Lecture Slides for the course Electronic Instrumentation-II delivered by me to Diploma in Electronics/Instrumentation Engineering.

1. BIE-601
Electronic Instrumentation-II
Unit-5
Miscellaneous Instruments
Lecture-1
Mohd. Umar Rehman
umar.ee.amu@gmail.com
22. 4. 2020
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2. Contents
Digital measurement of frequency (mains), digital measurement of high
frequency, digital pH meter, digital capacitance meter, digital tachome-
ter, measurement of power using Bolometer.
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3. Digital Measurement of Frequency
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4. Digital Measurement of Frequency
High frequency
AC Mains frequency
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5. Principle of Operation of Digital Frequency Meter
(DFM)
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6. Principle of Operation of Digital Frequency Meter
(DFM)
Frequency of a signal can be measured by counting the number of
cycles in one second.
This is facilitated by converting the signal into a pulse train which
is applied to one input of an AND gate
A pulse of one second duration is applied to the other input, hence
the output of the AND gate is a pulse train of duration 1 second.
The output of this AND gate can be measured by an electronic
counter. Since each pulse represents the cycle of the signal under
measurement, the number of counts directly indicates the signal
frequency.
Modern electronic counters have very high speed of operation,
hence HF signals can also be measured.
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7. Circuit (Block Diagram) of a DFM
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8. Circuit of DFM...Contd
The above figure shows the basic block diagram of a DFM.
The main component is the Schmitt Trigger which converts the
signal into a square wave with fast rise and fall times, which is
then differentiated and clipped.
The output of the schmitt trigger is the a train of pulses, one pulse
for each cycle. These pulses are fed to a START/STOP gate.
When this gate is enabled, the input pulses pass through this gate
and are fed to an electronic counter directly, which counts the
number of pulses.
When this gate is disabled, the counter stops counting, and the
displays the number of pulses that have passed through it in the
time interval between START and STOP.
If this interval is known, the unknown frequency can be
measured.
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9. Digital Measurement of High Frequency
The direct count range of a DFM extends from DC (0 Hz) to about
100 MHZ.
However, for further higher frequencies (∼ GHz), the counter
used cannot count at the speed demanded by HF measurement.
The following blocks/processes are included in a normal DFM to
extend its range to near about 40 GHz and above.
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10. HF Measurement...Contd
1. Pre-scaling
2. Heterodyne Converter
3. Transfer Oscillator
4. Automatic Divider
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11. 1. Pre-scaling
The frequency signal is divided by integer numbers 2, 4, 6, 8, etc. by
frequency dividing circuits (like asynchronous counter) to get the input
frequency within the desired range of the DFM.
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12. 2. Heterodyne Converter
It is a device that generates new frequencies by mixing two known
frequencies. The high frequency signal is reduced in frequency to a
range within that of the meter by using heterodyne techniques.
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13. 3. Transfer Oscillator
A harmonic or tunable LF oscillator is mixed (zero beat) to produce zero
frequency with the unknown HF signal. The LF oscillator frequency is
measured and then multiplied by an integer which is equal to the ratio,
of the two frequencies, in order to determine the value of the unknown
frequency.
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14. 4. Automatic Divider
The HF signal is reduced by a suitable factor such as 100 or 1000, by
using automatically tuned circuits which can generate an output fre-
quency equal to 1/100th or 1/1000th of the input frequency.
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