참조문헌
[1] KEYSIGHT OSCILLOSCOPE GLOSSARY, What is Oscilloscope Bandwidth?
[2] fff
https://blog.teledynelecroy.com/2020/08/how-do-you-choose-whether-to-use-50-ohm.html
How to Choose Between the Oscilloscope's 50 Ohm Input and 1 MOhm Input
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Bandwidth (대역폭)
대역폭은 오실로스코프가 정확하게 나타낼 수 있는 주파수의 영역을 측정한다. 이는 일반적으로 Hz, 또는 MHz 단위로 표현되며, 오실로스코프를 선택함에 있어 매우 중요한 파라미터이다. 오실로스코프의 대역폭은 신호의 심한 왜곡 또는 감쇠 없이 측정할 수 있는 주파수의 최대값과 최소값을 결정한다. 아날로그 신호와 디지털 신호는 모두 bandwidth로 특정될 수 있으며, 이는 주파수의 영역을 이해하는데 있어 주요한 파라미터이다. 높은 대역폭을 가지는 오실로스코프는 더 넓은 범위의 주파수 신호들을 정확하게 측정할 수 있으며, 이 대역폭이 소자의 frequency response를 결정한다(이는 다양한 전자소자의 문제를 진단하고 troubleshooting 하는데 주요하다).
What is Maximum Bandwidth?
Maximum bandwidth is the highest frequency an oscilloscope can measure with minimal attenuation while maintaining signal fidelity. This parameter is essential to ensure accurate measurements, especially when working with high-frequency signals.
What is Additional Bandwidth?
Additional bandwidth refers to the extra frequency range beyond the maximum bandwidth that an oscilloscope can still measure, albeit with increased signal attenuation and potential distortion. While not ideal for accurate measurements, this extra range can be useful for general troubleshooting and observing signal trends.
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Probe Bandwidth
The oscilloscope probe is an essential accessory for making accurate measurements. The probe bandwidth must match or exceed the oscilloscope bandwidth to ensure accurate signal representation. A probe with a lower bandwidth than the oscilloscope can cause signal distortion and limit the system's overall performance.
Typical Probe Bandwidths
| Probe Type | Bandwidth Range |
| Passive Probe | 6 MHz – 500 MHz |
| Active Probe | 100 MHz – 8 GHz |
| Differential Probe | 50 MHz – 6 GHz |
Input Coupling
Input coupling can be either AC or DC. AC coupling filters out the DC component of a signal, while DC coupling allows both AC and DC components to pass through. Using AC coupling with high-frequency signals can cause signal distortion and reduce the effective oscilloscope bandwidth.
Signal Conditioning
Signal conditioning is the modification of a signal to make it suitable for measurement. This can include:
- Filtering
- Amplification
- Attenuation
The signal conditioning circuitry within the oscilloscope can also affect the bandwidth.
Choosing the Right Oscilloscope Bandwidth
To choose the appropriate oscilloscope bandwidth for your application, consider the following steps:
- Determine the highest frequency component of interest in your signal.
- Apply the Nyquist theorem: The oscilloscope bandwidth should be at least twice the maximum frequency of interest.
Common Applications and Recommended Bandwidths
| Application | Recommended Bandwidth |
| Audio Electronics | 20 MHz |
| Switching Power Supplies | 100 MHz |
| Microcontroller Development | 100 MHz – 500 MHz |
| High-speed Digital Design | 1 GHz – 8 GHz |
Rule of Thumb: Bandwidth-to-Signal Frequency Ratio
As a general rule, we recommend choosing an oscilloscope bandwidth that is five times the highest frequency component of interest. This ensures the oscilloscope can accurately display the signal's harmonic content and maintain measurement fidelity.
Relationship between Rise Time and Bandwidth
Rise time and bandwidth are related through a fundamental relationship that can be expressed using the following formula:
- Rise Time (in seconds) ≈ 0.35 / Bandwidth (in Hertz)
This formula is an approximation based on a Gaussian frequency response and is valid when the oscilloscope has a predominantly single-pole response. According to this relationship, an oscilloscope with a higher bandwidth can measure signals with shorter rise times, providing more accurate representation of fast-changing signals.
Conversely, if you know the fastest rise time of the signals you need to measure, you can use this formula to estimate the minimum required bandwidth for your oscilloscope:
- Minimum Bandwidth (in Hertz) ≈ 0.35 / Fastest Rise Time (in seconds)
Understanding the relationship between rise time and bandwidth is crucial when selecting an oscilloscope or evaluating the performance of electronic devices and circuits. By considering both parameters, you can ensure that your oscilloscope provides accurate measurements and can handle the signals you need to analyze.
Gaussian Response and Bandwidth
Gaussian response is a frequency response characterized by a smooth roll-off at higher frequencies. This response is named after the Gaussian function, which has a bell-shaped curve. In oscilloscope bandwidth, Gaussian response plays a significant role in determining the oscilloscope's ability to accurately measure and display signals without distortion or overshoot.
When an oscilloscope exhibits Gaussian response, it maintains a relatively flat frequency response within its bandwidth and begins to attenuate higher frequencies smoothly.
Oscilloscope Bandwidth Limitations
It is essential to understand that oscilloscope bandwidth is not a single fixed value but rather a range of frequencies over which the oscilloscope's performance degrades. The performance degradation usually occurs gradually, with higher frequencies experiencing more signal attenuation and distortion. This degradation is due to the frequency-dependent characteristics of the oscilloscope's internal components, such as amplifiers and filters.
Effects of Aliasing on Bandwidth
Aliasing is a phenomenon that occurs when an oscilloscope's sample rate is insufficient to accurately represent a high-frequency signal. It causes the signal to appear at a lower frequency than it actually is, leading to incorrect measurements and potential misinterpretation. To avoid aliasing, ensure that the oscilloscope's sample rate is at least twice the maximum frequency component of interest, as dictated by the Nyquist theorem.
Considerations for Specific Applications
Certain applications may require specialized oscilloscopes with unique bandwidth requirements. For example, radio frequency (RF) and microwave applications typically require high-bandwidth oscilloscopes with frequency ranges in the gigahertz (GHz) region.
On the other hand, power electronics applications may require lower bandwidth oscilloscopes with high voltage input capabilities and specialized probes for safe and accurate measurements.
Real-time vs. Equivalent-time Sampling
Real-time sampling involves capturing a continuous stream of samples at the oscilloscope's maximum sample rate, allowing for an accurate representation of the input signal. Real-time sampling is best suited for measuring single-shot or non-repetitive signals.
Equivalent-time sampling, on the other hand, captures multiple snapshots of repetitive signals and reconstructs the waveform over time. This method allows for higher effective sample rates, making it suitable for measuring high-frequency signals exceeding the oscilloscope's real-time sample rate.
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하지만 샘플링 주파수보다 높은 주파수를 측정하면 아날로그 방식[12]과 달리 파형이 마구 꼬이는데, 이게 실제 파형이라 착각하면 큰 실수를 하게 될 수도 있으니 주의하자. 신호 측정이 정해진 주파수에서 한번씩 측정해서 발생하는 문제이며, 이를 에일리어싱이라 부른다
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