A rugged, low-cost RF probe can be made from materials readily available in the lab. This probe is useful for making relative measurements while troubleshooting a low-impedance circuit. Applications include locating the source of spurs, measuring relative spur and harmonic levels, and identifying a failed amplifier or SAW filter. When probing a 50 Ω point in a circuit, the probe is useful between about 400 kHz and 1 GHz.

The probe consists of an SMA connector followed by a series capacitor and resistor, which is further connected to a short semi-rigid coax that serves as the probe tip (Figure 1). The SMA connector connects to a spectrum analyzer through a standard SMA cable. The medium impedance (1 kΩ) of this probe enables its use in low impedance, i.e. 50 Ω, circuits without significant impact on 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 here are 1000 pF and 1 kΩ.

Figure 2 shows the response flatness of the probe is 1 dB from 400 kHz to 1 GHz. Resulting measurement error is small compared to normal discrepancies of more than 10 dB encountered during troubleshooting, such as when an amplifier fails. Furthermore, the maximum frequency of the probe is extended to 1.9 GHz by removing the coax and using the resistor itself as the probe tip. This reduces the resonance that peaks near 3 GHz, as seen in Figure 2. Although this modified probe is easily damaged, it illustrates the degree to which the coax parasitics affect response flatness. The 11 mm length of the coax in Figure 1 is selected as a trade off between better RF performance for a shorter coax and improved mechanical robustness for a longer coax, due to increased solder attachment surface.

The measured power level at the SMA connector on the probe is theoretically -26.4 dB below that at the probe tip, assuming a simple voltage divider of 50 Ω/1050 Ω. This matches the plot in Figure 2 quite well between 400 kHz and 1 GHz. The frequency response in Figure 2 is obtained by probing across a 50 Ω resistor that is connected across the output of a calibrated signal generator. Furthermore, an ideal 1 k Ω probe loads a 50 Ω circuit by only -0.2 dB, assuming 1 kΩ plus the 50 Ω spectrum analyzer in parallel with the 50 Ω circuit.

To assemble the probe shown in Figure 1, first cut a scrap PC board that includes a 50 Ω microstrip transmission line with an SMA bulkhead connector. Carefully cut gaps in the strip where a 1000 pF capacitor followed by a 1 kΩ resistor are soldered. Both components are 0603 size. Cut an 11 mm length of 0.086-inch diameter semi-rigid coax. Expose 2.5 mm of the center conductor on one end and 2 mm on the other end. The shorter end is the probe tip. The longer end is bent and soldered to the strip near the resistor, after removing the remainder of the strip under the coax. Further attach the coax by bridging solder from the outer conductor to nearby ground metalization. Use plenty of solder to attach the coax to the PC board robustly. If necessary, use copper tape to bridge the gap. Parallel to the coax solder a piece of coax center conductor to serve as the ground contact. Add a small glob of solder on the probe and ground tips to help them contact the circuit properly.

Bryan Irons is a strategic applications engineer with Maxim Integrated Products. Previously, Irons worked as an RF design engineer at Motorola for 10 years. He received a BSEE from Kansas State University in 1991 and an MSEE from the University of Kentucky in 1993. He may be reached by e-mail at bryan_inrons@or.mxim.com.

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