izsurk
Newbie level 6
[Basics] Why is (RF) current "flowing back" underneath the "forward line" ?
Hi,
Let's talk about a ground plane:
I have a "problem" in thinking about how "return current" will flow. In any signal integrity & EMC book / advice one can find the statement that "RF current will flow back on a GND plane directly under the signal line". That is why you try to minimize the "current-loop", avoid plane splits, ...
Obviously they are talking about the maximum of current density in the return path. Maximum current density will occure on the path where minimal resistance exists (for the particular frequency). But why does the RF current see minimal impedance there?
Obviously minimum / optimal (?) impedance exists in the (physically longer) path following the RF-forward-trace than the physicall shortest way on the copper plane which is intuitivly thought to have the lowest resistance.
Similar on a microstrip line. Mircrostrip impedance is defined by tracewidth and distance to groundplane (and eps_r) in most modells. So obviously return-current must be flowing underneath the microstrip otherwise the "distance to groundplane"-term would be invalid.
So why do I have the lowest impedance directly underneath the forward-line (e.g. microstrip). A im interested in the physical effect and some ressources which would handle this topic. Seem I am stuck somewhere in the basics ...
Thank you in advance
Hi,
Let's talk about a ground plane:
I have a "problem" in thinking about how "return current" will flow. In any signal integrity & EMC book / advice one can find the statement that "RF current will flow back on a GND plane directly under the signal line". That is why you try to minimize the "current-loop", avoid plane splits, ...
Obviously they are talking about the maximum of current density in the return path. Maximum current density will occure on the path where minimal resistance exists (for the particular frequency). But why does the RF current see minimal impedance there?
Obviously minimum / optimal (?) impedance exists in the (physically longer) path following the RF-forward-trace than the physicall shortest way on the copper plane which is intuitivly thought to have the lowest resistance.
Similar on a microstrip line. Mircrostrip impedance is defined by tracewidth and distance to groundplane (and eps_r) in most modells. So obviously return-current must be flowing underneath the microstrip otherwise the "distance to groundplane"-term would be invalid.
So why do I have the lowest impedance directly underneath the forward-line (e.g. microstrip). A im interested in the physical effect and some ressources which would handle this topic. Seem I am stuck somewhere in the basics ...
Thank you in advance