Basic principle and measuring method of oscilloscope
Oscilloscope is one kind of electrical measurement equipment with wide application. It could transfer invisible electrical signal to visible image to facilitate people research change processes of all kinds of electricity phenomenon. Oscilloscope use narrow electron beam which consists high-speed electronics and project onto screen coated with fluorescent substance to produce small spot (this is original working principle of analog oscilloscopes. Under the function of measured signal, electronics like the point of a pen which could on screen describe change curve of measured signal during instantaneous value. An oscilloscope can be used to observe the waveform curves of various signal amplitudes varying with time. It could also be used to measure various electricity, such as voltage, current, frequency, phase, amplitude modulation and etc.
(1) pre-adjusting: counterclockwise rotate the brightness button to the bottom, shift vertical and horizontal positions to the middle, set the attenuation gear at the highest level, place scan “outside X gear”
(2) turn on the power, do following steps waiting for one or two minutes after the indicator light is on
(3) adjust luminance first, then adjust the focus, adjust vertical and horizontal displacement to make the light point in center of suitable area.
(4) adjust scan, scan fine tuning and X gain, observe scan.
(5) adjust outside X gear to suitable scan range, observe the voltage waveform according to sine and cosines law on vertical direction inside the device.
(6) Connect the applied voltage to be studied to oscilloscope through Y input and earth. Adjust all gears to suitable position and observe waveform (the picture changes with time) (the synchronous polarity switch can make the starting point of the image start from positive half cycle or negative half cycle.)
(7) If you want to observe the vertical migration of light spot (with one added DC voltage), could adjust scan to “outside X gear”.
In general, the frequency of the calibration signal is 1kHz and the amplitude is 0.5v, which is used to calibrate the internal scanning oscillator frequency of the oscilloscope. If it is abnormal, the corresponding potentiometer of the oscilloscope (internal) should be adjusted until it is consistent.
Analog and digital oscilloscope in the observation of the details of the waveform, which has the advantage?
1. Which is more advantageous, analog or digital oscilloscope, in looking at the details of the waveform (e.g., looking at parasitic waveforms of less than 1% at zero and peak crossing)?
2. Digital oscilloscope generally provides online display of root mean square value, its accuracy is generally what?
Answer: 1) observe parasitic waveform of less than 1%. The observation accuracy is not very good either in analog oscilloscope or digital oscilloscope. The vertical accuracy of the analog oscilloscope may not be higher than that of the digital oscilloscope. For example, the vertical accuracy of the analog oscilloscope with a certain 500MHz bandwidth is ±3%, which is not more advantageous than that of the digital oscilloscope (usually the accuracy is 1-2%). Moreover, the automatic measurement function of the digital oscilloscope is more accurate than the manual measurement of the analog oscilloscope for details.
Main classification and and characteristics of oscilloscopes’ probes
The passive probe is made of wires and connectors and includes resistors and capacitors when compensation or attenuation is required. There are no active devices (transistors or amplifiers) in the probe, so no power supply is required for the probe. Passive probes are generally the strongest and most economical probes, and they are not only easy to use, but also widely used.
1.2 High resistance passive voltage probe
Actually, voltage probes are widely used among which high resistance passive probes are the most. Passive voltage probes provide attenuation 1x, 10x and 100x for different voltages. During these passive probes, 10x passive probes are most widely used probes. For applications where the signal amplitude is 1 v peak-to-peak or lower, a 1 x probe may be appropriate or even necessary. In application where low - and medium-amplitude signals are mixed (tens of millivolts to tens of volts), the switchable 1 x /10 x probe is much more convenient. However, the switchable 1 /10 probe is essentially two different probes in the same product, with not only different attenuation coefficients, but also different bandwidth, rise time and impedance (R and C) characteristics. Therefore, these probes do not exactly match the input of the oscilloscope and do not provide the optimal performance achieved by the standard 10 x probe.
1.3 Low resistance passive voltage probes
Bandwidth of most high resistance passive probes range from less than 100MHZ to 500 MHZ or more than. However, frequency characteristics of low resistance passive probes(also called 50 Ohm probe, Zo probe, voltage divider probe) are very good, adopting probe matching coaxial cable, bandwidth could reach 10GHZ and 100 psec or faster rise time. The probe is designed for use in 50 ohm environments such as high-speed equipment verification, microwave communications and time domain reflectometers (TDR).
1.4 Passive high voltage probes
High voltage is one relative concept. We can define a high voltage as any voltage that exceeds the voltage that a typical generic 10 x passive probe can safely handle. High voltage probes require great dielectric strength to ensure safety of user and probes.
2. Active voltage probe
2.1 Active probes
Active probes include or depends on active circuits, such as crystal valve. Most commonly, an active device is a field effect transistor (FET) that provides very low input capacitance, which leads to high input impedance over a wider frequency band.
2.2 Passive FET probe
Bandwidth of passive FET probes are usually during 500MHZ to 4GHZ. The high input impedance of an active FET probe allows measurements to be made at test points with unknown impedance, and the risk of a load effect is much lower. In addition, because low capacitance reduces the effect of ground wires, longer ground wires can be used.
Active FET probes have no passive probe voltage range. The linear dynamic range of active probe is generally between ±0.6v and ±10V
2.3 Active differential probe
Differential signals are signals that refer to one another rather than to ground. The differential probe can measure the signal of the floating device, which is essentially composed of two symmetrical voltage probes with good insulation and high impedance to the location, respectively. The differential probe provides a high common mode rejection ratio (CMRR) over a wider frequency range.
3. Current probe
In principle, the current value can be easily obtained by dividing the voltage measured by the impedance measured by a voltage probe. However, in practice this measurement introduces a large error, so generally do not use voltage conversion current method. The current probe can accurately measure the current waveform. The method is to use the current transformer input, the signal current magnetic flux is transformed into voltage by the mutual inductance transformer, and then amplified by the amplifier inside the probe and sent to the oscilloscope.
3.1 Ac current probe
The alternating current in the transformer will generate electric field and induce voltage with the change of current direction. The ac current probe is a passive device that requires no external power supply.
3.2 DC current probe
Conventional current probes can only measure ac and ac signals, because a stable dc current cannot induce current in a transformer. However, using the hall effect, a semiconductor device with a bias current will generate a voltage corresponding to the direct current field. Therefore, the dc current probe is an active device that needs external power supply.
So current probes are basically divided into two kinds: AC current probes and AC/DC current probes. AC current probes are usually passive probes and AC/DC active probes.
4. Logic probe
When observing and analyzing the analog characteristics of digital waveform with oscilloscope, logic probe is needed. In order to isolate the exact cause, digital designers usually need to check the specific data pulse that occurs under specific logic conditions, which requires logic trigger function.
5. Other probes
Because the application scope of oscilloscope is very wide, so in addition to the above types of probes there are a variety of special probes, these professional probes according to the different front-end sensors and have different functions, we introduce two of them below, only for readers to understand.
Photoelectric probe is a combination of common voltage probe and photoelectric conversion device in principle, which can directly measure optical device and optical signal transmitted by optical fiber.
Temperature probe is a combination of common voltage probe and temperature sensor, which can directly measure the temperature of an object.Temperature probe is a kind of sensor probe. Various sensor probes and oscilloscopes can be combined to measure a variety of physical quantities.