EEVblog 1445 - How to Simulate an Oscilloscope Probe in LTSPICE

Summary

In this video, the process of simulating a times-one oscilloscope probe using LTSpice is demonstrated. The video covers key aspects like probe loading, bandwidth limitations, and the role of compensation networks in ensuring accurate frequency response. By configuring lossy transmission lines and adjusting parameters like resistance, capacitance, and inductance, the presenter walks through common simulation challenges, such as handling floating nodes and fine-tuning compensation networks. Ultimately, the video offers a detailed approach to understanding and simulating oscilloscope probes, with an emphasis on achieving precise bandwidth and system response for high-fidelity measurements.

Takeaways

• 😀 Understanding oscilloscope probes: The script explains the impact of using a times 1 oscilloscope probe on circuit loading and bandwidth limitations.
• 😀 Low bandwidth in times 1 mode: A times 1 oscilloscope probe typically has much lower bandwidth (around 6.5 MHz) compared to the times 10 setting, which affects signal fidelity.
• 😀 LTSpice for simulation: The script demonstrates how to simulate a times 1 oscilloscope probe using LTSpice, a free and powerful circuit simulation tool.
• 😀 Importance of lossy transmission lines: The use of lossy transmission lines in the simulation is emphasized to reflect the real-world behavior of oscilloscope probes with coaxial cables.
• 😀 Spice directive usage: The necessity of incorporating Spice directives in LTSpice for accurate simulation is discussed, with an explanation of the parameters that need to be set (e.g., resistance, capacitance, inductance).
• 😀 Calculating parameters: The script walks through how to calculate the resistance, capacitance, and inductance of the probe’s transmission line using measurements and datasheet values.
• 😀 Compensation network: The probe's compensation network is crucial for maintaining accuracy, and the script shows how a compensation network with capacitors and resistors impacts the probe's performance.
• 😀 Dealing with simulation errors: The video addresses how to handle errors like 'matrix is singular' by adding appropriate ground connections or DC bias paths to the simulation.
• 😀 Importance of accurate probe modeling: It explains how the probe's response, including capacitance and compensation networks, affects the overall system performance in simulations.
• 😀 Practical applications: The script touches on how this simulation could be used for real-world scenarios such as analyzing the probe’s impact on circuit loading or evaluating ground coupling issues.

Q & A

What is the purpose of simulating an oscilloscope probe in the video?

The purpose of simulating an oscilloscope probe in the video is to understand how probing a circuit can affect its performance, specifically focusing on the loading effect and the bandwidth limitations of a times one oscilloscope probe.

What happens when you switch a probe to the 'times one' setting on a high-frequency oscilloscope?

When you switch a probe to the 'times one' setting, it dramatically reduces the bandwidth of the oscilloscope. In the example shown in the video, the bandwidth drops to about 6.5 MHz, even on a high-frequency oscilloscope rated for 100-200 MHz.

Why is it important to understand probe loading when using an oscilloscope?

Probe loading is important because it can affect the accuracy of the measurements. Probing a circuit can introduce additional resistance, capacitance, and inductance, altering the behavior of the circuit, especially at high frequencies.

Why does LTspice use a lossy transmission line model for simulating oscilloscope probes?

LTspice uses a lossy transmission line model to accurately simulate the behavior of a coaxial cable that is deliberately designed to be lossy. This is important for mimicking the real-world characteristics of oscilloscope probes, which are not ideal and have losses at higher frequencies.

What parameters must be specified when using the lossy transmission line model in LTspice?

The key parameters for the lossy transmission line model in LTspice include resistance (in ohms per unit length), inductance (in henries per unit length), conductance (in siemens), capacitance (in farads per unit length), and the length of the transmission line.

What is the significance of the 'spice directive' in LTspice?

The 'spice directive' in LTspice is a command that instructs the simulator to use specific parameters for a component, such as the lossy transmission line model. It allows users to customize simulations by including real-world characteristics like resistance, capacitance, and inductance in their models.

How do you calculate the inductance of a lossy transmission line in the simulation?

The inductance is calculated based on the capacitance value of the coaxial cable. Using the formula for characteristic impedance (Z = √(L/C)), you can rearrange it to solve for L (inductance) once the capacitance is known.

What was the cause of the simulation error 'matrix is singular' in LTspice?

The 'matrix is singular' error occurs when there is a floating node in the simulation, meaning there is no direct DC connection or bias to the component at one end of the transmission line. Adding a DC bias or another ground reference point resolves this issue.

Why is it necessary to include a compensation network in an oscilloscope probe simulation?

The compensation network is essential to match the input capacitance of the oscilloscope to the probe, ensuring accurate frequency response. Without proper compensation, the probe’s response could become distorted at higher frequencies, affecting measurement accuracy.

How did the author determine the compensation network's values for the simulation?

The author determined the compensation network values by referencing the probe's datasheet, which specified the expected range of values for the compensation capacitor. By measuring the resistance of the probe and considering the physical characteristics of the coaxial cable, the author adjusted the compensation network to match the expected bandwidth.