The Tech Blog

Kicking off an oscilloscope measurement and probe series with passive voltage probes. These are the most basic probes you see with any oscilloscope you buy. The only job of an oscilloscope probe is to transmit the measured signal(with all its freq components) reliably from the circuit to the front-end circuit of an oscilloscope without affecting the circuit it measures. How do you do that? Using your age-old voltage divider network with a high load impedance. This high impedance will cause the maximal signal to be dropped across the impedance so that you get the full signal. This is why you see most oscilloscopes with a 1MΩ input resistance.

In order to measure a large range of amplitudes(so as to not damage the oscilloscope front-end measurement circuits), probes usually come with an attenuation factor built in called 1x,10x 100x, or Any x for that matter. It means that the signal is divided(NOT multiplied) by the factor mentioned. That means if a signal with 1Vpp in amplitude is measured with a 1x probe, the oscilloscope front end sees 1Vpp. Whereas with a 10x probe, this value will be 0.1Vpp. The images show the probe circuitry for 1x, 10x, and switchable 1x/10x probes. A 10x probe contains a 9MΩ resistor in series. It becomes a voltage divider and the Reff of the measurement circuit is = 1M/(9M+1M) = 1/10 or a 10x attenuation. You have a tunable capacitor Cp in parallel with the 9MΩ resistor for compensation(which we will get to later on in the series).

When to use them? 1x probes have high sensitivity and are great for low amplitude and low freq signals but have low bandwidth(We will discuss Why later on) whereas 10x probes have higher bandwidth, higher input impedance means the circuit being measured is not affected(Or not “Loaded” as you might say) and you can measure higher voltage amplitude range. When seeing the signals on screen always make sure that the correct setting on screen is selected based on if a 1x or 10x probe is used by you. I can’t even begin to count the number of times I goofed up that setting while measuring in my early days.

Recently someone contacted me for troubleshooting their boards for noise problems. In one of the pics they send, they were probing the circuit with one of those long standard alligator clip-styled probes for ground leads. Let’s discuss that.

A probe’s primary job is to try to reliably transfer a signal from your circuit to the input circuitry of your oscilloscope for measurement. All the probes out there will have an input capacitance because of the compensation circuitry and the coaxial cable. You can represent this capacitor in a simplistic electrical circuit of the probe. Now the ground lead of your probe can be modeled as an inductor. When you are probing across a point in your circuit(Referenced to ground) with the long ground lead(for maybe ease of connection), what happens is that you are introducing a larger inductor in that return path. Or a fancy way of saying that is “Your loop inductance increases”. That inductance along with the probe capacitance can and will resonate when a signal with a high freq component is being measured by the probe. You will see ringing and overshoot at the edges of the signal. If you are let’s say probing a data signal(square waveform), the fundamental frequency or the data switching rate can be small, still, the ringing will be there because most circuits have a very fast-rising edge. That’s where all the high-frequency stuff lies and you will see your probe misbehaving.

So what’s the solution? Reduce the ground lead wire length to a bare minimum reducing the inductance of the leads. For that you have probes with tiny needle pins with the positive and earth leads very close to each other. This will ensure that resonance is at a much higher freq and your signal will be cleaner. You don’t have that tip in your oscilloscope kit, you can wind a small wire at the GND part as shown in the figure.

PS: Oscilloscope probes and measuring signals in itself is a fascinating topic with many nuances. If there is any interest in that, do let me know in the comments section. Might have to do a long series of posts to cover that.