SAMPLING AND STORAGE OSCILLOSCOPE

An oscilloscope, previously called an oscillograph, and informally known as a scope, CRO (for cathode-ray oscilloscope), or DSO ( digital storage oscilloscope), is a type of electronic test instrument that graphically displays varying signal voltages, usually as a two-dimensional plot of one or more signals as a function of time. Other signals (such as sound or vibration) can be converted to voltages and displayed.
Oscilloscopes are used in the sciences, medicine, engineering, automotive and the telecommunications industry. General-purpose instruments are used for maintenance of electronic equipment and laboratory work. Special-purpose oscilloscopes may be used for such purposes as analyzing an automotive ignition system or to display the waveform of the heartbeat as an electrocardiogram

1. SAMPLING OSCILLOSCOPE AND STORAGE OSCILLOSCOPE ELECTRONIC MEASUREMENT AND INSTRUMENTATION NAME- NITHIN KALLEPALLY

2. WHAT IS AN OSCILLOSCOPE An  oscilloscope , previously called an  oscillograph , and informally known as a  scope ,  CRO  (for cathode-ray oscilloscope), or  DSO  ( digital storage oscilloscope), is a type of electronic test instrument that graphically displays varying signal voltages, usually as a two-dimensional plot of one or more signals as a function of time. Other signals (such as sound or vibration) can be converted to voltages and displayed. Oscilloscopes are used in the sciences, medicine, engineering, automotive and the telecommunications industry. General-purpose instruments are used for maintenance of electronic equipment and laboratory work. Special-purpose oscilloscopes may be used for such purposes as analyzing an automotive ignition system or to display the waveform of the heartbeat as an  electrocardiogram

3. SAMPLING OSCILLOSCOPE Definition:  The sampling oscilloscope is a special type of digital sampling oscilloscope which is used to examine a very fast signal. In other words, The sampling oscilloscope receives various electrical signal and then togetherly display the signals on the screen. As it name suggests, it collects samples from several successive waveform and construct a complete picture of the waveform from the assembled data .The resultant waveform is amplified with low band pass filter and then shown on the screen. This waveform is made with the joining of many dots associated with each other to construct the whole shape

4. Each dot of the wave is the vertical deflection of the point of the progressive layer in each successive cycle of a staircase waveform. They are used to monitor high- frequency signals up to 50 GHz or more. The frequency of the displayed waveform is higher than the sample rate of the scope. It is about 10 pieces per division or more along with large bandwidth of amplifier about 15 GHz. At sampling stage, signals have low-frequency and to achieve large band-width it combines with an attenuator. Though, it reduces the dynamic range of the instrument. The sampling oscilloscope is limited to repetitive signals and not responsive to transient events. They only display high frequency within the range limit.

5. Sampling Method - Before each sampling cycle, the trigger pulse activates an oscillator and liner voltage is generated. When the amplitude of two voltages is equal, the staircase move one step and a sampling pulse is generated and it opens the sampling gate for a sample of the input voltage. The resolution of the waveform depends upon the dimension of the steps of the staircase generator. There are different ways of sample taking but two are commonly used. One is real-time sample and other is equivalent sample method .

6. Real-Time Sample Method In real-time method digitizer works at high-speed so it can register maximum points in one sweep. It’s main purpose to capture high-frequency transient events with accuracy. The transient waveform is so unique that its voltage or current level at any instant of time cannot be associated with its nearest ones. These events do not repeat themselves, so it must be registered in the same time frame as they occur. The frequency of samples is very high about 500 MHz and sample rate is about 100 samples per second. To store such a high-frequency waveform, a high-speed memory is required.

7. Equivalent Sample Method   Equivalent Sample Method Sampling in equivalent method works upon the principle of prophecy and estimation which is possible only with the repetitive waveform. In equivalent method digitizer get samples from many repetitions of signals.  It may take one or more samples from each repetition. By doing so, the accuracy in capturing signal gets increases. The frequency of the resultant waveform is much higher than the scope sample rate. This type of sampling can be done by two methods; Random method and sequential method .

8.   RANDOM METHOD OF SAMPLING Random method of sampling is the most common method of sampling. It uses an internal clock which adjusted in such a way that it runs with respect to input signals and the signal trigger samples are taken continuously, no matter where it was triggered.  Samples those are collected are regular with respect to time but random with respect to trigger. SEQUENTIAL METHOD OF SAMPLING In this technique, samples are taken with respect to triggered and it is independent of time setting. Whenever the trigger is detected, the sample is recorded with a small delay. When next trigger occurred, it gets registered with a little incremental time delay with respect to previous one. Let us suppose delay for the first time is ‘t” then the delay for the second time will be little more than ‘t’ and in this manner samples are being taken many times with added delay until the time window is filled.

9. ADVANTAGES Very High Frequency performance can be achieved. High speed electrical signals can be analyzed. A clear display is produced. Allows design of oscilloscope with wide bandwidth, high sensitivity even for low duty cycle pulses. LIMITATIONS: It can’t be used to display the transient waveforms  it can only measure the repetitive or continuous signal. 

10. DIGITAL STORAGE OSCILLOSCOPE A digital oscilloscope is an instrument which stores a digital copy of the waveform in the digital memory which it analyses further using digital signal processing techniques rather than using analogue techniques. It captures the non-repetitive signals and displays it consciously until the device gets reset. In digital storage oscilloscope, signals are received, stored and then displayed. The maximum frequency measured by digital oscilloscope depends upon two things: one is sampling rate of the scope, and the other is the nature of the converter. The main advantage of the digital oscilloscope is that it can display visual as well as numerical values by analyzing the stored traces. The displayed trace on the flat panel could be magnified and also we can change the brightness of the traces, and minute detailing can be done as per requirement after an acquisition.

11. Storage Oscilloscope Technology First the waveforms are conditioned by some analogue circuits then enter in the second stage which involves receiving the digital signals. To do so, samples have to pass through analogue - to – digital converter and output signals get recorded in digital memory at different interval of time. These recorded points together make a waveform. The set of points in a waveform show its length. The rate of samples defines the design of the oscilloscope. The recorded traces are then processed by the processing circuit and obtained traces are ready to display for visual assessment.

12. MODES OF OPERATION The digital storage oscilloscope has three modes of operation: i ). Roll mode ii) Store mode iii) Hold or save mode i) ROLL MODE : It is the most basic mode of operation which is similar to that of a general purpose CRO. When an input is applied, its trace is displayed on the screen. A user can use this mode to keep an eye on the waveform and its various characteristics. ii) STORE MODE: The most commonly used and called refresh mode. It is used when the sample rate of a waveform becomes too high, and when the waveform of interest is repetitive or nearly so. iii)   HOLD OR SAVE MODE: This is called automatic Refresh Mode.

13. ADVANTAGES 1. The cost of the digital storage oscilloscope is less. 2. We have store the 15 wave form in internal memory. 3. The stored wave form can be used for display for any large period by providing power only to the memory. 4. It stores the signal in digital form so prevents degradation. 5. Triggering DISADVANTAGES It can store the image only for a definite time. After that time period the image will be lost Supply to the C.R tube must be present for the period of storage The trace produced is not as sharp as is produced by conventional C.R tubes The writing speed is less compared to conventional C.R tubes one image can be stored in storage C.R tubes

14. APPLICATIONS Usage of Digital Storage Oscilloscope in educational institutes Used for testing signal voltage in circuit debugging. Testing in manufacturing. Designing. Testing of signals voltage in radio broadcasting equipment. In the field of research. Audio and video recording equipment.

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