2. **Understanding Impedance**
Let’s first clarify a few key concepts. We often encounter terms like impedance, characteristic impedance, and instantaneous impedance. While they are technically distinct, they often overlap in practice. Fundamentally, they all relate to the basic definition of impedance:
A) The input impedance at the beginning of a transmission line is commonly referred to as “impedance.”
B) The instantaneous impedance encountered by the signal at any given moment is called the “instantaneous impedance.”
C) If the transmission line maintains a constant instantaneous impedance, this is known as the “characteristic impedance” of the transmission line.
1. **Characteristic impedance** refers to the transient impedance encountered by a signal as it travels along a transmission line. This is a critical factor that influences the signal integrity within the transmission line circuit.
In the absence of specific instructions, characteristic impedance is typically used to refer to the impedance of the transmission line as a whole. The key factors that impact characteristic impedance include the dielectric constant, dielectric thickness, trace width, and copper foil thickness.
2. **Impedance continuity** is analogous to:
Water flowing steadily in a uniform channel, which suddenly bends and widens.
As the water flows around the bend, it sways and generates waves.
This represents the effect of impedance mismatch.
3. **Solutions to impedance discontinuities**
– **Corners**
When an RF signal trace takes a sharp right-angle turn, the effective trace width at the corner increases, causing impedance discontinuities that lead to signal reflections. To minimize these discontinuities, there are two common solutions: chamfering or rounding the corners. The radius of the rounded corner should be large enough to ensure that R > 3W, where W is the trace width.
– **Large pads**
Large pads on a 50-ohm microstrip line can act as distributed capacitance, disrupting the continuity of the line’s characteristic impedance. To mitigate this, two approaches can be employed simultaneously: first, increasing the thickness of the microstrip line’s dielectric, and second, hollowing out the ground plane beneath the pad. This reduces the distributed capacitance of the pad.
– **PCB vias**
Vias are one of the primary causes of impedance discontinuities in RF circuits. Factors such as via diameter, pad size, depth, and anti-pad dimensions all influence impedance, potentially causing discontinuities, reflections, and increased insertion loss. For frequencies above 1 GHz, the impact of vias must be carefully considered.
Common methods to reduce via-induced impedance discontinuities include: using a via-less process, selecting appropriate outlet techniques, and optimizing anti-pad diameter. Optimizing the anti-pad diameter is a widely used approach to minimize impedance mismatch.
– **PCB through-hole coaxial connectors**
Much like vias, PCB through-hole coaxial connectors can introduce impedance discontinuities. The solutions for these are similar to those used for vias. Common strategies to reduce the impedance discontinuity of through-hole coaxial connectors include: employing a via-less process, choosing suitable outlet techniques, and optimizing anti-pad diameter.
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