Microstrip Guideline by Vehicle Dynamic Module PCB Copying

Microstrip Guideline by Vehicle Dynamic Module PCB Copying

This example touches an interesting and quite handy point. Reference 17 discusses a useful guideline pertaining to microstrip impedance on Vehicle Dynamic Module PCB Copying. For a case of dielectric constant of 4.0 (FR-4), it turns out that when W/H is 2/1, the resulting impedance will be close to 50 Ù (as in the first example, with W=20 mils).

Careful readers will note that Eq. 9.21 predicts Zo to be about 46 Ù, generally consistent with accuracy quoted in Reference 17 (>5%).  The IPC microstrip equation is most accurate between 50 Ù and 100 Ù, but is substantially less so for lower (or higher) impedances. Reference 20 gives tabular results of various PCB industry impedance calculator tools.

The propagation delay of the microstrip line can also be calculated, as per Eq. 1-3. This is the one-way transit time for a microstrip signal trace. Interestingly, for a given geometry model, the delay constant in ns/ft is a function only of the dielectric constant, and not the trace dimensions (see Reference 21).

equation 1-3

Note that this is quite a convenient situation. It means that, with a given PCB laminate (and given ε r), the propagation delay constant is fixed for various impedance lines. This delay constant can also be expressed in terms of ps/in, a form which will be more practical for smaller PCBs. This is:

equation 1-4

Thus for an example PCB dielectric constant of 4.0, it can be noted that a microstrip’s delay constant is about 1.63 ns/ft, or 136 ps/in. These two additional guidelines can be useful in designing the timing of signals across PCB trace runs.