PCB foil odity question

[pandaemonium]

I was tearing apart a broken cam-corder today (for nifty parts) and noticed that in many places that the foil is folded in loops, that is to say, instead of going in a straight line from point A to B, it goes up and down and up and down several times before going to point B with no other components on the foil, they looked like a PCB implementation of antennae, but I'm fairly certain they where not. Is this maybe used for impedance matching?

 

[Exo]

It acts as a tiny capacitor...

 

[laroche73]

foil squiggles

It acts as a tiny capacitor...

and also as a tiny inductor. Some high end PCB layout programs offer this as an option to help control trace impedance.

 

[Roff]

It sounds like a delay line to me. This is commonly done on PCBs to match routing delays or to to match the delay of some component or circuit on the board.
A trace over a ground plane is a transmission line. If terminated in the characteristic impedance of the line, it will have delay equal to the length of the line divided by the propagation velocity. The propagation velocity is the speed of light divided by the square root of the effective dielectric constant of the board material (for stripline). See Measurements and Modelling of Delay Lines on Printed Circuit Boards.

 

[laroche73]

when to treat a trace as a transmission line

That makes sense. My brain's gone fuzzy with time. There was a useful rule of thumb about when you needed to be concerned about transmission line effects on PCB traces. It had to do with output edge (slew) rates vs. distance to the load. I don't remember it offhand, some help here Ron? Your description is accurate, but simple rules of thumb and catch phrases (like ElI the IcE man) usually stick in the memory better. In general, you're safe (within reasonable limits) ignoring transmission line effects with CD4000 & 74HC logic design. - Claude

 

[Roff]

You need to terminate the line (source and/or load) when the round trip delay exceeds the signal risetime. For stripline (buried line with ground plane on both sides), delay is about 180ps/inch for epoxy-glass board. For microstrip (surface line above ground plane), delay is about 140ps/inch. So, let's say you have a microstrip line that is 6" long. The round trip delay is Td=2*6*140=1.7ns. If your signal risetime is less than about 1.7ns, you should terminate the line or make it shorter.