How to make the impedance equal in length in Altium Designer?

littleshrimp

How to make the impedance equal in length in Altium Designer? [Copy link]

 

When drawing equal length in Altium Designer, some networks have resistors in series. How should I draw them?

lemonboard

This is a complete blind spot of knowledge.

damiaa

There is a good method: set the two network classes before and after the series resistor into two different classes and then set them to equal length in the equal length rule.

Finally, calculate the total length carefully and make fine adjustments if it doesn’t work.

littleshrimp

damiaa posted on 2023-12-13 09:33 There is a good way to operate: set the two network classes before and after the series resistor into two different classes and then set them to equal length in the equal length rule...

This is what I do now, but it feels rather troublesome. Also, do I need to take the length of the resistor itself into consideration?

damiaa

littleshrimp posted on 2023-12-13 11:06 This is what I do now, and it feels quite troublesome. In addition, should the length of the resistor itself also be taken into consideration?

The two resistor packages should be the same, right? The arrangement should also be symmetrical, right?

My personal feeling is: differential lines should be parallel and symmetrical as much as possible. They should also be of equal length. At least most of them should be parallel. If vias are necessary, they should be made in pairs. The same applies to changing layers.

However, some devices like USB 2.0 do not have such strict requirements and can be used.

littleshrimp

damiaa posted on 2023-12-13 17:02 The two resistor packages should be the same, right? The arrangement should also be symmetrical. Personally, I feel that the differential lines should be parallel and as symmetrical as possible. ...

You are right, if you use resistors, you don't need to calculate the length of the resistors.

Do we normally have to make the traces on both sides of the resistor equal in length? Or can the two sides be different in length and the overall length be the same?

Making both sides equal in length will waste space because the resistors cannot be in the middle of all the nets.

Nubility

In this case of series resistance, xsignal can be used to make the length equal. I have written such a post before https://en.eeworld.com/bbs/thread-1249394-1-1.html

littleshrimp

Nubility posted on 2023-12-13 17:52 This kind of series resistance can be made equal length by using xsignal. I have written such a post before https://en.eeworld.com/bbs/thre ...

Very useful thanks

damiaa

Nubility posted on 2023-12-13 17:52 In this case of series resistance, xsignal can be used to make equal length. I have written such a post before https://en.eeworld.com/bbs/thre ...

Thanks for sharing!

damiaa

littleshrimp posted on 2023-12-13 17:40 You are right. If you use resistors, you don't need to calculate the length of the resistors. Under normal circumstances, do you need to make the traces on both sides of the resistors equal in length? Or do you need to make the traces on both sides equal in length?

Personal feelings may not be correct. Without experience, I can only rely on my feelings. The differential impedance is actually an AC impedance . So the differential focuses on symmetry, just like a network cable. Every place may cause certain problems due to asymmetry (signal reflection may occur from every place on the line path). Including parallelism, vias, reference planes, length, etc. Some require equal length compensation to be added to the adjacent parts with incorrect lengths. So symmetry is important, and equal length is to compensate for the inconsistency of impedance. As for wasting length. I see that the wiring of many memory devices is winding. It is better to waste less.

littleshrimp

damiaa posted on 2023-12-14 09:34 Personal feeling may not be correct. I have no experience and can only rely on my feeling. The differential impedance is actually an AC impedance. So the difference focuses on symmetry...

What I mean is this, if the two ends of the resistor are made equal in length, when the resistor is not in the middle of multiple equal length lines, there will be extra space on the left and right to make the length equal.

damiaa

littleshrimp posted on 2023-12-14 11:38 What I mean is this, if the two ends of the resistor are separated and made of equal length, when the resistor is not in the middle of multiple equal length lines, there will be extra space on the left and right to make equal...

I have never done this before. Put the resistors on both sides. If they are equal in length when added together, then the equal length is achieved. But in principle, the differential lines of the PCB without resistors should be compensated on the side close to the larger error, in order to be as symmetrical as possible. The vias should be symmetrically punched in pairs, and the lines should also be run in pairs when changing layers. Why can't the resistors be put together?

littleshrimp

damiaa posted on 2023-12-14 16:52 I have never done this. Put the resistors on both sides. If they are equal in length when added together, the equal length is achieved. But in theory, the differential lines of the PCB without resistors should be close to the error...

The example in my picture may not be appropriate. In the picture below, the resistors are placed together, but because the pad positions or wiring methods of the two chips on the left and right are different, a large number of equal lengths have to be made on both ends of the resistor when there is no need to make too many equal lengths as a whole.