EMC Question of the Week: May 22, 2017
Some of the power in differential-mode signals propagating down a transmission line is converted to common-mode power when there is a change in the transmission line's,
- characteristic impedance
- propagation velocity
- electrical balance
- any of the above
Answer
The correct answer is "c". Differential-mode to common-mode conversion takes place if (and only if) there is a change in the electrical balance of the transmission line. For example, differential signals are typically conveyed on balanced transmission lines (each conductor in the conductor pair has the same impedance to ground). If there is a change in the impedance to ground that affects one of the conductors more than the other, mode conversion will occur at the point of the change.
Changes in the characteristic impedance will result in signal reflection at the point of the change, but there will be no mode conversion if the electrical balance is maintained.
Changes in the propagation velocity can occur if the dielectric changes, but this will not result in signal reflection or mode conversion if the characteristic impedance and electrical balance are maintained.
An important rule of thumb in situations where mode conversion would be undesirable is to route single-ended signals on unbalanced transmission lines (e.g. microstrip traces and coaxial cables) and to route differential signals on balanced transmission lines (e.g. differential trace pairs and twisted wire pairs).
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