Basic Elements of a Digital Oscilloscope

Every digital oscilloscope has four basic functional modules – vertical system, horizontal system, trigger system and display system. In order to understand the overall function of a digital oscilloscope, it is important to understand the function of each module.

Most of the front panel of a digital oscilloscope is used to control the vertical, horizontal, and trigger functions, as these are where most of the necessary adjustments are made. The vertical functions control the attenuation or amplification of the signal by changing the "volts per division" value so that the signal can be displayed at the appropriate amplitude. The horizontal controls are related to the instrument's time base, and its "seconds per division" control is used to determine the amount of time represented by each horizontal division on the display. The trigger system performs basic functions such as signal stabilization and oscilloscope initialization for signal acquisition, and the user can select and modify specific trigger types. The final display system includes the display itself and the display driver, as well as the software used to perform display functions.

Vertical System

The system (Figure 1) allows the user to vertically position and scale the waveform, select input coupling, and modify the signal characteristics to display it on the screen in a specific way. The user can vertically position the waveform at a precise location on the display and increase or decrease its size. All oscilloscopes have a grid on the display screen that divides the visible area on the screen into 8 or 10 vertical divisions, with each division representing a portion of the total voltage. In other words, for an oscilloscope with a 10-division display grid, if the total displayable voltage is 50 V, each division represents 5 V.

Figure 1: Vertical system

The choice of 8, 10, or some other grid is arbitrary, 10 is usually chosen for simplicity: 10 is easier to divide than 8. The probe also affects the display ratio, some probes do not attenuate the signal (1x probe), some have 10 times attenuation (10x probe), and some can even attenuate 1000 times. The issue of probes will be discussed later.

The input coupling mentioned above basically determines the signal transmission from the time the signal is captured by the probe to the entire process of being transmitted through the cable to the instrument. DC coupling provides an input coupling impedance of 1 m ohm or 50 ohm.

Selecting 50 ohm input coupling sends the input signal directly to the oscilloscope's longitudinal gain amplifier, thus achieving the widest bandwidth. Selecting AC or DC coupling mode (corresponding to 1m ohm terminal values) places an amplifier in front of the longitudinal gain amplifier, typically limiting the bandwidth to 500 mhz in all cases. The benefit of such high impedance is that it provides built-in high voltage protection. When "Ground" is selected on the front panel, the longitudinal system is disconnected and the 0-v point is displayed on the screen.

Other circuits associated with the vertical system include a bandwidth limiter, which is used to attenuate high-frequency signal components when de-noising the displayed waveform. Many oscilloscopes also use a dsp arbitrary equalization filter (anti-aliasing filter) to extend the instrument bandwidth by adjusting the phase and amplitude response of the oscilloscope channel to make the instrument bandwidth exceed the original response of the front end. However, these circuits require the sampling rate to meet the Nyquist theorem - the sampling rate must be greater than twice the maximum fundamental frequency of the signal. To achieve this, the instrument is usually locked at its maximum sampling rate, and the sampling rate cannot be reduced to observe a longer duration without disabling the filter.

Horizontal system

Compared with vertical systems, horizontal systems are more related to signal acquisition, emphasizing sampling rate, storage depth, and other performance indicators directly related to data acquisition and conversion.

The time interval between sampling points is called the sampling interval. The sample point value represents the value stored in the memory for generating the waveform. The time interval between waveform points is called the waveform interval. Since a waveform point may be based on multiple sampling points, the two are related and may sometimes have the same value.

The acquisition mode menu of a general oscilloscope is very limited, because a channel can only generate one waveform, and the user can only select one sampling type or one waveform algorithm type. However, some oscilloscopes can display three waveforms in parallel on one channel, and each waveform can combine the sampling type and waveform algorithm type.

Typical patterns include:

Sampling mode: For each waveform interval, one waveform point is generated by one sampling point.

High Resolution Mode: For each waveform interval, the average sample points of the waveform interval are displayed.

Peak Detect Mode: For each waveform interval, the minimum and maximum sample points within the waveform are displayed.

rms: Displays the rms value of the sampling points within the waveform interval. This is proportional to the instantaneous power.

Typical waveform algorithm modes include:

Envelope mode: Based on a waveform captured by at least two trigger events, the oscilloscope generates a boundary (envelope) to represent the maximum and minimum values ​​of the waveform.

Average mode: Get the average value of each waveform interval sample based on multiple sampling.

Trigger system

The trigger is one of the basic units of every digital oscilloscope, used to capture signal events for detailed analysis and provide a stable and repetitive waveform view. The accuracy of the trigger system and its flexibility determine how the measured signal is displayed and analyzed. As mentioned earlier, the digital trigger system brings significant advantages to oscilloscope users in terms of measurement accuracy, acquisition density, and functionality.

Analog trigger

An oscilloscope's trigger (Figure 2) ensures a stable waveform display for continuous monitoring of repetitive signals. Triggers are very useful in isolating and displaying specific signal characteristics such as "runt" logic levels and signal interference between channels caused by crosstalk, slow edges, or invalid timing in response to specific events. The number of trigger types and the flexibility of triggers have continued to improve over the years.

Figure 2: Analog trigger system

A "digital" oscilloscope is an instrument that samples the measured signal and saves it as discrete digital values, while the trigger system of a general oscilloscope has always been used to process the original measured analog signal, so it is called an analog trigger system.

The input amplifier adjusts the measured signal to match its amplitude with the working range of the ADC and the display. The conditioned signal is then sent to the analog-to-digital converter (ADC) and the trigger system in parallel from the amplifier output.

The ADC samples the measurement signal through one path, and the digitized sample values ​​are written to the acquisition memory; on the other path, the trigger system compares the signal with a valid trigger event (such as the signal crossing the trigger threshold of the "edge" trigger). When a valid trigger condition occurs, the oscilloscope will finally determine the sample of the ADC and process and display the required waveform. Once the measurement signal crosses the trigger level, it will cause a valid trigger event. However, in order for the signal to be accurately displayed on the display, accurate trigger point timing must be provided. Otherwise, the displayed waveform will not overlap with the trigger point (the intersection of the trigger level and the trigger position).

This can be caused by a number of factors. First, the signal in the trigger system is compared to the trigger threshold by a comparator, and the edge time at the comparator output must be accurately measured using a time-to-digital converter (TDC).

If the measurement result of tdc is inaccurate, the displayed waveform will be offset from the trigger point, and each trigger event will change this offset, resulting in trigger jitter.

Another factor is the presence of error sources in the two paths of the measured signal. The signal is processed through two different paths (the acquisition path of the ADC and the trigger system path), both of which contain different linear and nonlinear distortions. This results in a systematic mismatch between the displayed signal and the determined trigger point. In the worst case, the trigger will not respond to valid trigger events even though these trigger events can be seen on the display, or the trigger will respond to trigger events that the acquisition path cannot capture and display.

The final factor is that there are different noise sources in the two paths, including amplifiers with different noise levels. This will cause delays and amplitude differences, which will appear on the display as a shift in the trigger position (jitter). When operating in digital triggering mode, the trigger will not have these errors.

Digital trigger

In contrast to analog trigger systems, digital trigger systems (Figure 3) operate directly on samples collected by the ADC. The signal is not separated into two paths, but the same required signal is processed and displayed. As a result, signal damage that exists in analog trigger systems can be fundamentally avoided. To evaluate the trigger point, the digital trigger uses a precise DSP algorithm to detect valid trigger events and accurately measure the timestamp. The challenge of performing real-time signal processing is the need to monitor the measured signal seamlessly. For example, the digital trigger in the R&S RTO series oscilloscope uses an 8-bit ADC to sample at a rate of 10 GS/s and processes data at a rate of 80 Gb/s.