FPGA Central

Hello,

could you tell me if there`s an rule of thumb at which width of the trace, the trace is working as a plane depending on the signal frequency?

For example: the width of the trace for a signal-frequency 500kHz must be 0.15mm to get a 100 ohm impedance. If the width of the trace gets bigger, the impedance gets smaller -> you will get more radiation (no exact impedance matching).

But what will happen when the width of the trace gets more and more bigger....? At which point will the "trace width" work as plane and how do I have to calculate the correct impedance of this plane (microstrip calculator do not work with such width of trace)? Are there any kind of formulas?

best regards
Daniel

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Daniel,

The answer is dependent on your definition of "work as a plane."

My assumption, though, is that the transition from "working like a
transmission line"
to "working like a plane" is when the mode of signal propagation
transitions from transverse electromagnetic (TEM) to one of the
evanescent TE or TM modes. I forget the mode enumeration, but the first
evanescent mode of an infinitely long uniform stripline occurs when the
width of the trace approaches that of half-wavelength in the dielectric.
At this point, the electric field potential varies across the width of
the trace, and signals no longer adhere to TEM mode.

For example, a stripline in FR-4 with a relative permittivity of 4.0
will have a half-wavelength at 1 GHz of about 75 mm (3 inches). In
other words, striplines at 1 GHz need to be approaching 75 mm in width
before these modes prevail.

The relationship is inversely proportional. So, higher frequencies
imply narrower traces. e.g., 10 GHz equates to 7.5 mm (.3 inches).

Keep in mind, however, other issues can come up when you use very-wide
traces, and it is not recommended to come anywhere near these widths.
The above is simply a first-order theoretical limit, and other
second-/third-order issues can creep up long before this limit. For
example, if the dielectric thicknesses approach half-wavelength, other
modes will be propagated. Also, adjacent structures can excite yet
different modes. The moral of the story is to stay far away from
evanescent modes.

Good luck,
Pat Zabinski
Mayo Clinic




>
> Hello,
>
> could you tell me if there`s an rule of thumb at which width
> of the trace, the trace is working as a plane depending on
> the signal frequency?
>
> For example: the width of the trace for a signal-frequency
> 500kHz must be 0.15mm to get a 100 ohm impedance. If the
> width of the trace gets bigger, the impedance gets smaller ->
> you will get more radiation (no exact impedance matching).
>
> But what will happen when the width of the trace gets more
> and more bigger....? At which point will the "trace width"
> work as plane and how do I have to calculate the correct
> impedance of this plane (microstrip calculator do not work
> with such width of trace)? Are there any kind of formulas?
>
> best regards
> Daniel
>
>
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