Digital Electronics: Principles, Devices and Applications

674 Digital Electronics16.12.1.2 Vertical sensitivityThe vertical sensitivity specification tells us about the minimum signal amplitude that can fill theoscilloscope screen in the vertical direction. A 5 mV/div sensitivity is quite common. Oscilloscopeswith a sensitivity specification of 1 mV/div are also available. Sensitivity and bandwidth are oftentrade-offs. Although a higher bandwidth enables us to capture high-frequency signals, there is a goodpossibility of unwanted high-frequency noise being captured if the oscilloscope has a higher sensitivitytoo. That is why most of the high-sensitivity scopes have bandwidth limit controls to enable a clearview of low-level signals of moderate frequencies. It is also important that the oscilloscope we choosehas an adequate V/div range to make possible a full-screen or near-full-screen display for a wide rangeof signal amplitudes.16.12.1.3 AccuracyThe accuracy specification indicates the degree to which our measurement conforms to a true andaccepted standard value. An accuracy of ±1 − 3% is typical. Almost all oscilloscopes are providedwith a ×5 magnification in the V/div selector switch. This alters the nominal vertical deflection scalefrom say 5 mV/div–5 V/div to 1 mV/div–1 V/div. It may be mentioned here that the accuracy sufferswith the magnifier pull. Most of the manufacturers list accuracy specifications separately for the twocases for the oscilloscopes manufactured by them.16.12.2 Analogue Storage OscilloscopeWith the CRT storage-type oscilloscope, the stored writing speed is usually the main criterion forchoosing the instrument. The speed of a CRT storage scope depends on the speed of the input signal(signal frequency) and the size of the trace it draws.16.12.3 Digital Storage OscilloscopeJust like an analogue scope, the specification sheet of a digital oscilloscope contains scores ofspecifications that at first sight may appear quite confusing. A closer look at these specifications,particularly the decisive ones, will make one appreciate the performance capabilities of digitaloscilloscopes. The real strength of a digital oscilloscope lies in the following specifications: bandwidth,sampling rate, vertical resolution, accuracy and acquisition memory.16.12.3.1 Bandwidth and Sampling RateThe bandwidth is an important specification of digital oscilloscopes, just as it is for analogueoscilloscopes. The bandwidth, which is primarily determined by the frequency response of inputamplifiers and filters, must exceed the bandwidth of the signal if the sharp edges and peaks are to beaccurately recorded.The sampling rate is another vital digital scope specification. In fact, the sampling rate determinesthe true usable bandwidth of the scope. While the bandwidth is associated with the analogue front endof the scope (amplifiers, filters, etc.) and is specified in Hz, the sampling rate is associated with thedigitizing process and, if it is not adequate, degrades the bandwidth. A clear understanding of samplerate specification is thus important when it comes to establishing the adequacy of a particular sample