The constantly improving technology of electronics devices and circuits, the need of miniaturization of electronic products is in demand in today’s market place. Hence a complete product with all the components and circuitry on one single PCB has a great potential. This art of assembling all components in an optimum fashion so as to avoid any problems in performance of device and improve quality of performance of device is in high demand. The circuit designers and PCB layout design engineers with experience in this art are approached by high tech electronic engineering companies so as to become part of their R&D team and cope up with the design issues while also optimizing the design to minimize the dimension of product overall.
In the process of miniaturization of electronic product the low speed analog and high speed digital circuits are mostly placed on one single PCB, thus various issues arise like EMC / EMI and cross talk issues. According to the electromagnetic theory we know that any two current carrying conductors there is a mutual coupling between them that results in noise or interference to be transmitted from “aggressor” to the “victim” or we can say from transmitter to the receiver conductors. We will defined these terms in this article, although this article discusses about the effect on the cross talk intensity between two micro strip lines on PCB by changing various parameters as will be discussed in the section below.
What is Cross Talk..?
The cross talk is actually the interference caused by high speed or high frequency signal transmission lines like PCI Express, DDR2, HI-Speed USB circuits or SDRAM to other low speed analog circuitry or trace/path like OPAMP, transistor gate drive circuits etc on PCB.
Types of Cross Talks:
The cross talk is of two types. Near End Cross Talk (NEXT) and Far End Cross Talk (FEXT). Suppose two microstrip lines are fabricated on top of FR-4 dielectric based PCB (PCB dimensions 60mm x 20mm) with a distance between them is “d” and length of each strip is “L” and width is “w” and on bottom the image ground plane is fabricated. The distance between the microstrip lines and the image plane is “h”
So the cross talk will exist between the two ends of transmitter and receiver as shown in diagram below. The signal line or transmitter or aggressor or generator which is connected to the signal source (Vs) and has the characteristic impedance ZOG while the same is terminated by load impedance ZLG. On the other hand the receiver or victim has no source connected but the characteristic impedance is ZOR and this line is terminated by the load impedance ZLR. Now you have understood what is meant by aggressor and victim.
The cross talk between P1 and P3 is “Near End” and P1 and P4 is “Far End”. The resulting cross talk due to induced current in case of NEXT is the sum of the noise currents due to Cm and Lm while the resultant cross talk induced current is the difference of noise currents due to Cm and Lm. Where Cm and Lm are mutual capacitive and inductive coupling respectively.
Capacitive (Cm) and Inductive (Lm) Coupling:
Now as we have seen from below diagram that the noise current is induced due to mutual capacitive (Cm) and mutual inductive (Lm) coupling. The current flowing (1) from Vs to transmitter/aggressor load through microstrip line will cause current to flow in transmitter (High Speed Circuit) and will cause noise current to induce in other parallel microstrip line of receiver/victim.
The noise current (2) in receiver due to capacitive coupling (Cm) will flow in both direction (reverse and forward) along microstrip with same polarity while the noise current (3) caused by inductive coupling (Lm) will flow in both directions with opposite polarity
Characteristic Impedance of Microstrip Line:
The formula for the microstrip characteristic impedance in practical circuits while designing the PCB layout can be given as
=Dielectric Constant of FR-4 PCB = 4.5
h = Distance between the microstrip and image plane (power or ground) = 0.2mm
w = width of microstrip line = 0.37mm
t = thickness of microstrip line = 0.00023 mm 0
For our case we can simply put the values in above equation and find characteristic impedance of our microstrip line for simulation purpose.
Difference between the Microstrip Line, Strip Line and Coplanar Line:
The difference between the two is that in microstrip the FR-4 PCB substrate has no image plane on top of the microstrip line or we can say that the microstrip line is not in between the two image planes while in strip line the conductor line is in between the two image planes either ground or VCC or VDD i.e. power. The coplanar line are different because the top and bottom image planes are of same kind with both are GND or both VCC. And the top and bottom planes are internally interconnected with through holes in coplanar Line. The diagram below shows the difference in three types of conducting lines on PCB.
Simulation and Analysis:
Sweeping the frequency from 0GHz to 15GHz between Microstrip Lines:
Now that we have arranged the two microstrip lines on PCB as discussed in previous section. It is now time to check the behavior of cross talk between the two lines are different signal frequencies. We first check the cross talk signal (noise current) strength at range of frequencies from 0GHz to 15GHz. The circuit is actually four port network with P1, P2, P3 and P4 where we will take (P1, P4) as S14 and it is Far End cross Talk. (P1, P3) will be S13 and it is Near End cross talk. The S-parameters that will be analyzed are S13 and S14. As can be seen from the below graph from our simulation that S13 (NEXT) parameter have low cross talk signal strength while S14 (FEXT) parameter have higher cross talk strength. Also we can deduce that as the distance between the two microstrip lines is increased from d=1mm to d=2mm then the cross talk strength decreases for both S13 and S14.
Changing the length of Microstrip lines at various frequencies:
Now we check the influence of changing the microstrip length on cross talk strength. Microstrip line’s length interval is taken from 5mm to 40mm. The signal frequency of interest is 1GHz, 5GHz, 9GHz and 13GHz. We see that the increasing the length of microstrip line will increase the cross talk strength in dB as shown on vertical axis. Especially in FEXT S14 parameter the cross talk is increased dramatically as the length of microstrip is increased more than 35mm for 1 and 5 GHz frequency analysis. While the same is also observed for 9 and 13GHz frequency range.
Changing the distance between the microstrip lines:
As we can see that the increasing distance between the two microstrip lines is decreasing the cross talk strength at all four frequencies of interest i.e. 1GHz, 5GHz, 9GHz and 13GHz. However the S14 parameter at 5GHz has higher cross talk than S14 at 1GHz and S13 at 1GHz and 5 GHz.
Increasing the Thickness of PCB:
The increasing thickness of CB will increase the distance between the microstrip and image plane hence value of “h” is increase and this will result in increasing the cross talk strength. However we can see that S14 parameter at 5GHz has significantly larger cross talk than other three parameters. While S14 parameters at 9GHz and 13GHz are also having higher cross talk than S13 parameters at 9GHz and 13GHz. Note that the cross talk intensity is increasing by increasing the PCB board thickness
It can be concluded that, decreasing the signal edge rate for high speed digital circuits is not an option because the interval time is very small. So we have to go for the PCB layout microstrip spacing technique discussed above. From simulation results it is shown that higher frequency, closer gap between two microstrips, longer microstrips and thicker PCBs will cause greater cross talk strength and vice versa.
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