When you want to take a time-domain measurement, the standard tool that will be used is an oscilloscope. All oscilloscopes need probes to take high-precision measurements, and the probe needs to meet certain specifications to create an accurate signal representation above background noise.
When looking at probes, which attenuation value should be used and how will this affect signal recreation? We’ll examine this specification alongside the probe bandwidth to better understand what signals can be accurately represented in an oscilloscope trace.
Oscilloscope Probe Attenuation Defined
Attenuation in an oscilloscope probe refers to the amount by which the input signal being collected with the probe is reduced. The effect of attenuation is to reduce the input signal level to a lower value. Oscilloscope probe manufacturers will generally supply a probe with their systems that offers selectable 1x or 10x attenuation. In 10x mode, the signal being collected with the probe is reduced by a factor 10 (e.g., from 10 V to 1 V). Other probe attenuation values are available, which can reach 100x or even 1000x.
Typical probes supplied by oscilloscope vendors.
All oscilloscope probes are essentially two wires that are linked with some defined capacitance and resistance. The attenuation level is typically selectable on the body of a probe with a small switch. There can also be some additional compensation available on the probe that can be adjusted to ensure accurate signal reproduction during measurement.
The purpose of oscilloscope probe attenuation is not necessarily to reduce the signal level simply because the signal level is too large for the scope’s measurement range. Instead, the purpose is to modify the level of leading the tested circuit experience when the probe is connected. To see why this is important, we need to examine the front-end input of an oscilloscope and how this interacts with the probe.
The Front-End of an Oscilloscope
All oscilloscopes have some terminating impedance and input capacitance; probes have a nominal resistance rating and a capacitance, the former of which depends on the attenuation. When the probe is inserted into the oscilloscope, the probe + scope combination creates the following equivalent circuit arrangement. The attenuation arises due to simple voltage drop across the 9 MOhm resistor (when enabled for 10x attenuation).
The terminating impedance inside modern oscilloscopes is selectable between 1 MOhm (for low-impedance circuit measurements) and 50 Ohm (for RF measurements). Beyond the terminating input impedance on the oscilloscope, the input of modern digital oscilloscopes generally consists of a higher order filter stage, which could reach as high as 10th order. This cuts out the higher end of the bandwidth and increases the amount of bandwidth required to accurately sample a signal with a fast edge.
1x or 10x?
While it is true that attenuation reduces the signal level that enters the oscilloscope, this is not the primary reason for using high or low attenuation. The other important reason has to do with loading of the circuit under test.
When the probe is connected to the test circuit, the circuit’s output impedance is effectively in parallel with the front-end of the oscilloscope. This means that the portion of the circuit that is generating its output signal will pass that signal into a parallel impedance. In effect, the impedance seen by the output signal is:
When applying the probe, we want to ensure that this equivalent impedance is as close as possible to the real output impedance of the test circuit so that the real electrical behavior of the test circuit is not distorted during measurement. This requires that R(osc) >> R(circuit). In the 10x case, the input impedance into the oscilloscope is much higher, which reduces the loading on a test circuit, so we would be able to measure circuits with higher output impedances.
What About 50 Ohms Oscilloscope Input?
For systems requiring an impedance matched input, such as an RF circuit with 50 Ohm output impedance, the situation is different. In this case, we can still have something like 10x attenuation in the signal level reaching the input of the oscilloscope, but this requires a special probe that has an input impedance matched to 50 Ohms. An example probe for measuring RF circuits is shown below.
Example 50 Ohm probe with high attenuation.
Here the role of the 50 Ohms matching impedance is to prevent reflections into the oscilloscope front-end so that the signal can be accurately sampled without reflection within the signal’s bandwidth.
Technically, it is possible to modify a standard vendor-supplied probe to produce a 50 Ohm impedance, but this will most likely be plagued with parasitics that will not allow accurate sampling of high-frequency signals. It is better to just use the correct probe, or to connect a 50 Ohm coax directly from the scope to the test circuit.
What About Probe Bandwidth?
The other important specifications for all oscilloscope probes is their bandwidth. The bandwidth of a probe is exactly as its name suggests: it is the frequency band within which the oscilloscope can collect accurate measurements. Because the oscilloscope is basically a 1st order low-pass filter, it will limit the bandwidth that can be accurately sampled. This is defined by the equivalent RC time constant of the entire input stage of the oscilloscope, and it is an important topic we will examine in an upcoming article.
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