PCB Layout - Ground Shielding of High Speed Internal Traces
PCB Layout - Ground Shielding of High Speed Internal Traces
(OP)
I have a 10-layer PCB design with a controlled impedance of 50 Ohm +/- 10% for all four internal signal planes. On one of the signal planes, I have a signal that carries a 25 MHz clock signal for an LCD. This signal appears to contain several other frequencies and is far from the intended square wave I was hoping for. The trace itself is routed away from other high frequency signals and switching circuits and any other signals that are routed by it are perpendicular. Other associated LCD data signals look good.
I'm wrestling with the idea of running a ground shield trace on both sides of this signal to help protect it from any unwanted EMI. I've seen this done on external layers, but never on an internal layer. I have concerns with it also interfering with the controlled impedance.
Is shielding this signal an appropriate move? Will it interfere with the controlled impedance? Is this typically avoided on internal signal layers?
I'm wrestling with the idea of running a ground shield trace on both sides of this signal to help protect it from any unwanted EMI. I've seen this done on external layers, but never on an internal layer. I have concerns with it also interfering with the controlled impedance.
Is shielding this signal an appropriate move? Will it interfere with the controlled impedance? Is this typically avoided on internal signal layers?





RE: PCB Layout - Ground Shielding of High Speed Internal Traces
Mike Halloran
Pembroke Pines, FL, USA
RE: PCB Layout - Ground Shielding of High Speed Internal Traces
RE: PCB Layout - Ground Shielding of High Speed Internal Traces
I have made the effort to eliminate potential noise sources from the circuit by disabling and bypassing the few power supplies I have available. I have also disabled other potential sources including memories and communication buses. All has had little to no effect on the signal. All other LCD RGB data signals that are routed in the same area with the same length look very clean. Regarding the ~3" flex cable for the LCD, I do not know if it has a controlled impedance. I can tell you that the signal looks the same regardless of whether the cable is connected or not.
So far, I've been able to clean up the signal through a simple RC filter. It looks better, but it's ultimately not the clean signal I'm after. At the moment, I'm modifying the design to include a couple new circuits, including a spread spectrum clock IC and a high-speed comparator just to try out. The LCD works fine as it is, but I'm concerned with any reliability or stability issues this signal may cause under unknown or noisy conditions.
RE: PCB Layout - Ground Shielding of High Speed Internal Traces
When you say, "far from the intended square wave", what does this signal look like? Stair stepping and ringing, as well as runt or inverted pulses are strong indications of a reflections problem.
Also, is your driver capable of delivering into a 50 ohm load? In my experience, 50 ohms is pretty stiff. What are the dimensions of the trace (width, height to reference)? How much variation is there and do you do things like make 90 degree turns?
RE: PCB Layout - Ground Shielding of High Speed Internal Traces
RE: PCB Layout - Ground Shielding of High Speed Internal Traces
Best to you,
Goober Dave
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RE: PCB Layout - Ground Shielding of High Speed Internal Traces
RE: PCB Layout - Ground Shielding of High Speed Internal Traces
RE: PCB Layout - Ground Shielding of High Speed Internal Traces
With (suspect) difficult scope probing situations, I've found using a dual trace scope, using two scope inputs with "Add & Invert" options set between the two scope channels (to create a fully isolated, differential scope input), one probe tip on your ground (or signal) reference point, and the other probe tip on the signal of interest..
Scope probe ground leads are NOT connected.
This eliminates any ground loop and common mode noise between the scope and the circuit of interest, subject to the longitudinal balance specs of the scope which should be very high...