When measuring ripple and noise on the same power supply, different oscilloscopes can yield varying results. Even using different probes can influence the readings. Why is this the case?
Understanding Ripple and Noise
Ripple
In a switching power supply, the high-frequency operation of the switching transistor causes energy to be transferred from the input to the output capacitor, creating a charging and discharging cycle. This results in voltage fluctuations on the output, known as ripple. The frequency of this ripple matches the switching frequency of the transistor. Ripple is an AC component superimposed on the DC output, and its amplitude is measured as the peak-to-peak value between the highest and lowest points of the AC signal.
Noise
Noise, on the other hand, arises from the sharp pulses generated during the switching process. Unlike ripple, which has a lower frequency, noise occurs at much higher frequencies. The magnitude of noise is influenced by various factors such as the power supply’s topology, transformer winding design, parasitic elements in the circuit, external electromagnetic interference, and PCB layout.
Now that we understand the difference between ripple and noise, let's explore how measurement techniques affect the results.
Why Oscilloscope Readings Differ
Differences in oscilloscope measurements can arise due to several factors:
- Not Using a 20MHz Bandwidth Limit: Higher bandwidths allow more noise to pass through, affecting the peak-to-peak value of the signal.
- Improper Grounding: Long ground leads or large loops can introduce significant electromagnetic interference into the measurement.
- Uncorrected Probe Compensation: A poorly compensated probe can lead to inaccurate amplitude readings.
It's important to note that no single oscilloscope or brand should be considered the "gold standard." Oscilloscopes have limited vertical resolution (often 8-bit ADC), and their frequency responses vary slightly. Therefore, small differences in readings are normal and expected.
Is 20MHz a Scientific Limit?
The 20MHz limit was originally introduced as a simple hardware filter to reduce high-frequency interference. While it may serve as a practical guideline, it’s not a strict theoretical requirement. In modern digital oscilloscopes, you don’t need to rely solely on this limit.
How to Measure Ripple and Noise More Accurately
Modern digital oscilloscopes offer advanced functions like FFT, digital filtering, and statistical analysis. These tools can help you get more accurate and detailed results.
1. FFT Function
FFT (Fast Fourier Transform) allows you to analyze the frequency components of a signal. You can identify harmonic distortion, noise sources, and other characteristics. For example, the ZDS2000 series offers 4Mpts FFT with a frequency resolution of 250Hz, enabling precise analysis of signal interference.
2. Digital Filtering
Digital filters, such as IIR or FIR, can effectively remove unwanted noise. The ZDS2024 oscilloscope provides low-pass and high-pass filtering options up to 100MHz, offering excellent stopband attenuation and passband flatness. As shown in Figure 1, the amplitude-frequency response of a low-pass Butterworth filter is clearly defined.
Figure 1: Amplitude-Frequency Response of a Low-Pass Filter
3. Automatic Measurement
Modern oscilloscopes also support automatic parameter measurement. They can calculate the current, maximum, minimum, average, and standard deviation values across all captured waveforms. This helps identify anomalies quickly and assess signal behavior efficiently.
Summary of Key Functions
- FFT: Analyze frequency domain characteristics of noise.
- Digital Filtering: Remove unwanted noise components.
- Automatic Measurement: Evaluate waveform parameters after processing.
By leveraging these features, engineers can achieve more accurate and reliable ripple and noise measurements, ensuring better performance and stability in power supply designs.
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