Constructing rugged, low-cost RF probe

The RF probe consists of a SMA connector, followed by a series capacitor and resistor, then by a short semi-rigid coax that serves as the probe tip. The SMA connector connects to a spectrum analyser through a standard SMA cable. The medium impedance (1kΩ) of this probe enables its use in low-impedance, i.e., 50Ω, circuits without significant impact to the circuit. The resistor value is chosen large compared to the impedance of the circuit—on the order of 20:1 works well. The blocking capacitor value is selected to be self-resonant in the middle of the frequency band of interest. This ensures low impedance relative to the resistor value. The 0603 size components selected in this application note are 1000pF and 1kΩ.

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