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The next important thing to consider when using a probe is its Bandwidth. Fundamentally Bandwidth, from the probe’s perspective, is nothing but the range of frequencies a probe can reliably transport from the circuit being measured to the input of the oscilloscope. It’s defined as the range till the point amplitude of the signal falls to 0.707 times or -3dB from the input. This means that a probe with a BW of let’s say, 50MHz will have the amplitude 3dB lower at 50MHz. This will introduce a 30% measurement error if you are pushing a probe to its end limits. So there is usually a rule of thumb that says when trying to measure a circuit signal with Freq f, use a probe with around 5*f as the bandwidth. This will ensure that the amplitude error is less than 3% as you are way within the spec.

Again, for a square signal, the amount of freq(BW) content is not usually determined by the time period but rather by the rise/fall edges. Bandwith=0.35/RiseTime. So when you are trying to measure a signal with a fast rise time, if the probe BW is not high, the value of the rise time measured will be longer. The same 5 times rule above applies for the rise time case also just to be well within the spec for low measurement error.

You have to always ensure that the probe bandwidth is larger than the oscilloscope input bandwidth. If your scope is limited, even if your probe transports the entire signal, the scope can’t display it without some error. The rule of thumb I use is, Probe BW>=1.5*Scope BW. But do note that some manufacturers underrate the values of probe BW, so you may be able to get away with using a 50MHz probe with 50MHz scope if it’s by the same manufacturer and is matched.

When using the 1x/10x probes do read the probe’s specs carefully, BW is drastically different for 1x and 10x. 10x mode has an order of magnitude higher BW than 1x(Why? That’s a long explanation. Maybe for the future if there is interest). So please do not use them interchangeably when measuring if you don’t know the BW of the signal you are measuring. It’s very easy to screw up on this point.

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