What is characteristic impedance?
Characteristic impedance:It is not a DC resistance, but a concept in long line transmission. In the high frequency range, the signal transmission process, the signal along the arrival place, the signal trace and the reference plane (power or ground plane) between the establishment of the electric field, there will be a momentary current.
If the transmission line is isotropic, a current I is always present as long as the signal is being transmitted, and if the output voltage of the signal is V, the transmission line is equated to a resistance of size V/I during signal transmission, calling this equivalent resistance the characteristic impedance Z of the transmission line.
During the transmission of the signal, if the characteristic impedance on the transmission path changes, the signal will be reflected at the junction where the impedance is not continuous.
Factors affecting the characteristic impedance?
The factors that affect the characteristic impedance are: Dielectric constant, Dielectric thickness, Wire width, and Copper foil thickness.
Solutions To Impedance Discontinuity Problems In PCB Design
1. Tapering line
Some RF component packages are small, SMD pad width may be as small as 12 mils, while the RF signal line width may reach more than 50 mils, so we need to use the gradient line, to prohibit abrupt changes in line width.
If RF signal lines at right angles, the effective line width of the corner will increase and impedance discontinuity, causing signal reflection. In order to reduce the discontinuity, the corner should be processed, there are two methods: tangent and rounded corners. The radius of the rounded corner should be large enough, in general, to ensure that: R>3W.
3. Large pads
When there are large pads on a 50-ohm microstrip line, the large pads are equivalent to the distributed capacitance, which disrupts the characteristic impedance continuity of the microstrip line. Two methods can be taken at the same time to improve: first of all, the microstrip line dielectric thickened, and secondly, the ground plane below the pad will be hollowed out, both can reduce the distributed capacitance of the pad.
4. Tenting Vias
The tenting vias are metal cylinders plated outside the through holes between the top and bottom layers of the board. Signal vias connect transmission lines on different layers. The vias stubs are the unused parts of the vias. The vias pads are circular spacers that connect the vias to the top or internal transmission lines. Isolation disks are annular gaps within each power or grounding layer to prevent short circuits to the power and grounding layers.
5. Through-hole coaxial connectors
Similar to through-hole structure, through-hole coaxial connectors also have impedance discontinuity, so the solution is the same as through-hole. The common methods to reduce the impedance discontinuity of through-hole coaxial connectors are the same: using diskless process, suitable wire exit, and optimizing the counter pad diameter.
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