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PCB meander line

By Melvin Reynolds,2014-08-08 21:39
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PCB meander line

     Meander Length L=5mm Meander Length L=10mm

    S/h=w/h 72% 69%

    S/h=2w/h 32% 32%

    S/h=3 w/h 17% 19%

    S/h=4 w/h 7% 11%

    Table 1 Effect of trace separation on relative crosstalk

     S/h=w/h S/h=2w/h

    2 meanders 19% 11%

    4 meanders 43% 32%

    6 meanders 79% 48%

    Table 2 Effect of number of meanders on relative crosstalk

    ; Start out with the propagation delay calculated by a hand calculation or a simple

    transmission line analysis. The delay is an easy variable to adjust.

    ; Focus on the signal quality. For good digital signal quality, keep the mplitude of the

    leading crosstalk peak limited to around 15% of the signal swing.

    ; Use Tables 1 and 2 as starting points for the trace separation to height ratio for the

    required signal quality.

    ; Keep the number of meanders to a minimum. Fewer long meanders are better than

    many short meanders.

    ; Minimize the number of sharp corners. While one corner will not degrade the signal

    significantly, many corners can make a significant difference. Use chamfering when

    required.

    ; If the routing area is seriously restricted, use bifilar structures, such as the bifilar spiral. ; Avoid overdesigning the meander. Many overdesigned meanders lead to poor board

    area utilization.

    ; If full wave analysis software is available, use it to select routing strategy and

    especially to verify and fine tune the final structure.

    Special thanks to those who responded to my e-mail question, especially the pointers towards IEEE papers about the subject. The specific papers I found most useful were:

    00496044, Flat Spiral Delay Line Design with Minimum Crosstalk Penalty: talks about how to create a flat spiral to replace the traditional serpentine to reduce the problem. Gives a good qualitative (readable) description of the problem. It deals with LONG traces, however (serpentine lengths in the inch range), so the effect is not quite the same as what we're seeing. I don't know about the effect of the spiral at the short times we're interested in.

    00853205, Characterization of Microstrip Meanders in PCB Interconnects: describes the problem with shorter serpentines (0.4" range) in an understandable format. Gives some recommendation. I'm not sure about one of them - "Keep the number of

    meanders to a minimum. Fewer long meanders are better than many short meanders." And again, I don't know if a flat spiral (AKA "bifilar spiral") will help us.

    00868994, Study of Meander Line Delay in Circuit Boards and 00475270, Laddering Wave in Serpentine Delay Line: quantitative analysis of the phenomena. Have all those funny greek symbols that make my head swim. I didn't find any use in their conclusions. Pure candy for those who like differential equations, poison for the rest of us.

    00410843, Timing Skew of the Equal-Length Serpentines Routing: quoted as a reference by everybody else. If you find anything of value in it, let me know - I'm missing something.

    Increasing trace length with serpentines (AKA "meanders") does not give an increase in flight-time directly proportional to the increase in trace length. Coupling across the serpentine legs causes part of the wave to bypass the serpentine (I would refer to it as a "barreling through the switchbacks" phenomena), reducing the flight-time. What's most surprising is the absence of any mention of the effect, when layout guidelines talk about matching trace lengths to 5mils! The speed-up effects are reproducible in simulations and seem to be only weakly tied to rise-time.

    The effect can be lessened by separating serpentine legs, or routing in stripline. A "flat spiral" (AKA "bifilar spiral") may be an option to lessen the effect, but this needs to be studied at the lengths/times we're concerned with (pS). I don't know whether many short serpentines are better or worse than a few long serpentines. My recommendations would be:

    Increasing trace length with serpentines (AKA "meanders") does not give an increase in flight-time directly proportional to the increase in trace length.

    Coupling across the serpentine legs causes part of the wave to bypass the serpentine (I would refer to it as a "barreling through the switchbacks" phenomena), reducing the flight-time. What's most surprising is the absence of any mention of the effect, when layout guidelines talk about matching trace lengths to 5mils! The speed-up effects are reproducible in simulations and seem to be only weakly tied to rise-time.

    The effect can be lessened by separating serpentine legs, or routing in stripline. A "flat spiral" (AKA "bifilar spiral") may be an option to lessen the effect, but this needs to be studied at the lengths/times we're concerned with (pS). I don't know whether many short serpentines are better or worse than a few long serpentines. My recommendations would be:

    Take pains to avoid serpentines - they're not free.

    If serpentining is necessary, follow ****** layout guidelines which try to address the issue directly by recommending an S/H (trace separation to dielectric thickness) ratio of about 5 to 1.

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