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.
Oscilloscope chooses appropriate storage depth, also called record length
Storage depth is a measure of how many sampling points an oscilloscope can store. If you need to capture a pulse string continuously, you need the oscilloscope to have enough memory to capture the entire event. The required storage depth can be calculated by dividing the length of time to be captured by the sampling rate needed to accurately reproduce the signal.
Storage depth is closely related to sampling rate.The depth of storage you need depends on the total time span to be measured and the required time resolution.
Modern oscilloscopes allow users to select record lengths to optimize the details of some operations. Analysis of a very stable sinusoidal signal requires only 500 points of record length;But to parse a complex stream of digital data, you need a million points or more of record length.
Capturing the effective trigger of the signal in the right position can usually reduce the storage capacity of the oscilloscope.
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.