"Logic pen, ordinary oscilloscope, storage oscilloscope, logic analyzer, etc. The multimeter and logic pen use are relatively simple, and the logic analyzer and storage oscilloscope is currently not very common in digital circuit teaching experiments. Oscilloscope is a very useful A relatively complex instrument. This chapter introduces the principle and method of use of the oscilloscope from the perspective of use.
01, oscilloscope working principle
The oscilloscope is a characteristic of electron oscillators that convert an altering electromatal signal that cannot be observed. The electronic measuring instrument is displayed on the fluorescent screen to measure. It is an important instrument that observes digital circuit experiments and analyzes experiments. The oscilloscope consists of an oscilloscope and a power supply system, a synchronization system, an X-axis deflecting system, a y-axis deflection system, a delayed scanning system, a standard signal source.
1, oscillating tube
Cathode Ray Tube (CRT) is abbreviated as an oscillobe, which is the core of the oscilloscope. It converts the electrical signal into a light signal. As shown in Figure 1, the electron gun, deflection system and the fluorescent screen are sealed in a vacuum glass shell, which constitutes a complete oscillating tube.
Figure 1 Interior structure and power supply icon of the oscillator
(1) Fluorescence screen
The current oscillator screen is usually a rectangular plane, and a layer of phosphorescent material is deposited in the inner surface constituting a fluorescent film. A layer of evaporated aluminum film is often increased on the fluorescent film. High-speed electrons pass through an aluminum film, impact fluorescent powder, and illuminate forming a bright spot. The aluminum film has internal reflection, which is conducive to improving the brightness of the bright spot. Aluminum film also has other functions such as heat dissipation.
When the electronic stops bombard, the highlight cannot disappear immediately and to keep a period of time. The time passed by the bright point of 10% of the bright spot to the original value is called "Yu Hui Time". The glow time is short of 10μs for extreme short hair, 10μs-1 ms is short, 1 ms-0.1s is medium Yuji, 0.1s-1s is long, greater than 1s is extremely long. The general oscilloscope is equipped with the overcome wave tube, and the high frequency oscilloscope selects a short resort, and the low frequency oscilloscope selects long-term.
Different colors can be emitted on the phosphorescent material from the phosphorescent material used. Generally, the oscilloscope uses green light-oriented oscillators to protect people's eyes.
(2) Electron gun and focus
The electron gun consists of a filament (F), the cathode (K), the gate (G1), the front accelerator (G2) (or the second gate), the first anode (A1), and the second anode (A2). Its role is to transmit electrons and form a very fine high-speed electron beam. The filament is powered by the cathode, and the cathode is thermally transmitted.
The gate is a metal garden cartridge with a small hole with a sleeve in the cathode. Since the gate potential is lower than the cathode, the electron is controlled to the cathode, generally only a small amount of electrons having a large motion initial speed, and can pass through the gate small hole under the action of the anode voltage, and run to the fluorescent screen. The electron with small initial speed is still returned to the cathode.
If the gate potential is too low, all electrons return the cathode, ie the tube is turned off. The W1 potentiometer in the regulating circuit can change the gate potential to control the electron flow density of the inverse fluorescent screen, thereby achieving the brightness of the highlight. The first anode, the second anode and the front acceleration are extremely three metal cylinders on the same axis with the cathode. The front accelerator G2 is connected to the A2, and the added potential is higher than A1. The positive potential of G2 has accelerated the cathode electron.
During the process of running from the cathode to the phosphor screen, after two focusing processes. The first focus is completed by K, G1, G2, and K, K, G1, G2 is called the first electron lens of the oscillator. The second focus occurs in the G2, A1, A2 region, adjusting the potential of the second anode A2, which enables the electron beam to be a point on the phosphor screen, which is the second focus. The voltage on the A1 is called a focus voltage, and A1 is also called focusing poles. Sometimes the adjustment A1 voltage still does not satisfy good focus, it is necessary to fine-tune the voltage of the second anode A2, and A2 is called auxiliary focus.
(3) deflection system
The deflection system controls the direction of electron ray, so that the variation of the spot on the fluorescent screen depicts the waveform of the measured signal. In Fig. 8.1, Y1, Y2, and XL, X2 two pairs of mutually perpendicular deflection plates constitute a deflection system. The Y-axis deflecting plate is before, the X-axis deflector plate is behind, so the Y-axis sensitivity is high (the signal is added to the Y axis after being processed). The two pairs of deflection plates add a voltage, so that the two pairs of deflection plates each form an electric field, and the electron beam is controlled in the vertical direction and the horizontal direction.
(4) Power supply of oscillating tube
In order to make the oscillator work normally, there is a certain amount of demand for power supply. It is specified that the potential between the second anode and the deflection plate is similar, and the average potential of the deflection plate is zero or close to zero. The cathode must work on a negative potential. The gate G1 is relative to the negative potential (-30V ~ -100V), and is adjustable to achieve luminance adjustment. The first male is positive (about + 100V ~ + 600V), and it should also be adjustable to adjust the focus adjustment.
The second anode is connected to the front acceleration, and the cathode is positive and high pressure (about + 1000V), and the adjustable range of the ground potential is ± 50V. Since the electrode currents of the oscillate tube are small, the public high pressure can be powered by the resistance divider.
02, the basic composition of the oscilloscope
As can be seen from the previous section, the pattern shape of the oscillator display can be controlled as long as the voltage on the X-axis deflection plate and the Y-axis deflection plate can be controlled. We know that an electronic signal is a function f (t) of time, which varies over time. Therefore, as long as a voltage proportional to the time variable is added to the X-axis deflection plate of the oscillator, the measured signal is added to the Y-axis, and the ratio is displayed. The signal varies over time. In the electrical signal, the signal proportional to the time variable in a period of time is a sawtooth wave.
The basic composition block diagram of the oscilloscope is shown in Figure 2. It consists of five parts: oscillator, Y-axis system, X-axis system, Z-axis system, and power supply.
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Figure 2 oscilloscope basic composition block diagram
The measured signal 1 receives the "Y" "input, sent to the Y1 amplifier (front amplification), push-pull output signal 2 and 3 after the Y-axis attenuator (preamplifier), the latency delay г1 time, to the Y2 amplifier. After amplifying a sufficiently large signal 4 and 5, add it to the Y-axis deflection plate of the oscillator. In order to display a complete stable waveform on the screen, the measured signal of the Y-axis is introduced into the trigger circuit of the X-axis system. A level of trigger pulse 6 is generated at a level value of the positive (or negative) polarity of the introduction signal, and the zigzag scanning circuit (time base generator) is activated to generate a scan voltage 7.
Since the X-axis starts scanning before the X-axis starts scanning from the trigger to the start-up scan, the Y-axis starts scanning before the Y-axis signal reaches the fluorescent screen, the Y-axis delay time г1 should be slightly larger than the delay time г2 of the X-axis. The scan voltage 7 is amplified by the X-axis amplifier, generating push-pull output ⑨ and ⑩, adding to the oscillate of the x-axis deflection plate. The Z-axis system is used to enlarge the scan voltage direction and become a forward rectangular wave, sent to the oscillator gate. This makes a certain fixed luminance in the waveform displayed in the scan, and scanning backhaul.
The above is the basic working principle of the oscilloscope. Double trace shows that two different measured signals input by the Y-axis input by electronic switches are displayed on the phosphor screen, respectively. Due to the visual suspension of the human eye, when the conversion frequency is high to a certain extent, two stable, clear signal waveforms are seen.
The oscilloscope often has a precise and stable square wave signal generator for the calibrator.
03, oscilloscope use
This section describes how the oscilloscope is used. The oscilloscope type, a lot of models, and the function is different. More than 20 MHz or 40MHz dual tracers are used in digital circuit experiments. These oscilloscopes are similar to those with similar. This section is not targeting an oscilloscope for a model, just introducing the common functions of oscilloscopes in digital circuit experiments.
1, fluorescent screen
The fluorescent screen is a display portion of the oscillator. The horizontal direction and the vertical direction of the screen each have multiple ticks, indicating the relationship between the voltage and time between the signal waveforms. The horizontal direction indicates the time, the vertical direction indicates the voltage. The horizontal direction is divided into 10 grids, and the vertical direction is divided into 8 grids, and each piece is divided into 5 copies. The vertical direction is marked with 0%, 10%, 90%, 100% or the like, and the horizontal direction is marked with 10%, 90% signs, and the parameters such as DC level, AC signal amplitude, delay time are used. The voltage value and time value can be obtained by multiplying the grid of the measured signal on the screen by appropriate proportion constant (V / DIV, TIME / DIV).
2, oscillating tube and power system
(1) Power (Power)
Oscilloscope main power switch. When this switch presses, the power indicator is on, indicating that the power is turned on.
(2) Intensity
Rotate this knob to change the brightness of the spot and the scan line. Observing the low frequency signal is small, and the high frequency signal is large.
It is generally not too bright to protect the fluorescent screen.
(3) Focus (FOCUS)
The focus knob adjusts the size of the electron beam, focusing the scan line into the clearest state.
(4) Scale brightness (Illuminance)
This knob adjusts the brightness of the light after the fluorescent screen. Under normal indoor light, the lighting is dark. In an environment in which indoor light is insufficient, it can be properly tightened.
3, vertical deflection factor and horizontal deflection factor
(1) Vertical deflection factor selection (Volts / DIV) and fine-tuning
Under the unit input signal, the distance from the spot on the screen is referred to as offset sensitivity, which is applicable to the X-axis and the Y-axis. The reciprome of sensitivity is called the deflection factor. The unit of vertical sensitivity is cm / v, cm / mv or Div / MV, DIV / V, and the number of vertical deflection factors is V / cm, MV / CM or V / DIV, MV / DIV. In fact, due to the convenience of habits and measurement voltage readings, the deflection factor is sometimes sensitive.
Each passage in the trace is a vertical deflection factor selecting a band switch. Generally, according to 1, 2, 5, from 5 mV / div / div / div to 10 files. The value indicated by the band switch represents a voltage value of the vertical direction on the fluorescent screen. For example, when the wave switch is placed in a 1V / DIV file, if the signal spot on the screen moves on the screen, represent the input signal voltage change 1V.
Each band switch often has a small knob that fine-tunes the number of vertical deflection per gear. Spin it in the clockwise direction, in the "calibration" position, at which point the vertical deflection factor is consistent with the value indicated by the band switch. Rotate this knob counterclockwise, you can fine-tune the number of vertical deflection. After the vertical deflection factor is fine-tuning, it will cause inconsistency with the indication value of the band switch, which should be noted. Many oscilloscopes have vertical extensions, and when the fine-tuning knob is pulled out, the vertical sensitivity expands several times (the deflection factor reduces several times). For example, if the deflection factor indicated by the band switch is 1V / DIV, the vertical deflection factor is 0.2V / DIV when using the × 5 extended state.
When doing digital circuit experiments, the ratio of the vertical moving distance of the signal measured on the screen is often used to determine the voltage value of the measured signal.
(2) Time base selection (TIME / DIV) and fine-tuning
The use of time base selection and fine-tuning is similar to the vertical deflection factor. Time base selection is also achieved by a band switch, and the time base is divided into several gears in 1, 2, 5. The indication value of the band switch represents the time value of the spot moving in the horizontal direction. For example, in the 1μs / DIV file, the spot moves on the screen to represent the time value 1 μs.
The "fine tune" knob is used for time base calibration and fine-tuning. When the clockwise direction is rotated in the calibration position, the time base value displayed on the screen coincides with the nominal value shown in the band switch. Rotate the knob counterclockwise, then the time base is finely adjusted. After the knob is pulled out, it is in the scanned extension. It is usually × 10 extension, i.e., horizontal sensitivity is 10 times, and the time base is reduced to 1/10. For example, at 2μs / DIV gear, the time value of the highlights on the fluorescent screen in the scan extension state is equal to
2μs × (1/10) = 0.2μs
There are 10MHz, 1 MHz, 500kHz, 100kHz clock signal on the TDS test station, generated by quartz crystal oscillator and divider, high accuracy, can be used to calibrate the oscilloscope.
The standard signal source CAL of the oscilloscope is specifically used to calibrate the time base and vertical deflection factor of the oscilloscope. For example, a COS5041 oscilloscope standard source provides a square wave signal of VP-P = 2V, F = 1 kHz.
The Position knob on the front panel of the oscilloscope adjusts the position of the signal waveform on the phosphor screen. The rotation horizontal displacement knob (marked with a horizontal bidirectional arrow) moves the signal waveform, rotating the vertical displacement knob (labeled a vertical bidirectional arrow) up and down movement signal waveform.
4, input channel and input coupling selection
(1) Input channel selection
There are at least three optional modes of the input channel: channel 1 (CH1), channel 2 (CH2), dual channel (DUAL). When the channel 1 is selected, the oscilloscope only displays the signal of the channel 1. When the channel 2 is selected, the oscilloscope only displays the signal of the channel 2. When the dual channel is selected, the oscilloscope simultaneously displays the channel 1 signal and channel 2 signal. When testing signals, first connect the oscilloscope to the measured circuit.
Depending on the selection of the input channel, insert the oscilloscope probe to the corresponding channel socket, the oscilloscope probe is connected to the measured circuit, and the oscilloscope probe is in contact with the point. There is a pair of digits on the oscilloscope probe. When this switch is turned to the "× 1" position, the measured signal is not attenuated to the oscilloscope, and the voltage value read from the phosple screen is the actual voltage value of the signal. When this is turned to the "× 10" position, the measured signal attenuation is 1/10, and then sent to the oscilloscope, multiplied by the voltage value read from the phosphor screen to the actual voltage value of the signal.
(2) Enter a coupling method
There are three options in the input coupling: AC (AC), GND, DC (DC). When "ground" is selected, the scan line displays the position of the "oscilloscope" on the phosphor screen. DC coupling is used to determine signal DC absolute values and observation extreme low frequency signals. AC coupling is used to observe the AC signals containing DC components. In digital circuit experiments, "DC" mode is generally selected to obtue the absolute voltage value of the signal.
5, trigger
The first section indicates that after the measured signal is input from the Y-axis, a portion is sent to the Y-axis deflecting plate of the oscillator, and the drive light is moved in the vertical direction on the fluorescent screen; the other part is divided into X-axis deflection system generation trigger Pulse, trigger the scan generator, generates a repeated zigzag voltage to the X deflection plate of the oscillator, moving the spot in the horizontal direction, and the two combination, the graphic drawn on the fluorescent screen is the measured signal Graphic.
It can be seen that the correct trigger method directly affects the effective operation of the oscilloscope. In order to obtain a stable, clear signal waveform on the fluorescent screen, it is important to master basic triggering functions and its operational methods.
(1) Trigger source (SOURCE)
To display a stable waveform on the screen, you need to add the measured signal itself or a trigger signal that has a certain time relationship with the measured signal to a trigger circuit. The trigger source selects to determine where the trigger signal is supplied. There are usually three triggers: internal trigger (int), power trigger(Line), external trigger EXT).
The internal trigger uses the measured signal as a trigger signal is a triggering method that is often used. Since the trigger signal itself is part of the measured signal, a very stable waveform can be displayed on the screen. The channel 1 or channel 2 can be selected as a trigger signal in a dual trace oscilloscope.
The power supply trigger uses the AC power frequency signal as the trigger signal. This method is effective when measuring the signal related to the AC power frequency. Especially in the measurement of the audio circuit, the low level AC noise of the gate flow tube is more effective.
External triggering uses an add-on signal as a trigger signal, the plus signal is input from the external contact input. The external trigger signal and the measured signal should have periodic relationship. Since the measured signal is not used as a trigger signal, when the scan is not related to the measured signal.
Correctly selecting the trigger signal to the stability of the waveform display, clear and clear. For example, in the measurement of the digital circuit, for a simple periodic signal, the selection can be selected, and for a signal having a complex period, there is a signal with which there is a cyclical relationship, it may be more it is good.
(2) Trigger coupling mode
There are a variety of ways to trigger signals to trigger circuits, and the purpose is to trigger the stability and reliability of the signal. Here are several commonly used.
AC coupling is also known as capacitive coupling. It only allows the AC component of the trigger signal to be triggered, the DC component of the trigger signal is partitioned. 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 10 Hz, it will cause difficulty triggering.
DC coupling (DC) does not partition the DC component of the trigger signal. When the frequency of the trigger signal is low, the DC coupling is preferably used when the duty cycle of the trigger signal is large.
When the low frequency suppression (LFR) is triggered, the trigger signal is added to the trigger circuit, and the low frequency component of the trigger signal is suppressed; when the high frequency suppression (HFR) is triggered, the trigger signal is added to the trigger circuit, trigger the signal. High frequency components are inhibited. In addition, there is also a TV synchronization (TV) triggering for television maintenance. These trigger coupling methods have their own scope and need to be in use.
(3) Triggering level (Level) and triggering polarity (SLOPE)
Touch the hair level adjustment is also called synchronous adjustment, which makes the scan synchronize 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. Clockwise rotate the knob clockwise, the truncation level rises; counterclockwise rotates the knob, and the trip power is lowered. When the level knob is transferred to the level lock position, the trigger level is automatically kept within the amplitude of the trigger signal, and a stable trigger can be generated without the need to level adjustment. When the signal waveform is complicated, when the level knob cannot be stably triggered, the Holdoff is adjusted with the HOLDOFF knob to adjust the waveform, and the scan is stabilized to synchronize the scan.
Polar switches are used to select the polarity of the trigger signal. When dialing in the "+" position, in the direction of the signal, the trigger is generated when the trigger signal exceeds the trigger hour. When all of the "-" is dial, in the direction of the signal reduction, triggering is generated when the trigger signal exceeds the trigger. Trigger polarity and trigger levels to determine the trigger points of the trigger signal.
6, scanning mode (SWEEPMODE)
Scanning has three types of scanning methods for automatic (AUTO), normal (NORM) and Single.
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Automatic: When there is no signal input, or when the trigger signal frequency is less than 50 Hz, the scan is self-excited.
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Normal: When there is no signal input, the scan is in the preparation state, no scan line. Trigger the scan after triggering the signal arrival.
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Single: Single button is similar to a reset switch. In a single scan mode, the scan circuit is reset when pressing a single button. At this time, it is ready to be on. Trigger the signal to generate a scan. After the single scan is over, the light is prepared. Single scan for observing non-periodic signals or single transient signals, often need to take pictures of waveforms.
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The basic functions and operations of the oscilloscope should be described above. The oscilloscope has some more complex features such as delay scanning, trigger delays, X-Y work mode, etc., this will not be introduced. The oscilloscope entry operation is easy, and it is truly skilled to be in the application. It is worth noting that although the oscilloscope is more functions, many other instruments are better in many cases. For example, in digital circuit experiments, it is judged that a single pulse having narrow pulse is more than a logical pen, and the logical pen is more simple; when the single pulse width is measured, it is better to use a logic analyzer.
04, digital oscilloscope must pay attention to problems
1 Introduction
The digital oscilloscope is increasingly popular with unique advantages such as waveform triggering, storage, display, measurement, waveform data analysis processing. Since there is a large performance difference between the digital oscilloscope and the analog oscilloscope, if the use is improper, a large measurement error is generated, thereby affecting the test task.
2, distinguish analog bandwidth and digital real-time bandwidth
Bandwidth is one of the most important indicators of oscilloscopes. The bandwidth of the analog oscilloscope is a fixed value, while the bandwidth of the digital oscilloscope has two types of analog bandwidth and digital real-time bandwidth. Digital oscilloscopes use sequential samples or random sampling techniques that can achieve digital real-time bandwidth, digital real-time bandwidth and maximum digitized frequency and waveform reconstruction technology factor K correlation (digital real-time bandwidth = maximum digitization rate / K) It is generally not directly given as an indicator.
As can be seen from the definition of two bandwidths, the analog bandwidth is only suitable for repeated periodic signals, while digital real-time bandwidth is suitable for repetitive signals and single signal measurements. The manufacturer claims that the bandwidth can reach how many megabytes, actually refers to analog bandwidth, and the digital real-time bandwidth is lower than this value. For example, TEK's TES520B has a bandwidth of 500 MHz, which is actually referred to as its analog bandwidth is 500MHz, while the highest digital real-time bandwidth can only reach 400MHz far below analog bandwidth. So when you measure a single signal, you must refer to the digital real-time bandwidth of the digital oscilloscope, otherwise it will bring unexpected errors to the measurement.
3. About sampling rate
The sampling rate is also referred to as a digitization rate, refers to the number of sampling of an analog input signal within a unit time, often in MS / S. Sampling rate is an important indicator of a digital oscilloscope.
(1) If the sampling rate is not enough, it is easy to have a mixed phenomenon.
If the oscilloscope's luminos signal is a 100 kHz sinusoidal signal, the signal frequency displayed by the oscilloscope is 50 kHz. What is going on? This is because the sampling rate of the oscilloscope is too slow, producing a mixed phenomenon. Aligning is that the waveform frequency displayed on the screen is lower than the actual frequency of the signal, or even if the trigger indicator on the oscillier is already on, the displayed waveform is still unstable. The result is made as shown in Figure 1.
Then, for a waveform of an unknown frequency, how to determine if the displayed waveform has been mixed? It can be slowly changed by slowing the sweep speed T / DIV to the faster time frame, whether the frequency parameters of the waveform changes sharply, if yes, the waveform is mixed; or the waveform of the shake is in a faster time. Stable, also shows that the waveform is mixed. According to the Nyquist theorem, the sampling rate is at least twice the signal high frequency component without deterioration, such as a 500 MHz signal, at least 1 Gs / s sampling rate. There are several ways to simply prevent mixing:
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Adjust the sweep;
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AutoSet;
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Try to switch the collection method to the envelope or peak detection method, because the envelope is to find extreme values in multiple collection records, and peak detection mode is looking for maximum minimum in a single collection record, both methods Fast signal changes can be detected.
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If the oscilloscope has an Instavu collection method, it can be used because this mode is collected fast, and the waveform displayed by this method is similar to the waveform displayed with analog oscilloscope.
(2) The relationship between sampling rate and T / DIV
The maximum sampling rate of each digital oscilloscope is a fixed value. However, at any of the scan time T / DIV, the sampling rate fs is given by the following formula:
FS = n / (t / div) n is a sample point
When the sample point N is a certain value, the FS is inversely proportional to T / DIV, the larger the sweep, the lower the sampling rate.
In summary, when using a digital oscilloscope, in order to avoid aliasing, the sweeping speed is preferably placed in a fast sweep. If you want to capture a fleeting burr, the spanning speed is preferably placed in a slower position. "
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