UNIT-IV�Electronic Measuring Instruments

Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: EEM

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CRO & its Construction

• The CRO is a type of electrical instrument which is used for showing measurement & analysis ofwaveforms.
• Construction of CRO
• The main parts of CRO are as follows:
• 1. Cathode ray tube.
• 2. Electronic Gun Assembly.
• 3. Deflecting plate.
• 4. Fluorescent Screen for CRT
• 5. Glass Envelop.

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Working of CRO.

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• Digital storage Oscilloscope(DSO)
• The DSO is defined as the oscilloscope which stores and analysis the signal digitally, i.e in the form of 1 or 0 .
• The digital oscilloscope takes an input signal , store them and then display it on the screen .
• The digital oscilloscope has advanced features of storage , triggering and measurement . Also it displays the signal visually as well as numerically.

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Digital Instruments

• Digital Instruments: The analog instruments display the quantity which is to be measured in terms of a deflection of a pointer.
• The digital instruments indicate the value of measured quantity in the form of decimal number.
• The digital meters work on the principle of quantization.
• The analog quantity which is to be measured is first subdivided or quantized into a number of small intervals upto many decimal places.

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• The objective of the digital instrument is then to determine (in which portion of the subdivision) the measurand can be identified(as an integral multiple of the smallest unit called the quantum(chosen for subdivision).
• The reading accuracy can be increased by increasing the number of decimal places or by increasing the quantizing levels.
• The advantages of digital instruments are:

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• (i) The digital instruments indicate the readings directly in the decimal numbers and therefore errors on account of human factors like errors due to parallex and approximation are eliminated.
• (ii) The readings may be carried to any number of significant numbers by merely positioning the decimal point.
• (iii) Since the output of digital instruments is in digital form and therefore the output may be directly fed into memory devices like tape recorders , printers hard disc and digital computers etc. for storage and for future computations.

Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: EEM

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• The power requirements of digital instruments are considerably smaller.
• The chief advantages of analog instruments are that they are cheap and simple and accordingly for ordinary purposes these(analog) instruments will not be completely displaced by digital instruments.
• But however cost considerations and complexity of digital instruments are not of main consequence then there the digital instruments are certainily preferred over analog instruments.

Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: EEM

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• At present digital instruments are costlier than the corresponding analog instruments but with the developments in modern techniques, the cost gap will be narrowed down.

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•What Are Counters?

•Counters are specially designed synchronous sequential circuits, in which , the state of the counter is equal to the count held in the circuit by the flip flops. Counters calculate or note down the number that how many times an event occurred.

•Counters are the crucial hard ware components, and are defined as “The digital circuit which is used to count the number of pulses”. Counters are well known to us as “Timers”. Counter circuits are the best example for the flip flop applications. Counters are designed by grouping of flip flops and applying a single clock signal to them. In simple words, the counters are those, which have the group of storage elements like flip flops to hold the count.

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Department of Instrumentation & Control Engineering, BVCOE New Delhi Subject: EEM

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COUNTER

•A Counter is a device which stores (and sometimes displays) the number of times a particular event or process has occurred, often in relationship to a clock signal. Counters are used in digital electronics for counting purpose, they can count specific event happening in the circuit. For example, in UP counter a counter increases count for every rising edge of clock. Not only counting, a counter can follow the certain sequence based on our design like any random sequence 0,1,3,2… .They can also be designed with the help of flip flops. They are used as frequency dividers where the frequency of given pulse waveform is divided. Counters are sequential circuit that count the number of pulses can be either in binary code or BCD form. The main properties of a counter are timing , sequencing , and counting. Counter works in two modes

•Up counter

•Down counter

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•Counters are used not only for counting but also for measuring frequency and time ; increment memory addresses.

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•Need of Counters

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•Counting means incrementing or decrementing the values of an operator, with respect to its previous state value. So to perform the mathematical operation we use no devices other than counters. We cannot perform this action (counting) with any other logic devices rather than counters.

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•Counters are broadly divided into two categories

1.Asynchronous counter

2.Synchronous counter

•1. Asynchronous Counter

•In asynchronous counter we don’t use universal clock, only first flip flop is driven by main clock and the clock input of rest of the following flip flop is driven by output of previous flip flops. We can understand it by following diagram-

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•It is evident from timing diagram that Q0 is changing as soon as the rising edge of clock pulse is encountered, Q1 is changing when rising edge of Q0 is encountered(because Q0 is like clock pulse for second flip flop) and so on. In this way ripples are generated through Q0,Q1,Q2,Q3 hence it is also called RIPPLE counter and serial counter. A ripple counter is a cascaded arrangement of flip flops where the output of one flip flop drives the clock input of the following flip flop

•2. Synchronous Counter

•Unlike the asynchronous counter, synchronous counter has one global clock which drives each flip flop so output changes in parallel. The one advantage of synchronous counter over asynchronous counter is, it can operate on higher frequency than asynchronous counter as it does not have cumulative delay because of same clock is given to each flip flop. It is also called as parallel counter.

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Synchronous counter circuit

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•From circuit diagram we see that Q0 bit gives response to each falling edge of clock while Q1 is dependent on Q0, Q2 is dependent on Q1 and Q0 , Q3 is dependent on Q2,Q1 and Q0.

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REFERENCE

• A.K Sawhney Electrical & Electronic Measurement & Instrumentation.

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