How to determine the bandwidth of an oscilloscope?

Bandwidth is known as the first indicator of an oscilloscope and is also the most valuable indicator of an oscilloscope. The oscilloscope market is often divided based on bandwidth as the primary basis. When engineers choose an oscilloscope, the first thing they need to determine is bandwidth. In the sales process, there are many stories about bandwidth.

The bandwidth usually mentioned without special explanation refers to the bandwidth of the analog front-end amplifier of the oscilloscope, which is often called the -3dB cutoff frequency point. In addition, there are also concepts such as digital bandwidth and trigger bandwidth.

We often say that digital oscilloscopes have five major functions, namely capture, view, measurement, analysis and documentation. The principle block diagram of these five major functions is shown in Figure 1.

Figure 1. Schematic diagram of a digital oscilloscope

The capture part is mainly composed of three chips and a circuit, namely, the amplifier chip, A/D chip, memory chip and trigger circuit. The principle block diagram is shown in Figure 2. The measured signal is first converted into a voltage range that the ADC can receive after passing through the probe and amplifier and normalization. The sampling and holding circuit divides the signal into independent sampling levels at a fixed sampling rate. The ADC converts these levels into digital sampling points, which are stored in the acquisition memory and sent to display and measurement analysis processing.

Figure 2, oscilloscope capture circuit block diagram

The typical circuit of an oscilloscope amplifier is shown in Figure 3. This circuit can be seen everywhere in analog circuit textbooks. This amplifier can be equivalent to an RC low-pass filter as shown in Figure 4. The relationship between the output voltage and the input voltage is derived from this equivalent circuit, and the Bode plot of the ideal amplitude-frequency characteristic is shown in Figure 5.

Figure 3. Typical circuit of an amplifier