capable devices, it still requires considerable design skill to maximise its RF performance. In particular, the engineer needs to consider factors such as power supply filtering, external crystal timing circuits, antenna design and placement, and crucially, impedance matching. The key parameter that differentiates a good RF circuit from a poor one is its impedance (Z). At high frequencies, such as the 2.4GHz used by a short-range radio, the impedance at a given
point on an RF trace is related to the characteristic impedance of that trace, which in turn depends on the printed circuit (pc) board substrate, dimensions of the trace, its distance from the load, and the load’s impedance. It turns out that when the load impedance – which for a transmitting system will be the antenna and for a receiving system is the transceiver SoC – is equal to the characteristic impedance, the measured impedance remains the same at any distance along the
trace from the load. As a result, line losses are minimised, and maximum power is transferred from the transmitter to the antenna, thereby boosting robustness and range. That makes it good design practice to build a matching network that ensures an RF device’s impedance is equal to the pc board trace’s characteristic impedance. (See, Bluetooth 4.1, 4.2 and 5 Compatible Bluetooth Low Energy SoCs and Tools Meet IoT Challenges (Part 2) .)
The matching network comprises
Figure 2: The Nordic nRF52840 requires external circuitry to exploit its functionality. External circuits include input voltage filtering, support for external crystal timing, and connected to the SoC’s antenna (ANT) pin, impedance matching circuitry between the SoC and an antenna. Image source: Nordic Semiconductor
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