How to Calibrate an Oscilloscope Probe

An oscilloscope is a common electronic device in electronic testing equipment. Electronic engineers use it to measure the signal output of related circuits and the corresponding voltage and current changes.

In the application scenarios of oscilloscopes, except for some RF or high-speed digital scenarios where cables are used for direct measurement, many debugging tasks on boards are completed with the help of probes.

However, before we officially start using the probe, we need to calibrate it. So how do we calibrate the probe of the oscilloscope?

The probe is part of the oscilloscope measurement system. Many high-bandwidth probes must be active probes. The gain and offset of the active amplifier inside the active probe may drift with temperature or aging. In order to compensate for this drift, the probe needs to be calibrated regularly.

There are three common methods for calibrating oscilloscope probes:

(1) DC gain and offset calibration

DC calibration is the most commonly used calibration method for oscilloscopes. It compares the calibration signal output (standard DC voltage) with the calibration signal voltage actually tested by the oscilloscope to correct the gain and offset deviation of the probe test DC voltage. The DC calibration process is to determine the values ​​of the coefficients m and b of the linear equation y=mx+b. The DC calibration of the probe needs to be performed at least once a year, and more frequently every few months or even every day.

(2) AC calibration

For high-performance oscilloscopes that test high-speed signals, it is difficult to ensure that the amplitude-frequency and phase-frequency responses within the band are absolutely flat due to their very wide bandwidth. In order to improve the measurement accuracy, it is necessary to calibrate the frequency response within the band so that the oscilloscope and probe test system have consistent amplitude and frequency responses at different frequency points within the entire bandwidth.

DC calibration cannot correct the frequency response. The probe AC ​​calibration method uses a network analyzer to test the S parameters of the active probe amplifier, usually testing the loss at each frequency point and correcting the probe frequency response.

Oscilloscope manufacturers test the S parameters of each probe amplifier and store them in the probe's internal memory before shipment. When the user uses the probe, the oscilloscope reads the probe's S parameters for AC calibration.

(3) User-side AC calibration

The above probe AC ​​calibration process uses the fixed S parameters provided by the manufacturer for calibration, which cannot fully take into account the loss of probe connection accessories in different actual situations. In fact, the user's usage environment varies greatly, such as different probe connection front end lengths. For oscilloscopes and probes with bandwidths of tens of GHz, it is very necessary to perform AC calibration based on the user's usage environment and test accessories.

The process of testing S parameters using a network analyzer is very complicated and is not suitable for use in field environments. At present, Agilent's oscilloscope based on phosphide smoke materials can provide a signal with a rising edge of less than 15ps as a calibration source. Since the fast rising edge contains enough high-frequency components, it is reasonable and feasible to use the fast edge signal as a calibration source. (Although traditional high-speed oscilloscopes also have fast edge outputs, their rising edges are usually tens of ps or even slower, so they are mainly used for delay calibration, but not enough for accurate frequency response calibration).

As shown in the figure, the oscilloscope cal out outputs a fast edge signal. The two channels of the oscilloscope test the calibration signal cal out at the probe input front-end signal Vin and the probe measurement output signal Vout. The frequency response within the calibration is corrected by the Vout/Vin.

After on-site AC calibration, users can obtain a flatter frequency response, improving the test accuracy under actual test conditions of high-speed circuits.