[Sticky] أساسيات راسم الإشارة Oscilloscope basics
أساسيات راسم الإشارة Oscilloscope basics
Oscilloscope is an instrument that displays the characteristics of the signal waveform over time.
The oscilloscope can transform electrical signals that are invisible to the naked eye into visible images (waveforms), which is convenient for people
Study the changing process of various electrical phenomena
راسم الإشارة هو أداة تعرض خصائص شكل موجة الإشارة مع مرور الوقت.
يمكن أن يقوم راسم الإشارة (الذبذبات) بتحويل الإشارات الكهربائية غير المرئية بالعين المجردة إلى صور مرئية (أشكال موجية) ، وهي ملائمة للأشخاص
الذين يريدون دراسة العملية المتغيرة لمختلف الظواهر الكهربائية
**Description and function
We can simply think of an oscilloscope as a voltmeter with a graphic display.
يمكننا ببساطة التفكير في راسم الإشارة كمقياس فولت ميتر مع عرض رسومي(جرافيك)
A common voltmeter is a pointer or digital display that moves on its dial to give a measurement reading of the signal voltage. While the oscilloscope
It's different. The oscilloscope has a screen, which can graphically display the change of the signal voltage over time, that is, the waveform.
The main differences between an oscilloscope and a voltmeter are الفرق بين الآفو ميتر وراسم الإشارة:
1. The voltmeter can give the value of the measured signal, which is usually the effective value, that is, the RMS value. But the voltmeter cannot give the relevant information
Number shape information. Some voltmeters can also measure the peak voltage and frequency of the signal. However, the oscilloscope can be displayed graphically.
Shows the historical situation of the signal over time.
2. A voltmeter can usually only measure one signal, while an oscilloscope can display two or more signals at the same time.
The display device of the oscilloscope is a cathode ray tube, abbreviated as CRT,
The basis of an polar tube is a system capable of generating electrons, called an electron gun. Electron gun
Emit electrons to the screen. The electrons emitted by the electron gun are focused to form an electron beam and hit the electron beam.
On the center of the screen. The inner surface of the screen is coated with a fluorescent substance so that the electron beam strikes
The center point emits light.
The electrons pass through the deflection system on the way from the electron gun to the screen. Deflection system
Applying a voltage to it can cause the light spot to move on the screen. The deflection system consists of a horizontal (X) deflection plate and a vertical (Y) deflection plate.
This type of deflection is called electrostatic deflection.
A number of horizontal and vertical straight lines are formed on the inner surface of the screen by means of scoring or etching, called a ruler. Ruler
Usually there are 8 in the vertical direction and 10 in the horizontal direction, and each grid is 1cm. Some ruler lines are further divided into small cells, and
There are also special lines marked 0% and 100%. These special lines are used with 10% and 90% rulers for ascent
Measurement of time. We will discuss this later.
As mentioned above, the fluorescent substance on the CRT emits light after being bombarded by electrons. When the electron beam is removed, the fluorescent substance
Will continue to glow for a short time. This time is called afterglow time. The length of afterglow time varies with different fluorescent materials.
The most commonly used fluorescent substance is P 31, and the afterglow time is less than one millisecond (ms). The fluorescent substance P 7 has a longer afterglow time, about
300ms, which is useful for observing slower signals. The P 31 material emits green light, while the P 7 material emits yellow-green color.
The input signal is applied to the Y-axis deflection plate, and the oscilloscope itself scans the electron beam along the X-axis direction. This makes the light point at
The waveform of the input signal is drawn on the screen. The signal waveform swept out in this way is called a waveform trace.
The control mechanisms that affect the screen are:
The brightness control is used to adjust the brightness of the waveform display. The oscilloscope used as an example in this book can
Scan speed automatically adjusts brightness. When the electron beam moves faster, the time that the fluorescent substance is excited becomes shorter, so it must be
Increase the brightness to see the trace clearly. In contrast, when the electron beam moves slowly, the light spot on the screen becomes very bright, so the brightness must be reduced
So as not to burn out the fluorescent substance. Thereby extending the life of the oscilloscope.
For the text portion on the screen, there is a separate brightness control mechanism.
The focus control mechanism is used to control the size of the light spot on the screen in order to obtain a clear waveform trace. Some oscilloscopes, such as this
On an oscilloscope used as an example, the focusing is also optimally controlled by the oscilloscope itself.
Keep a clear waveform trace while scanning. Manually adjustable focus control is also provided.
This control mechanism aligns the X-axis scan line with the horizontal ruler line. Since the Earth's magnetic field is different everywhere, this will
Will affect the scanning line displayed by the oscilloscope. The trace rotation function is used to compensate for this. The scan rotation function is preset.
Usually only need to adjust after the oscilloscope is moved.
— Ruler lighting
Ruler brightness can be controlled individually. This is useful for screen photography or when working in low light conditions.
The brightness of the scan can be changed electrically by an external signal. This is for horizontal offsets generated by external signals.
It is very useful in applications that use X-Y display to find frequency relationships.
This signal input is usually a BNC socket on the rear panel of the oscilloscope.
CRT is the foundation of all oscilloscopes. Now we know something about it. Let's take a look at how the oscilloscope works
For the heart of the oscilloscope.
We have already seen that the oscilloscope has two vertical deflection plates, two horizontal deflection plates and an electron gun. Emitted from an electron gun
The intensity of the electron beam can be controlled electrically.
On the basis of the above technique, add the circuit described below to form a complete oscilloscope (see Figure 2)
Figure 2 Block diagram of an analog oscilloscope
The oscilloscope's vertical deflection system includes:
—Input attenuator (one per channel)
—Preamp (one per channel)
— An electronic switch to select which input channel to use
Oscilloscope's horizontal deflection system includes: time base, trigger circuit and horizontal deflection amplifier
The brightness control circuit uses electronics to turn on and off the trace at the appropriate time.
For all these circuits to work, the oscilloscope needs a power source. This power comes from AC mains or from inside or outside the machine.
The cell gets energy to make the oscilloscope work. The basic performance of any oscilloscope is determined by the characteristics of its vertical deflection system.
So let's first examine this part in detail.
1.3 vertical deflection
The vertical deflection system scales the input signal so that it can be displayed on the screen. Oscilloscope can show peak-to-peak
The voltage is a signal from a few millivolts to tens of volts. Therefore, signals of different amplitudes must be transformed to fit the display range of the screen.
Then you can measure the waveform according to the ruler scale. Therefore, it is required to attenuate large signals and amplify small signals. Show
The sensitivity or attenuator control of the wave filter is set for this purpose.
Sensitivity is measured in volts per division. Looking at Figure 3, you can know that the sensitivity is set to 1V / div. Therefore, peak to peak
A signal with a value of 6V causes the trace to deflect within 6 divisions in the vertical direction. Know the sensitivity settings and electronics of the oscilloscopeThe number of grids scanned by the beam in the vertical direction, we can measure the peak-to-peak voltage of the signal.
On most oscilloscopes, the sensitivity control is changed in steps of 1-2-5. That is sensitivity. Set upside down to
10mV / div, 20mV / div, 50mV /, 100mV / div and so on. Sensitivity is usually controlled using the amplitude up / down buttons, while
In some oscilloscopes, this is done by turning the vertical sensitivity knob.
If you cannot adjust the signal using these sensitivity steps so that it can be accurately displayed on the screen as required, then you can
Use variable (VAR) control.
A good example. The variable control can continuously adjust the sensitivity in steps of 1-2-5. Usually when using
When changing the control, the exact sensitivity value is unknown. We only know that the sensitivity of the oscilloscope is two in the 1-2-5 sequence.
A value between the step values. At this point we call the Y deflection of the channel uncalibrated or expressed as "uncal". This is not calibrated
The status is usually indicated on the oscilloscope's front panel or screen.
In more modern oscilloscopes, such as the oscilloscope we use as an example, due to the use of modern advanced technology to control and
calibration. Therefore, the sensitivity of the oscilloscope can be continuously changed between the minimum and maximum values, and it is always kept in the calibration state.
On older oscilloscopes, the channel sensitivity setting is read from the scale around the sensitivity control knob. While in new
On a type of oscilloscope, the channel sensitivity setting value is clearly displayed on the screen, as shown in Figure 3, or displayed on a separate CD.
The display shows.
Figure 3 With a sensitivity of 1v / div, a signal with a peak-to-peak value of 6v deflects the electron beam by 6 divisions in the vertical direction.
The coupling control mechanism determines the input signal from the BNC input on the front panel of the oscilloscope to the channel's vertical deflection system.
Part of the way. Coupling control can be set in two ways, namely DC coupling and AC coupling.
DC coupling provides a direct connection path for signals. The signal therefore provides a direct connection path. So all of the signal
The components (AC and: DC) affect the waveform display of the oscilloscope.
In AC coupling, a capacitor is connected in series between the BDC terminal and the attenuator. In this way, the DC component of the signal is blocked, and the signal
The low-frequency AC component of the signal will also be blocked or greatly attenuated. The low frequency cutoff frequency of the oscilloscope means that the signal amplitude displayed by the oscilloscope is only
The signal frequency is 71% straight and solid. The low frequency cutoff frequency of the oscilloscope is mainly determined by the value of its input coupling capacitance.
The low frequency cutoff frequency of the oscilloscope is typically 10Hz, as shown in Figure 4.
Figure 4 illustrates a simplified input circuit with AC and DC coupling, input ground, and 50? input impedance function selection
Another function related to the coupling control mechanism is the input ground function. At this time, the input signal and the attenuator are disconnected and attenuated.
The input of the amplifier is connected to the ground level of the oscilloscope. When selecting ground, you will see a straight line at 0V level on the screen. This
You can use the position control mechanism to adjust this reference level or scan the position of the baseline.
The input impedance of most oscilloscopes is 1M? and is associated with approximately 25pF. This is sufficient for most applications, because
It has minimal loading effect on most circuits.
Some signals come from 50? output impedance sources. In order to accurately measure these signals and avoid distortion, these
The signal is correctly transmitted and terminated. In this case, a 50? characteristic impedance cable should be used and terminated with a 50? load. certain
Some oscilloscopes, such as the PM3094 and PM3394A, have a 50? load inside to provide a user-selectable function. for
To avoid misoperation, you need to confirm again when selecting this function. For the same reason, 50? input impedance cannot work with some probes.
With the use of.
The vertical position control or POS control mechanism controls the position of the trace on the screen's Y axis. Select ground in the input coupling control, this
When the input signal is disconnected, the position of the ground level can be found. Separate ground level indicator on more advanced oscilloscopes
Indicator, which allows the user to continuously obtain the reference level of the waveform.
The dynamic range is the maximum amplitude of the signal that the oscilloscope can display without distortion. Under this signal amplitude, just adjust the oscilloscope's
The full position of the waveform can still be observed in the vertical position. For Fluke oscilloscopes, the typical dynamic range is 24 (3
Add and reverse
It doesn't seem to make sense to simply add the two signals together. Of course, reverse one of the two related signals, and then
Adding the two actually achieves the subtraction of the two signals. This is useful for eliminating common-mode interference (ie, hum) or for differential
Measurements are very useful.
Subtract the input signal from the output signal of a system, and then perform the appropriate ratio transformation to detect the cause of the system under test.
Because many electronic systems have their own reverse characteristics, this can be achieved by simply adding the two input signals of the oscilloscope.
Subtract our desired signals.
Alternate and intermittent
The oscilloscope CRT itself can only display one trace at a time. However, in many oscilloscope applications, signal comparisons are often performed.
Compare, for example, the relationship between input / output signals, or the delay of a system to a signal. This requires the oscilloscope to actually
Can display more than one signal at the same time.
To achieve this, there are two ways to control the electron beam:
1. Alternately finish one sweep and then another. This method is called alternating mode, or ALT mode for short.
2. You can quickly switch on or off between the two traces to draw the two traces in sections. This is called intermittent
Mode or CHOP mode. The result is that two traces are drawn one after the other during a scan.
The discontinuous mode is suitable for displaying low-frequency signals at a low time base rate, because the chopper switch can be quickly switched at this time.
Alternate mode is suitable for the display of high-frequency signals that require a faster time base setting. The oscilloscope we use as an example in this book
Automatically ALT or CHOP mode at different scanning speeds to give the best display results. Users can also select ALT manually
Or CHOP mode to suit the needs of special signals.
The most rooted specification of an oscilloscope is bandwidth. The scope's bandwidth indicates the frequency response of the scope's vertical system. Show
The bandwidth of the wave filter is defined as the highest frequency at which the oscilloscope can display the signal on the screen with an amplitude not less than 3dB.
The frequency at the -3dB point is the signal amplitude "Vdisp" displayed by the oscilloscope as the actual signal value "Vinput"
The signal frequency at 71% is as follows:
dB (volt) = 20log (voltage ratio)
—3Db = 20log (Vdisp / Vinput)
—0.15 = log (Vdisp / Vinput)
-0.15 = Vdisp / Vinput
Vdisp = 0.7Vinput
Figure 5 shows a typical frequency response curve for a 100MHz oscilloscope.
For practical reasons, the bandwidth is usually imagined as the t-reverb curve extending flat to its cut-off frequency, and then from that frequency
Decreases with a slope of -20dB / + octave. Of course, this is a simplified consideration. In fact, the sensitivity of
The frequency starts to decrease, and its cut-off frequency reaches -3dB. The simplified frequency response curve and the actual frequency are given in Figure 5.
Frequency response curve.
The use of a bandwidth limiter can reduce the bandwidth of a broadband oscilloscope with a typical bandwidth above 100MHz to a typical value of 20MHz.
This reduces noise levels and interference, which is very useful for making highly sensitive measurements.
Rise time is directly related to bandwidth. Rise time is usually specified as the time it takes for the signal to go from 10% to 90% of its steady state maximum
Rise time is the fastest transient time an oscilloscope can theoretically display. High-frequency response curve of oscilloscope
It was carefully arranged. This ensures that signals with high harmonic content, such as square waves, can be accurately reproduced on the screen. Such as
If the frequency response curve drops too quickly, ringing will occur on the fast rising edge of the signal. If the frequency response curve drops too slowly, ie
The drop in the frequency response curve starts too early, and the overall high-frequency response of the oscilloscope is affected, causing the square wave to lose its "square" characteristic.
For various general oscilloscopes, the high-frequency response curves are similar. From this curve we can get an oscilloscope band
Simple relationship formula for width and rise time. This formula is:
tr (s) = 0.35 / BW (Hz)
For a high-frequency oscilloscope, this formula can be expressed as:
tr (ns) = 350 / BW (MHz)
For a 100MHz oscilloscope, the rise time is 3.5 (ns = nanosecond 10-9 seconds)
There are special lines marked with 0% and 100% on the scale of the oscilloscope to measure the rise time. When measuring me
We first used the VAR sensitivity control mechanism to align the top and bottom of the measured identification number with the lines marked 0% and 100%, respectively.
Then find the intersection of the signal and the two lines labeled 10% and 90% on the ruler. In this way, the rise time can be changed from these two
Intersections are read at time intervals along the X axis.
To measure the rise time of an oscilloscope, we use the same method as above, except that the rise time of the test signal is required
The time should be much shorter than the rise time of this oscilloscope. To obtain a 2% measurement error, the rise time of the test signal should be at least less than
One fifth of the oscilloscope rise time. The rise time displayed on the oscilloscope should be the oscilloscope rise time and signal rise time and
Combined functions. Its relationship is
t rdisplayed = ? (t rsignal
2 + t
Remember this formula and you will find it useful.