DVM

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https://electronicscoach.com/digital-voltmeter-dvm.html
Digital Voltmeter (DVM)
Definition: Digital Voltmeter is a voltage-sensitive device. It measures
AC or DC voltage and displays the value directly in numeric form instead of
pointer deflection. DVM is an acronym for Digital Voltmeter. DVM was first
invented in 1954 by Andrew Kay.
There exist many factors that affect the measurement accuracy of a digital
voltmeter (DVM). These are basically temperature, input impedance, variation
in power supply voltage etc.
As we know that an analogue instrument provides results by pointer
deflection on a continuous scale. On contrary, a digital instrument provides
results as discrete numerals. Thus, providing accuracy and versatility
together.
Block diagram of Digital Voltmeter
The figure below shows the block diagram of a typical digital voltmeter.
As we can see, the block diagram consists of an attenuator with an analogue to
digital converter after it. This ADC unit basically distinguishes between various
types of Digital Voltmeters which we will discuss later.
A counter section is also employed in the circuitry that is usually a decade
counter. The read-out system is used to display the digital voltage of the
input signal.
Working of Digital Voltmeter
Let us now have a look at the detailed diagram of a Digital Voltmeter. The figure
below will help you to understand the working of a DVM.

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The attenuator section consists of a series resistance that attenuates the input
signal. The voltage at point A will be exactly the same as the input voltage Vin.
By the voltage division rule, it is clear that the voltage at B will be less than the
voltage at A. Similarly, the voltage at C will definitely be less than the voltage at
both points A and B. So, in this way an attenuator section works.
Now, the thing that comes to our mind is what is the need for this attenuator at
the beginning of the circuit?
So, the reason for this is, that an attenuator placed here minimizes the excess
voltage that can damage the other components of the device. It is basically
a predefined resistive network that performs attenuation for circuit protection.
Here, the attenuation is termed decade attenuation as it is a digital voltmeter
and we require a decimal count. That means if attenuation occurs it will be
in powers of 10.
So, at the input of ADC, we will have Vin/N as the input voltage.
: N = 1, 10, 100, 1000
The ADC employed in the circuit converts an analogue signal into a digital one
in order to provide the digital output. A digital signal is one having 2
levels i.e., 0 and 1.
So, the input at ADC Vin/N gets converted into a digital signal. This will provide
us with a sequence of digital pulses. One pulse for 1 mV as at the ADC we have
taken resolution as 1 mV/step.

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These digital pulses are then fed to the counter unit. Here, we have used
a decade counter. A decade counter counts in decimal in spite of binary.
The counter unit on a whole consists of 3-decade counters that are cascaded
together. These decade counters have the ability to count from 0-9 i.e., 10
counts. Hence the three will count up to 1000.
DC 7447 converts the BCD value into 7 segment display. That means now the
input value will be displayed here in digital format. The decimal point
selector at the end of the circuit will select the position of the
decimal according to the magnitude of the voltage.
Types of Digital Voltmeter
The classification of DVM is based on different ADC conversion methods-
1. Ramp type DVM
In a ramp type DVM, the operation basically depends on the measurement of
time. The time that a ramp voltage takes to change from the level of the input
voltage to that of 0 voltage or vice versa. An electronic time interval counter is
used to measure the time interval and the count is displayed in digits as
voltmeter output.
Let us look at the block diagram and operating principle of a ramp-type DVM.
Here, as we can see in the figure below a negative-going ramp voltage is shown.
This ramp voltage is compared with the unknown voltage. An input comparator
employed in the circuit generates a pulse when ramp voltage becomes equal to
the voltage under measurement.

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Now, the ramp voltage falls to reach 0 value. The ground comparator employed
in the circuit generates a stop pulse. This stop pulse closes the gate.
The gate opening time duration is proportional to the value of the input
voltage. The sample rate Multivibrator employed here is used to find the rate
by which the measurement cycle begins.
2. Dual slope integrating type DVM
The figure below shows the block diagram of dual slope integrating type DVM.
For a fixed time, interval, the analogue input is applied to the integrator via
switch S. The level of the input voltage is raised in the comparator to some
desired positive value as we can see in the figure below.

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At the end of a fixed time interval, the rate of increase in voltage will be
proportional to the input voltage.
At this time the count is set at 0 and the switch gets shifted to reference
voltage. Now, the output of the integrator will start decreasing and drop until it
reaches below the comparator reference voltage. At this time control logic will
receive a signal in order to stop the count.
The count shown by the counter that is proportional to input voltage will be the
measured value and hence displayed at the Digital read-out.
3. Integrating type DVM
In this category of Digital Voltmeter, the true value of input voltage is measured
over a fixed measuring time.
Here, an integration technique is employed that uses voltage to frequency
conversion. This voltage to frequency converter act as a feedback control
system. This basically governs the pulse generation rate is proportional to the
magnitude of applied input voltage.

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In the voltage to frequency conversion technique, a train of pulses is
generated. The frequency of these pulses depends on the voltage being
measured.
Then these pulses are counted that appear in a definite time interval. After all,
the frequency of pulses is a function of input voltage, the number of pulses is
an indication of the input voltage.
4. Successive Approximation DVM
In this category of DVM, the ADC employed makes use of a successive
approximation converter. Thus, it is named as so. These are capable of 1000
readings per second.

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In the beginning, a start pulse is applied at the start/stop Multivibrator. Due to
this, the MSB of the control register is set to high and all other bits to low. So,
for an 8-bit control register, the reading would be 10000000.
Thus, causing the output of DAC to be half of the reference voltage.
Now, the comparator compares the output of the converter from the input
voltage and produces an output that will cause the control register to retain 1 in
its MSB.
The ring converter employed in the circuit advances one count next thus shifting
a 1 in the second. This will cause the MSB of the control register and its reading
to be 11000000.
Thus, DAC increases its reference by one increment and another comparison
of input voltage with that of reference takes place. In this way through
successive approximation the measurement cycle proceeds. On reaching the
last count, the measurement cycle stops.
The output in digital format at the control register shows the final
approximation of input voltage.
Advantages of Digital Voltmeter
1. DVM provides numerical readouts that eliminate observational
errors. Thus, providing better readability.
2. DVM offers better accuracy and versatility as compared to analog
voltmeters.
3. DVM has a greater speed of taking voltage readings as compared
to analog instruments.
4. The output of DVM can be fed to memory devices for further
computations.
5. The decreased size of DVM increases the portability of the
instrument.
DVM offers an accuracy of 0.5% + 1 digit and the operating temperature range
is -5 ⁰C to 55 ⁰C.

8. 8
https://circuitglobe.com/q-meter.html
Q Meter
Definition: The instrument which measures the storage factor or quality factor of the
electrical circuit at radio frequencies, such type of device is known as the Q-meter.
The quality factor is one of the parameters of the oscillatory system, which shows
the relation between the storage and dissipated energy.
The Q meter measures the quality factor of the circuit which shows the total energy dissipated
by it. It also explains the properties of the coil and capacitor. The Q meter uses in a laboratory
for testing the radio frequency of the coils.
Working Principle of Q meter
The Q meter works on series resonant. The resonance is the condition that exists
in the circuit when their inductance and capacitance reactance is of equal
magnitude. They induce energy which is oscillating between the electric and
magnetic field of the capacitor and inductor respectively.
The Q-meter is based on the characteristic of the resistance, inductance, and
capacitance of the resonant series circuit. The figure below shows a coil of
resistance, inductance, and capacitance connected in series with the circuit.
At resonant frequency f0,
The value of capacitance reactance is

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At inductive reactance,
At the resonant frequency,
and current at resonance becomes
The phasor diagram of the resonance is shown in the figure
The voltage across the capacitor is expressed as

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Input voltage
The above equation shows that the input voltage E is Q times the voltage that appears across
the capacitor. The voltmeter is calibrated for finding the value of the Q factor.
Applications of the Q-meter
The following are the applications of the Q-meter.
1. Measurement of Q – The circuit used for the measurement of Q is shown in the figure.
The oscillator and tuning capacitor adjust to the desired frequency for obtaining the maximum
value of E0. Under this condition, the value of the quality factor is expressed as
True value is given as

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The value of the quality factor is obtained by the voltmeter which is connected across the
capacitor. The measured value is the Q factor of the whole circuit and not only of the coil.
Thus, errors occur in the reading because of the shunt resistance and distributed capacitance.
The above equations show that the measured value of the Q is smaller than the true value.
2. Measurement of Inductance – The inductance is measured by the equation shown
below.
The value of f0 & C is required for calculating the value of inductance.
3. Measurement of Effective resistance – The equation computes the value of effective
resistance
4. Measurement of Self-Capacitance – The self-capacitance is determined by measuring
the two capacitances at different frequencies. The capacitor is adjusted to the high
value, and the circuit is resonated by adjusting the oscillator frequency. The resonance
of the circuit is determined by the Q meter.
Thus,

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or distributed capacitance
5. Measurement of Bandwidth – The equation below calculates the bandwidth
6. Measurement of Capacitance – The capacitance is determined by connecting the
dummy coil across the terminals T1 and T2. Let the capacitor under test is connected
across the terminal T3 and T4. The circuit is again resonated by varying the value of
tuning capacitor C2. The value of testing capacitance is determined by subtracting the
C1 and C2.