Digital Oscilloscope Principle

The following is an example of Agilent's 90000A series digital oscilloscope to introduce the principle of digital oscilloscope.

Figure 1. Internal structure of digital oscilloscope

Figure 2. Agilent 90000A series oscilloscope capture board
Figure 1 is the internal structure of the digital oscilloscope. The internal structure of the oscilloscope mainly includes the following parts:
1) Signal conditioning part: mainly composed of attenuators and amplifiers;
2) Acquisition and storage part: mainly composed of analog-to-digital converter ADC, memory controller and memory;
3) Trigger part: mainly composed of trigger circuits;
4) Software processing part: composed of a computer.
After the signal enters the oscilloscope, it must be attenuated first and then amplified. Why is this?
It turns out that the attenuator is an adjustable attenuator. When the attenuation ratio is adjusted to a large value, it allows us to test large-amplitude signals. When the attenuation ratio is adjusted to a small value or 0dB attenuation, the amplification effect of the amplifier allows us to test small-amplitude signals. When we usually adjust the vertical sensitivity of the oscilloscope, we are actually adjusting the attenuation ratio of the attenuator. Through the signal conditioning circuit, the signal can be ideally converted to digital by the ADC. The waveform displayed on the oscilloscope screen is as large as possible to reach more than 2/3 of the screen (but not exceeding the screen).
The amplifier amplifies the signal on the one hand, and provides a matching circuit to drive the ADC and trigger circuit on the other hand. The amplifier determines the analog bandwidth of the oscilloscope, which is the first important indicator of the oscilloscope.
After the signal passes through the ADC, the points need to be stored in the memory first. When the memory is full, the sample points are transferred to the computer. Why is this?
It turns out that the sampling rate of the ADC is relatively high (for example, 20G samples per second), and each sample point is represented by 8 bits (modern digital oscilloscopes usually have 8 bits of ADC). The bus bandwidth behind the ADC reaches 160Gbps, which is impossible to transfer the sample points to the computer in real time. Therefore, it is necessary to adopt the Block working mode, store the points first, and then slowly transfer the data to the computer after they are full. Moreover, this time is generally longer than the acquisition time, so the dead time of the digital oscilloscope is still relatively large (generally more than 95%). So how can we ensure that the oscilloscope captures the signal we are interested in? This depends on triggering, which can solve the contradiction between acquisition and transmission.
The second most important indicator of an oscilloscope is determined by the ADC, which is the real-time sampling rate. The third most important indicator is the storage depth, which is determined by the memory controller and memory. The fourth most important indicator is the trigger capability, which is determined by the trigger circuit.
Figure 2 is the capture board of the Agilent 90000A series oscilloscope (the 90000A oscilloscope includes 2 capture boards). The signal is connected to the front end of the capture board through an SMA coaxial cable. The front end includes an attenuator, an amplifier, and a part of the trigger circuit. These devices are sealed on a MCM chip. The front end circuit drives two ADC chips. The sampling rate of each ADC chip is 20GSa/s. The two chips use cross-collection to achieve a sampling rate of 40GSa/s. Behind the ADC is the memory controller IDA, which performs data storage allocation and some operations, such as amplitude, phase compensation, trigger jitter compensation, etc. IDA is connected to the computer through the PCI-Express bus.
So what else needs to be done after the data is transferred to the computer? Figure 3 is a block diagram of the computer processing structure.

Figure 3. Computer data processing structure diagram of oscilloscope
After the collected data is transferred to the computer, it must first be reconstructed by Sin(x)/x sinusoidal interpolation or linear interpolation. The reconstructed waveform can be used for various parameter measurements, signal operations and analysis. The final result or the original sample point can be directly displayed on the screen.
For the selection and use of oscilloscopes, bandwidth and sampling rate are the most critical indicators. So how to quantify and calculate bandwidth and sampling rate? Please refer to Table 1. Why is there such a conclusion?
Please refer to the next article: Analysis of the signal fidelity of digital oscilloscopes.

Table 1. Quantitative selection of bandwidth and sampling rate of oscilloscopes