In oscilloscope applications, many measurements are performed directly via cables for RF or high-speed digital signals. However, a significant amount of on-board debugging is carried out using probes. These probes are essential components of the oscilloscope measurement system, especially when dealing with high-bandwidth signals. Most high-speed probes are active probes, which contain internal amplifiers that can experience gain and bias drift due to temperature changes or long-term aging. To maintain accuracy, it's crucial to perform regular probe calibration.
Currently, there are three main methods used for calibrating oscilloscope probes:
1. **DC Gain and Offset Calibration**
This is the most common form of calibration. It involves comparing a known DC voltage from the calibration source with the actual measured voltage by the oscilloscope. This process helps correct the probe’s gain and offset errors. The DC calibration determines the linear equation y = mx + b, where m represents the gain and b represents the offset. Typically, this type of calibration is done once a year, but in some cases, it may be required more frequently—sometimes even daily.
2. **AC Calibration**
For high-performance oscilloscopes used in high-speed signal testing, the wide bandwidth makes it challenging to ensure a flat amplitude and phase response across the entire frequency range. To improve measurement accuracy, AC calibration is necessary to align the amplitude and frequency response of the oscilloscope and probe system across the full bandwidth. Unlike DC calibration, this method addresses frequency-dependent variations. AC calibration typically involves a network analyzer to measure the S-parameters of the active probe amplifier. The manufacturer tests each probe and stores these parameters internally. When the user connects the probe, the oscilloscope automatically reads and applies the S-parameters for accurate AC calibration.
3. **User Site AC Calibration**
The standard AC calibration method relies on pre-stored S-parameters from the manufacturer. However, this approach doesn’t account for variations caused by different connection accessories or environmental conditions. For example, the length of the probe cable or the type of connector used can affect the overall performance. For high-bandwidth systems (up to tens of GHz), it's important to perform AC calibration based on the specific test setup and environment used by the user.
The traditional method of measuring S-parameters with a network analyzer is complex and not practical for field use. However, modern solutions like Agilent’s indium phosphide-based oscilloscopes now offer a fast edge signal with a rise time of less than 15 ps. This fast edge contains sufficient high-frequency content to serve as an effective calibration source. While traditional high-speed oscilloscopes also have fast-edge outputs, their rise times are usually in the tens of picoseconds or slower, making them suitable mainly for delay calibration rather than precise frequency response calibration.
As shown in the diagram, the oscilloscope's calibration output sends a fast edge signal to the probe input. Two channels of the oscilloscope then capture the input signal (Vin) and the output signal (Vout) from the probe. By analyzing the ratio Vout/Vin across the frequency band, the system can adjust and flatten the frequency response.
After performing user site AC calibration, the frequency response becomes flatter, significantly improving the accuracy of high-speed circuit measurements under real-world conditions.
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