Coplanar Strip
Understanding Coplanar Strip
CPS is the natural choice when a balanced (differential) transmission line is needed in a planar circuit. Unlike microstrip (unbalanced) or CPW (semi-balanced), CPS inherently supports the odd mode that differential circuits require.
CPS Properties
- Balanced mode: Two strips carry equal and opposite currents. The electric field crosses the gap, similar to a two-wire line.
- No ground plane: CPS does not require a backside ground. Simplifies substrate via requirements.
- Impedance range: 50-250 ohms. Higher impedance than microstrip, similar to slotline.
- Radiation: More open structure radiates more than enclosed transmission lines.
Applications
- Balanced mixer feeds: Naturally balanced structure for diode pairs.
- Antenna feeds: Feeding balanced antennas (dipoles, patches in differential mode).
- Balun transitions: CPS-to-microstrip transition acts as a balun.
Frequently Asked Questions
What is coplanar strip?
CPS is a balanced transmission line with two parallel conductors on the same substrate side without a ground plane. It is the balanced complement of CPW. Used for balanced mixer feeds, antenna transitions, and balun designs from DC to 100+ GHz.
What is the difference between CPS and CPW?
CPW has a center conductor between two ground planes (unbalanced structure). CPS has two equal conductors with no surrounding ground (balanced structure). CPW is more common for general interconnect; CPS is preferred when a balanced interface is needed.
Where is CPS used?
Balanced mixer circuits, differential antenna feeds, Vivaldi antenna feeds, and as intermediate structures in balun designs. CPS is also used in some lens and planar antenna arrays.