The Tech Blog

When you start probing(in 10x mode) known signals with higher freq, you will start seeing cases when the oscilloscope is not showing the right amplitude even though the probe is rated for that signal freq & amp. That is due to the fact that your 10x probes are not probe compensated. Let’s delve into what and why?

There is always a capacitance(Cin) at the input circuitry inside the scope. It varies between scope models and is usually in the 10pF-20pF range. At higher freq, the impedance of this capacitor drops, if we take a value of 10pF@10MHz (X=1/2*Pi*f*Cin), eff. Impedance is 1.6KΩ only. This low impedance dominates as it is parallel with the 1MΩ at the input and the probe attenuation values go for a toss. Effectively the high freq signal components use the path mentioned in the figure through the tip capacitor. It’s a classic low-pass filter. Whereas the capacitor(Cp) across the 9MΩ is starting to pass high freq signals and is trying to be a high pass filter. In order to fix this, we use a variable capacitor in the probe. It can be placed in 2 spots, one at the tip end and the other at the connector end(Varies in diff models, in some both maybe present). This capacitor is exposed as a tiny knob on the probe which can be rotated to adjust its value. We want the effectively “cancel” out the effects of both RC circuits so that we get flat freq response across the entire range. That is possible only when Rp*Cp = Rin*[Cin + Cb] . This adjustment is called Probe Compensation.

How do you do it in a scope? You connect the probe to a special pin in front of the scope which generates a 1KHz Square wave signal. It’s a square wave becoz it has a low fundamental freq of 1KHz and a sharp rise takes care that high freq components are also present in the signal. Now when you turn the variable cap knob you will see the output waveform changing to undershoot or overshoot. Adjust it just enough to get a perfect square waveform. Probes need to be compensated everytime you change the probe or you change a channel as there will be variations. Do take of that when you probe around next time.

#BacktoBasics#Electronics#Circuits

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.