Issue link: https://iconnect007.uberflip.com/i/1337116
FEBRUARY 2021 I DESIGN007 MAGAZINE 61 trace (1.5V @ 1GHz). In this case, the traces are 4 mils wide, 40 ohms impedance with a 4-mil spacing. Crosstalk falls off rapidly with the square of the distance and the degree of impact is related to the aggressor signal volt- age, the proximity of trace segments, and proximity to the plane(s). In an outer layer microstrip configuration, the mutual capacitive coupling between adja- cent traces is generally weaker than the mutu- ally inductive coupling, driving the FEXT co-efficient negative, as can be seen in the simulation. However, forward crosstalk does not exist in the stripline configuration. e fine balance between inductive and capacitive coupled crosstalk produces almost no observ- able forward crosstalk (Figure 3). is shows the near-end crosstalk of a stripline configura- tion for 4 mil wide, 40 ohms impedance traces with a 4-mil spacing. Notice how there is no FEXT component of the noise. Also, the peak amplitude of the crosstalk has been consider- ably reduced. All other factors being equal, here is just another good reason why one should always route high-speed signals on the inner layers of a multilayer PCB. Stripline edge coupled signals can also be placed closer to each other compared to the microstrip equiva- lent leaving more space for routing, which is always welcomed. e easiest way to reduce crosstalk from a nearby aggressor signal is, of course, by increasing the spacing between the signals in question. Crosstalk falls off very rapidly with distance. Crosstalk plummets roughly quadratically with increased separation. By doubling the spacing, it cuts the crosstalk to roughly a quarter of its original level. A good rule of thumb for this is Gap = 3X trace width. However, in today's complex designs it is not always possible to use up valuable real estate to satisfy the above. Also, different technologies should not be mixed as higher voltages create higher crosstalk. And long parallel trace seg- ments should be avoided. Figure 4 shows the effect of the edge coupling on the crosstalk for both microstrip (outer lay- ers) and stripline (inner layers). Note that the stripline has about one-quarter the crosstalk of the microstrip. Also, microstrip crosstalk Figure 3: Crosstalk for stripline with 4/4 mil trace width/clearance.