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.
Digital oscilloscope generally provides online display of root mean square value, its accuracy is generally what?
For oscilloscope amplitude measurement accuracy, many people use A/D digit to measure. In fact, it varies depending on the oscilloscope bandwidth you use, the actual sampling rate Settings, and so on. If the bandwidth is insufficient, the amplitude measurement error itself will be very large; if the bandwidth is sufficient, the sampling setting is very high, and the actual amplitude measurement precision is not as good as that of the sampling rate when the sampling rate is low (you may refer to the user's manual of the oscilloscope, which may give the actual effective digits of an oscilloscope at different sampling rates). In general, oscilloscope measurement amplitude, including root mean square value accuracy is often less than the multimeter, similarly, measuring frequency it is less than the frequency counter.
Digital oscilloscope test accuracy
Any designer who chooses oscilloscope for parameter measurement will know its test accuracy through the product index to ensure enough tolerance error and measurement margin.Jitter testing is no exception. For example, the tekker TDS6804B oscilloscope indicates the precision and specifies the typical value of jitter measurement capability.Jitter measurement accuracy is affected by many factors, including timing stability of oscilloscope, sampling noise, instrument amplitude background noise and interpolation error.
Interpolation error is caused by linear interpolation between actual voltage samples.This error is less than 0.3ps RMS when measuring 100ps rise time signal and oscilloscope detects at 50% voltage threshold with 20GSa/s sampling rate.In many cases, this error can be improved by sine (X)/X sine interpolation in oscilloscope and other methods, such as making full use of the vertical dynamic range of oscilloscope, so that the input signal amplitude reaches the oscilloscope full scale.In most cases, the error for this reason is much smaller than for other error sources, and can be further reduced by using interpolation such as Sin(X)/X or Sinc.