is connected to the GND lead of the SMA connector placed at both ends of the wiring. Based on the board without slits on the reference plane, a slit of 5 mm width was provided only in the second layer in the center of the board longitudinal direction. Created a board.
The electrical characteristics were measured using Agilent Technologies’ 86100C+54754A+86112A as TDR and TDT, and E5071A from Agilent Technologies as a network analyzer.
Results and Discussion
Measurements at a rise time of about 40 ps at the probe tip resulted in a Z diff for substrates with Z com =30Ω that increased by about 6Ω across the substrate slit. It was confirmed that the amount of increase in Z diff at the slit decreased as Z com increased.
3.2 Characteristic impedance of the slit
Z diff is the characteristic impedance during odd mode transmission, and Z com is that during even mode transmission, which are expressed by Equations 1 and 2, respectively.
v odd is the transmission speed during odd mode transmission, v even is the transmission speed during even mode transmission, C1 is the capacitance between the wiring and the solid plane, and C12 is the capacitance between the wirings .
Since Z diff , Z com , v odd and v even without a slit can be measured from TDR and TDT, C 12 can be calculated from Equations 1 and 2 . On the other hand, the characteristic impedance Z diff ‘ of the slit portion can be expressed by Equation.
Here, from the rising edge of the TDR waveform, it can be considered that the change in the propagation velocity due to the presence or absence of the slit is very small, and can be approximated as v odd ‘≈v odd .
In general differential wiring, the electrostatic capacitance C 1 ‘ between the wiring in the slit section and the solid plane is much smaller than C 12 , so it can be approximated as C 12 + C 1 ‘/2 ≈ C 12 . can. Therefore, Z diff ‘ can be approximated by Equation 4 as a function of Z diff , Z com , v odd and v even .
3.3 Prediction of transmission characteristics of slit
When the experimental circuit this time is shown in Fig. 4, the ABCD matrix of the slit part can be expressed as Equation 5.
where β is the transmission constant and d is the slit gap. Equation 5 can be approximated to Equation 6 because βdZ com =25Ω is a fractional number equal to 0.
Also, Equations 7 to 9 can be derived from FIG.
V 2MAX is the voltage across the terminating resistor without slits. If I 1 , I 2 , and V 1 are eliminated from Equations 6 to 9 , the change in attenuation due to the slit is expressed by Equation 10.
3.4 Comparison of calculated and measured values
Fig. 6 shows the calculation result of ⊿S dd21 of the slit part with respect to the change of Z com in the substrate with Z diff =100Ω and slit width of 5 mm . The fact that the smaller Z com is, the greater the attenuation is, is consistent with the trend of the TDR measurement results in. FIG. 6 shows Sdd21 calculated by adding the slit values calculated by Equation 9 to the substrate without slits, the substrate with slits, and the substrate without slits.
As the Z com increases, the actual measurement results show a periodic decrease in S dd21 corresponding to the length between the connector mounting area and the slit due to the impedance mismatch in the slit. No, but the ⊿S dd21 of the slit obtained by the calculation this time almost matches the measured value.
It was found that ⊿S dd21 is attenuated as Z com is smaller when there is a slit in the reference plane of the microstrip differential wiring and the differential wiring crosses , compared to the case without the slit. The characteristic impedance of the differential wiring on the substrate without slits and the attenuation at the slits obtained from the propagation velocities in the odd and even modes agreed well with the measured values.
In the future, we plan to conduct experiments on substrates with different slit lengths to verify the versatility of this prediction method.