How to choose digital oscilloscope?
How to choose oscilloscope reasonably as the price difference of oscilloscope nearly 50 times between low-grade with high-grade.
The relationship between the bandwidth selection and the highest signal frequency to be measured is needed if accurate measurements are required. Let’s check below example, there is one pulse signal with 50 MHZ, to ensure scope of signal and accuracy of rising delay, the bandwidth of oscilloscope should be 3-5 times of measured signal’s frequency and the accurate measurement should be 8-10 times or more.
Sine wave: as usual, need more than 5 sampling sites during one period as the waveform will be more close to the actual one with more sampling sites.
Pulse wave: rising delay should be more than 5 sampling sites.
Accurate measurement rising delay should be more than 10 sampling sites.
3. Record length: record length=sample*scanning speed*10, it could also be called waveform observation time.
4. Trigger function: ensure that the measured signal can be captured and synchronized to facilitate the observation and analysis of the measured waveform
Trigger methods: automatic trigger, normal trigger and single trigger
Two kinds of trigger function
1) Edge trigger: all digital oscilloscopes have this function, It refers to positive edge, negative edge trigger, window trigger, before trigger and after trigger.
2）Smart trigger：This is considered perfectly in high-grade oscilloscopes. currently there are: delay trigger, sequence trigger, burr trigger, interval trigger, leakage logic surface trigger, TV trigger, this trigger......
5. Analysis function: Should have very strong automatic processing, computation, test and analysis ability of signal
1）Shape and parameters pass/fail automatic test function
2) Advanced function processing: average, differential, polar, exponential, logarithm, power, square root, envelope, high score system, etc.
3）FFT spectrum operation function from 10k-4m point, with power spectrum, power density, phase vector, imaginary part, real part and other measurements;
4）The direct square analysis can be used for the stability calculation of the direct square test signal from 500 points to 8M points according to various parameters
5）Waveform parameter Trend analysis function, Jitter and time analysis;
6) Could open 2-8 windows to observe original waveform and processed waveform simultaneously.
X-Y display and X-Y+X-T and Y-T display are provided, can also carry on the vernier measurement makes it specially suitable for vector diagram analysis of digital communication signals.
6. Record and print signal
1) Can be stored in a test line on a floppy and hard disk, and can be read on a PC. Some digital oscilloscopes are equipped with built-in, convenient to print and analyze long time signal;
2) Some oscilloscopes also provide VGA interface.
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.
Twelve tips of oscilloscope you must know
(1). The most valuable index of oscilloscope: Bandwidth, class parameter
(2). How high is the sampling rate?
Generally speaking, the sampling rate is 5 times the bandwidth, such as 200M oscilloscope, with 1G sampling rate is ok.
The pursuit of higher sampling rate is nothing more than to catch small burrs, but these high frequency burrs have been filtered out in the bandwidth layer, and higher sampling rate cannot bring good benefits.
(3) What is the difference between normal trigger and automatic trigger?
Normal trigger, it will sample the waveform only with trigger event satisfies the condition.
Automatic trigger, after wait for a moment, if there is no eligible trigger events occur, the oscilloscope will force the trigger function and start sampling. As random trigger position, the waveform will show unsteady. If above sequence occurs, please adopt normal mode.
(4) How to catch abnormal signal under uncertain condition?
You could catch the signal with template trigger.
When the template trigger is turned on, the template actually comes as a layer. It will constantly detect whether there is waveform that will touch the area of the template. When there is waveform that touches the template, a signal will be detected and it will be filtered and displayed.
(5) Are those channels isolated of oscilloscope?
No. Bottom of oscilloscope is connected with the earth, it could not be connected directly with the zero line.
To isolation, transformer could directly measure 220V city power, however, this is not recommended, and the safest and correct method it to use difference probes.
(6) Which is more accurate between oscilloscope and multimeter when measure voltage?
The vertical resolution ratio of general oscilloscopes is 8 bits. The measure accuracy is related to vertical resolution ratio and vertical gear.
The accuracy of multimeter is much higher. ADC of six and a half meter is up to 24 bits. That of handheld meter is far more than 8 bits. The voltage measured by the multimeter is more reliable.
(7) Differences between probe X1 and gear X10?
Two important points
Different attenuate bandwidth, signal x1 gear amplitude does not decay. X10 gear signal decay to 1/10 of original signal.
Different bandwidth, bandwidth of X1 gear is almost 10M, and 250M for X10 gear.
(8) Does the general oscilloscope probe need to be calibrated periodically?
The standard does not have a specific metering specification for the probe, but for passive probes, at least the probe compensation must be adjusted when the probe is replaced and the probe switches channels.
(9) What is main factor determines the price of oscilloscope probe ?
The main determinants of price are, of course, bandwidth and functionality.
There are many types of oscilloscope probe, different performance, such as high voltage, difference, active high-speed probe and so on, the price also from a few hundred yuan to nearly ten thousand dollars.
The probe is part of the oscilloscope contact circuit. A good probe can provide the fidelity required for testing.To do this, even passive probes must have a very large number of passive device compensation circuits (RC networks) inside.
(10) Is there any way to measure the working condition of high frequency transformer or inductor core by oscilloscope?
Can rely on the oscilloscope's own power analysis software to do, there is a function - B-H curve analysis, it can reflect the working state of the magnetic core, but also can measure the dynamic inductance value, and get the core loss.
(11) Can an oscilloscope perform Fourier decomposition?
Modern digital oscilloscope mostly has FFT function, but need to pay attention to FFT can analyze the number of points, which directly determines the accuracy of FFT analysis results, avoid spectrum leakage.ZDS2000 series oscilloscope is equipped with 4M point FFT analysis.
(12) Can oscilloscope filter? Such as low pass filtering of PWM wave?
Oscilloscopes typically have a 20MHz bandwidth limit, and this is a hardware filter.Some oscilloscopes also support software filters with adjustable cutoff frequencies.The ZDS2024Plus is adjustable from 100Hz to 100MHz.