Oscilloscope Classroom: The Composition of an Oscilloscope

Oscilloscope basic block diagram

　　The measured signal ① is connected to the "Y" input terminal, and after being properly attenuated by the Y-axis attenuator, it is sent to the Y1 amplifier (preamplifier) ​​to push-pull output signals ② and ③. After being delayed by the delay stage for Г1 time, it reaches the Y2 amplifier. After amplification, sufficiently large signals ④ and ⑤ are generated and added to the Y-axis deflection plate of the oscilloscope. In order to display a complete and stable waveform on the screen, the measured signal ③ of the Y-axis is introduced into the trigger circuit of the X-axis system, and a trigger pulse ⑥ is generated at a certain level value of the positive (or negative) polarity of the introduced signal, which starts the sawtooth wave scanning circuit (time base generator) and generates a scanning voltage ⑦. Since there is a time delay Г2 from triggering to starting the scan, in order to ensure that the X-axis starts scanning before the Y-axis signal reaches the fluorescent screen, the delay time Г1 of the Y-axis should be slightly greater than the delay time Г2 of the X-axis. The scanning voltage ⑦ is amplified by the X-axis amplifier to generate push-pull outputs ⑨ and ⑩, which are added to the X-axis deflection plate of the oscilloscope. The z-axis system is used to amplify the positive course of the scanning voltage and convert it into a positive rectangular wave and send it to the oscilloscope grid. This makes the waveform displayed on the forward scanning path have a certain fixed brightness, while smearing occurs on the return scanning path.

　　The above is the basic working principle of the oscilloscope. The dual-trace display uses an electronic switch to display two different measured signals input from the Y axis on the fluorescent screen. Due to the visual persistence of the human eye, when the conversion frequency reaches a certain level, two stable and clear signal waveforms are seen. The oscilloscope often has an accurate and stable square wave signal generator for calibrating the oscilloscope.

　　What is a trigger?

　　Any oscilloscope has limited memory, so all oscilloscopes must use triggers. A trigger is an event of interest that the oscilloscope should find. In other words, it is something that the user wants to find in the waveform. A trigger can be an event (i.e., a problem in the waveform), but not all triggers are events. Examples of triggers include edge triggers, glitch signal triggers, and digital pattern triggers.

　　The reason oscilloscopes must use triggers is that they have limited memory. For example, the Agilent 90000 Series oscilloscopes have a memory depth of 2 billion samples. However, even with such a large memory, the oscilloscope still needs some events to distinguish which of the 2 billion samples it needs to display to the user. Although 2 billion samples may sound like a lot, it is still not enough to ensure that the oscilloscope memory can capture the event of interest.

　　The memory of an oscilloscope can be thought of as a conveyor belt. Whenever a new sample is taken, it is stored in memory. When memory is full, the oldest samples are deleted to allow the newest samples to be stored. When a trigger event occurs, the oscilloscope captures enough samples to store the trigger event in the required location in memory (usually in the middle), and then displays this data to the user.

　　The first section points out that after the measured signal is input from the Y axis, a part of it is sent to the Y axis deflection plate of the oscilloscope, driving the light spot to move vertically in proportion on the fluorescent screen; the other part is diverted to the x axis deflection system to generate a trigger pulse, triggering the scanning generator, generating a repetitive sawtooth wave voltage to be added to the X deflection plate of the oscilloscope, so that the light spot moves horizontally. The two are combined, and the pattern drawn by the light spot on the fluorescent screen is the measured signal pattern. It can be seen that the correct triggering method directly affects the effective operation of the oscilloscope. In order to obtain a stable and clear signal waveform on the fluorescent screen, it is very important to master the basic triggering function and its operation method.

　　Automatic mode and trigger mode

　　What happens if no trigger event occurs? This is a very good question. In this case, the waveform on the screen will not update. This is not what we want because the user may not know how to change the trigger to get the waveform on the screen. For example, if the probe slips off, the oscilloscope will stop triggering. However, if the screen does not update, the loss of signal will not be obvious.

　　To solve this problem, oscilloscopes have a mode called "Auto" triggering. In this mode, if a trigger cannot be found within a period of time, the oscilloscope will automatically trigger to update the screen. Usually, there is some indicator on the oscilloscope (such as an LED on the front panel) to indicate whether the last trigger was a real trigger or an automatic trigger. This way, if the user sees the "Auto" indicator, they will know that the trigger they set did not occur. For example, if the user set the trigger to be a glitch signal, they will know that the oscilloscope did not detect the glitch signal.

　　However, as you will recall from the previous paragraph, when auto-triggering occurs, it means that after each trigger, there is a hang time while the oscilloscope re-arms. To avoid this time entirely, the oscilloscope should be changed to "triggered" mode. (This is called "normal" mode on some oscilloscopes.) In "triggered" mode, the oscilloscope will not trigger unless a trigger event is found. Therefore, if the user sets the trigger mode to glitch and the oscilloscope never triggers, the user can be confident that the glitch did not occur (at least as far as the oscilloscope can detect).

　　1. Trigger source selection

　　To display a stable waveform on the screen, the measured signal itself or a trigger signal with a certain time relationship with the measured signal needs to be added to the trigger circuit. The trigger source selection determines where the trigger signal is supplied. There are usually three trigger sources: internal trigger (INT), power trigger (LINE), and external trigger (EXT).

　　Internal triggering uses the measured signal as the trigger signal, which is a commonly used triggering method. Since the trigger signal itself is part of the measured signal, a very stable waveform can be displayed on the screen. In a dual-trace oscilloscope, either channel 1 or channel 2 can be selected as the trigger signal.

　　Power triggering uses the AC power frequency signal as the trigger signal. This method is effective when measuring signals related to the AC power frequency. It is especially effective when measuring low-level AC noise in audio circuits and thyristors.

　　External triggering uses an external signal as a trigger signal, and the external signal is input from the external trigger input terminal. There should be a periodic relationship between the external trigger signal and the measured signal. Since the measured signal is not used as a trigger signal, when to start scanning has nothing to do with the measured signal.

　　Correctly selecting the trigger signal has a great impact on the stability and clarity of the waveform display. For example, in the measurement of digital circuits, for a simple periodic signal, it may be better to choose an internal trigger, while for a signal with a complex period and a signal with a periodic relationship with it, it may be better to choose an external trigger.

　　(1) Trigger coupling mode selection

　　There are many ways to couple the trigger signal to the trigger circuit in order to ensure the stability and reliability of the trigger signal. Here are some commonly used methods.

　　AC coupling is also called capacitive coupling. It only allows the AC component of the trigger signal to trigger, and the DC component of the trigger signal is blocked. This coupling method is usually used when the DC component is not considered to form a stable trigger. However, if the frequency of the trigger signal is less than 10Hz, it will cause triggering difficulties.

　　DC coupling (DC) does not block the DC component of the trigger signal. When the frequency of the trigger signal is low or the duty cycle of the trigger signal is large, it is better to use DC coupling.

　　When low frequency suppression (LFR) is triggered, the trigger signal is added to the trigger circuit through a high-pass filter, and the low-frequency component of the trigger signal is suppressed; when high frequency suppression (HFR) is triggered, the trigger signal is added to the trigger circuit through a low-pass filter, and the high-frequency component of the trigger signal is suppressed. In addition, there is a TV synchronization (TV) trigger for TV maintenance. These trigger coupling methods each have their own scope of application, which needs to be experienced in use.

　　(2) Trigger level (Level) and trigger polarity (Slope)

　　Trigger level adjustment is also called synchronization adjustment, which synchronizes the scan with the measured signal. The level adjustment knob adjusts the trigger level of the trigger signal. Once the trigger signal exceeds the trigger level set by the knob, the scan is triggered. Turn the knob clockwise to increase the trigger level; turn the knob counterclockwise to decrease the trigger level. When the level knob is adjusted to the level lock position, the trigger level automatically remains within the amplitude of the trigger signal, and a stable trigger can be generated without level adjustment. When the signal waveform is complex and the level knob cannot be used to stably trigger, use the Hold Off knob to adjust the waveform's holdoff time (scan pause time) to synchronize the scan with the waveform.

　　The polarity switch is used to select the polarity of the trigger signal. When the switch is in the "+" position, a trigger is generated when the trigger signal exceeds the trigger level in the direction of signal increase. When the switch is in the "-" position, a trigger is generated when the trigger signal exceeds the trigger level in the direction of signal decrease. The trigger polarity and the trigger level jointly determine the trigger point of the trigger signal.