Column: Embedded design


In praise of the resistive probe


By Myk Dormer, RF Engineer and Director, Smallwireless Limited I


n an ideal world, RF circuit elements are neat, self-contained blocks, connected together by well- behaved 50Ω-matched signal paths, which can be uncoupled as needed, to connect each circuit to test


equipment for accurate measurements. Unfortunately, this ideal of easy ‘plug-


together’ elements might exist in colleges or advertisements for connectorised modules, but, in the real world of small, low-power wireless designs, it’s not so simple. Circuits are tiny and closely packed.


T ey are complex, and convenient “breaks” in the signal path rarely present themselves. Well-behaved 50Ω-matched ports are unusual, with high and low impedance nodes being more common. Debugging real circuits is a trial.


Sometimes you can remove a part and solder in a coax “tail” to connect your test instrument – usually a spectrum analyser or a ‘scope. More oſt en you will need to monitor a point in the circuit where you can’t break in a 50Ω port. Here we must use a (voltage) probe to access the signal, hopefully without disturbing the rest of the circuit. Just soldering in a 50Ω coax doesn’t


cut it. Conventionally, either a high impedance (‘scope) probe, or an “active” RF probe would be used. Both have their limits: Oscilloscope probes are compatible with the very high resistance inputs of typical bench ‘scopes (1MΩ resistance and tens of picofarads). T e probe then uses a complex reactive divider and an esoteric resistive core coax to present 10MΩ and about 10-15pF at the tip, with a -3dB bandwidth from 100MHz to over


10 May 2026 www.electronicsworld.co.uk


Well-behaved 50Ω-matched ports are unusual, with high and


low impedance nodes being more common


350MHz, depending on the probe. T ese have been staple tools of the bench


engineer for many decades. T e problem is that they are incompatible with the 50R input ports of RF test equipment and, at higher frequencies, 10pF capacitive load has a serious eff ect on the probed circuit. T is is more than enough to de-tune a VHF resonant circuit and, at 800MHz, that’s a reactance of only 20Ω. Active RF probes are the obvious


answer. T ey package a wide bandwidth RF amplifi er into a slightly bulkier probe body and, taking the 500MHz bandwidth Keysight 41800A as an example, deliver an at-the-tip impedance of 1MΩ and under 1pF. Even better, these probes have 50Ω outputs and connect directly to spectrum analysers. T is is a perfect solution if you can aff ord it. Luckily, for practical engineers on a


realistic budget, there’s an alternative: Make your own resistive probe. Commercial designs can be on sale, but you might as well pick up your iron and build one. Start with a coax “tail”: RF connector on one


end, stripped open end at the other. Add a 51Ω resistor from the central conductor to ground, roughly matching the coax. Put a 1nF cap in series with a resistor (see later) between the coax centre wire to the tip – which can be just the sharpened end of a component lead, or a spring-loaded test probe. An earth connection (especially at UHF)


is optional. You can build out of leaded parts “in the air” and stabilise with epoxy. You can use SMT parts on a small patch of FR4. You can even make a PCB. A series resistor isn’t especially critical.


I use anything from 470R (lower loss, but more loading), up to 4K7, with 1K5 being a fair compromise. It’s not highly accurate, it doesn’t


compare with a real active probe, but, within understood limits, it is a valuable tool. It has a lot of loss (1K5 in series, and


50R to ground is a 1:31 divider), and it measures RF voltage, not power (so the “dBm” on your analyser won’t mean much). T ere is no attempt to compensate for stray reactance, so the bandwidth won’t be particularly fl at. It’s not a precision instrument; however,


for comparative signal level tests up and down an RF signal chain, for comparing the signal levels between suspect and known-good circuits, or for sniffi ng off a local oscillator signal and inspecting its spectrum, it is invaluable. And if you lose or break it, just make


another one!


T is column on embedded design prepared by Myk Dormer continues in the next month’s edition of Electronics World.


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