PCB Tech - PCB design signal isometric analysis

PCB design signal isometric analysis
What is PCB signal isometric processing
In the PCB design, in order to meet the total length of all signal lines in a certain group within a tolerance range, it is usually necessary to use serpentine traces to wind the signal line with a shorter total length to the longest signal line length in the group. Within the tolerance range, this process of winding long signal lines with serpentine traces is what we commonly call PCB signal equal length processing.

Why is the length equal? Generally, there are several reasons for the signal equal length processing during PCB design. 1. Generally, the equal length is to satisfy the system's isochronous time for the signal group, that is, to meet the system requirements for the sequence of signals in this group. For example, for DDR, each 8-bit data signal is processed by +/-25mil. If the same length of the signal of this group is not within this tolerance range, the signal line length differs too much, which will cause its relative delay to be longer, and finally As a result, the DDR operating speed is not high. However, when we design, we sometimes find that the DDR device is not equalized, and the finished product can also run normally without affecting it. The reason is generally that the system software delays the signal and the software does timing control. For the stripline, the trace length corresponding to each 1ps delay is about 6mil, so the general signal group length differs by 6mil, and the total delay is 1ps. Generally, we don’t need to control the length of the equal length to be too small when designing. It is good to control it to about +/-10mil. +/-10mil equal length and +/-1mil equal length, the difference in time does not exceed 4ps, and the general IC signal margin is more than 4ps, so there is no need to control too small when doing the equal length, which leads to your own design. The line is difficult. 2. Differential signal (for analysis of differential signal, please refer to previous articles on our public account). The equal length is to meet the phase. A pair of differential signals are 180 degrees apart in phase. If the length difference is too large, the phase offset will be too large. Isometric processing method in PCB design]

1. When designing, we must first look at the data sheet of the device, and obtain the signal of equal length and its equal length range according to the data sheet. For conventional signals, such as DDR, network ports, HDMI signals, etc., the length can be equalized based on design experience. 2. Before making equal lengths, first find the longest signal line in the signal group that needs equal length, and find ways to shorten it to shorten the length of all signal lines in this group and other signal lines that need to be wound. The length of the line. 3. When processing the equal length, consider the winding of the signal in a small space, try to transfer the nearby signal first, and process this part of the signal, so as to avoid the end of the winding space being insufficient and the equal length cannot be obtained. 4. When winding the same length, the edge spacing of the serpentine line should generally be kept at 3W, that is, 3 times the line width. If the space is limited, at least 2W should be required.
5. For differential signals, the equal length error is generally controlled to +/-5mil, and the equal length position is at the end where the length error occurs, and the winding waveform is a small waveform.
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