Basic functions and technical parameters of universal digital storage oscilloscope

This course first introduces you to some basic functions of a general oscilloscope. Our engineers will perform actual operation demonstrations to fully understand the most basic functions and corresponding parameters that constitute an oscilloscope, and implement them in our own DIY oscilloscope. Correspondingly, some of the most basic and core corresponding functions.

The reference is Tektronix's TBS1202B, which is an economical general-purpose digital storage oscilloscope. Other general-purpose digital oscilloscopes have slight differences in indicators, functions and operation interfaces, but their main functions, control mechanisms and interface methods are basically similar.

Physical picture

Official website pictures

Let’s first take a look at the main functions and indicators of the oscilloscope:

It can be seen from the panel of the instrument that it is a dual-channel 200MHz, 2Gsps digital oscilloscope. 200MHz (Hz is the unit of frequency) indicates that the bandwidth (3dB) of the analog signal that this oscilloscope can observe is 200MHz, which is 2Gsps ( sps - samples per second) indicates that the internal ADC sampling rate of this oscilloscope is up to 2Gsps. These two indicators are related to a certain extent. The higher the analog bandwidth, the higher the sampling rate required, but there is no one-to-one correspondence. For example, 2Gsps sampling rate oscilloscopes have 50MHz analog bandwidth, as well as 100MHz and 200MHz bandwidth. These two indicators are very important. Sometimes we say this is a 100M oscilloscope. We need to clarify whether the 100M here refers to the analog bandwidth of 100MHz or the sampling rate of 100Msps.

We will introduce the relationship between the two in detail in later courses.

From the product introduction on the official website, we can see that its record length is 2500 points, which means that one acquisition can acquire and retain 2500 points in the memory, which can be used for later processing, zoom display, etc.

So how much can our DIY oscilloscope do?

dual channel

Analog bandwidth - 100KHz

ADC sampling rate - 2.5Msps (single channel), 1.25Msps (dual channel)

Record length: 256 points (the display screen is 128*128 points, the display area is set to 100*100)

Our actual boards for DIY oscilloscopes

Our functional block diagram for the DIY oscilloscope

The indicators are very low in comparison, but although the sparrow is small and has all the internal organs, it is enough to understand the construction principles of the oscilloscope. The 100KHz analog bandwidth and 2.5Msps sampling rate are also enough for the audio signals we commonly use, and it is enough for learning analog circuits. Principles can also come in handy.

Let’s take a look at the more specific indicator parameters from the product’s data sheet:

We know that any electrical signal can be decomposed into a superposition of multiple single-frequency signals, and the two main parameters of each single-frequency signal are amplitude and frequency (phase). The main function of the oscilloscope is to analyze the electrical signals that change with time. It is displayed graphically through a two-dimensional display screen, showing changes in voltage on the vertical axis and changes over time on the horizontal axis.

The knobs around the display screen are used to adjust the horizontal and vertical scale of these graphics, zoom and move the collected signals, and perform other setting functions.

In addition to vividly displaying waveforms, the oscilloscope also has the function of measuring waveform parameters, which can be measured automatically or manually. It mainly measures the following parameters:

Voltage-related characteristic parameters:

Amplitude

Maximum and minimum voltage

average voltage

Time-related characteristic parameters:

The frequency and period of the signal

duty cycle

rise time and fall time

Setting menu for parameter measurement

Let’s take a look at the voltage indicators first:

Vertical resolution - 8 bits, that is, an 8-bit ADC is used internally to quantize the analog signal

Input signal sensitivity range: - 2mV to 5 V/div

DC gain accuracy: ±3%, from 10mV/div to 5 V/div

Maximum input voltage range: 300 VRMS CAT II; derated at 20 dB/decade above 100 kHz to 13 Vp-p AC at 3 MHz and above

Offset voltage range: 2 mV to 200 mV/div: ±1.8 V, >200 mV to 5 V/div: ±45 V

Bandwidth limit: 20 MHz

Input coupling method: AC, DC, GND

Input impedance: 1 MΩ in parallel 20 pF

Let’s look at the indicators related to time, frequency, and phase in the horizontal axis direction:

Time base range: 2.5 ns to 50 s/div

Time base accuracy: 50 ppm

Let’s take a look at the corresponding functions of the main components of the oscilloscope:

Functional block diagram of a typical digital oscilloscope

data collection system:

Acquisition mode

Peak detection - for capturing high-frequency signals or random glitch signals, capable of capturing signals as narrow as 12ns

sampling

Average: Waveform average, you can choose 4, 16, 64, or 128 waveforms for averaging.

Single collection

Rolling collection

Trigger system:

External trigger input

Trigger mode: automatic, normal, single sequence

Trigger type:

edge triggered

video trigger

Pulse width trigger (or glitch)

Trigger source: Dual channel mode: CH1, CH2, Ext, Ext/5, AC Line

Trigger view: Read out the frequency of the trigger signal

Waveform measurement:

Measure manually using cursors

Type: amplitude, time

Able to measure: ΔT, 1/ΔT, ΔV

Automatic measurement: period, frequency, pulse width, peak-to-peak value. . . . . .

Waveform calculation

Arithmetic: addition, subtraction, multiplication

Math function: FFT, 2048 sample points

FFT: Windowing: Hanning, Flat Top, Rectangular

Source: Dual channel mode: CH1 - CH2, CH2 - CH1, CH1 + CH2, CH1 × CH2

Time domain waveform and corresponding spectrum

auto configuration

Auto setup menu

square wave

sine wave

Video (NTSC, PAL, SECAM)

Automatically adjust the range - adopt the most appropriate sampling rate, gain, and display scale

frequency counter

Resolution: 6 digits

Accuracy: + 51 parts per million including all frequency reference errors and +1 count errors

Frequency range: AC coupled, 10 Hz minimum to rated bandwidth

Frequency counter signal source

display system

Interpolation: Sin (x)/x

Wave style: dot, vector

Duration: off, 1 second, 2 seconds, 5 seconds, unlimited

Format: YT and XY

Based on the above summary of functions of conventional oscilloscopes, we can set the functions and corresponding technical indicators to be implemented by our DIY oscilloscope:

In the subsequent courses on embedded system programming, we will take you step by step to implement the functions in the table above. Completing these functions means realizing the functions of a complete oscilloscope. From the perspective of software programming, more functions can be supported through programming, but technical indicators are limited by the following factors:

Analog link performance - Designed according to the 100KHz analog bandwidth, the gain adjustment is set to 1:1 and 10:1. However, since the ADC we use has 12-bit accuracy, it can also handle analog signals ranging from 2mVpp to 50Vpp. ;

The ADC sampling rate inside STM32G031 is 2.5Msps. If dual channels work at the same time, it is reduced to 1.25Msps/channel, and a 100KHz sine wave is collected, which can have 12.5 sampling points in one cycle;

The internal RAM of STM32G031 is only 8KB, which is used for both data acquisition and waveform display and other data processing. Therefore, the waveform that can be stored in each acquisition is controlled at 256 points, which is also the number of points used in FFT;

Display area: Commercial oscilloscope displays are relatively large. The resolution of TBS1202B is 800*480, and the amount of displayed information is relatively large. We use a 128*128 OLED display, which has a small display interval. The more appropriate display area is 100 *100;

Input control: There are many knobs on the universal oscilloscope panel, which can support multiple functions. However, our DIY oscilloscope only has one rotary encoder and two buttons. It is relatively simple and needs to display more functions on the small display screen. In the menu The design requires a lot of effort, and the supported functions can only be simplified.

Before the formal course, you can watch the series of oscilloscope videos produced by Hard Wo Academy and Tek.

In the next section, let’s take a look at the basic indicators of “electrical signals”.