## Saturday, 31 August 2019 ## Frequency and Period Measurement using Universal Counter

(a) Measurement of Frequency:

The part of the circuit of the universal counter that is utilized for frequency measurement is presented in figure.

The input signal whose frequency is to be measured (counted) is given to the first channel of the universal counter. It reaches the preamplifier through the attenuator. The amplified signal reaches the Schmitt trigger where it is converted in to a square wave. A differentiator circuit, along with a clipper (not shown in the diagram) produces a train of pulses. This train of pulses is separated by the period of the input signal. These pulses are applied to the A input of the AND gate.

The time base selector switch is selected to the appropriate range. The time base pulses are given to the Schmitt trigger from where they reach the gate control flip-flop. The first time base pulse triggers the gate control flip-flop and its output goes high. Therefore the B input of the AND gate will be held at high state, thus enabling the AND gate. Therefore the input signal pulses present at the A input of the AND gate are allowed to reach the decimal counter. Therefore the count starts. The AND gate will be enabled as long as its B input is high. For the second time base pulse, the gate control flip-flop toggles and its output goes low. Therefore the B input of the AND gate goes low. Hence the AND gate is disabled, preventing the input signal pulses from reaching the decimal counter.

Thus the decimal counter will count the input pulses only for the period between the first time base pulse and the second time base pulse. If we select the time base pulse with 1 second duration, the decimal counter counts the number of input pulses presented during that 1 second. This count will be displayed on the display which is the frequency of the input signal.

(b) Time Period Measurement:

From the time period measurement block diagram, it can be seen that the gate control flip-flop is controlled by the input signal presenting the start and stop signals through the two input channels. That is the gating signal is obtained from the input signal whose time period is not known. This input signal controls the enabling and disabling of the main gate.

The other input to the main gate is derived from the decade dividers connected through the Schmitt trigger to the clock oscillator. Now the gate will pass the trigger pulses from the divider to the decimal counter, which counts and displays. The decimal counter counts the number of pulses that occur during one period of the unknown input signal.

(c) Measurement of Frequency Ratio:

The frequency ratio measurement section is presented in the figure. The aim is to measure the ratio of two frequencies. Normally one frequency will be higher than the other. The low frequency will be employed as the gating signal and high frequency signal is used for counting.

From the block diagram it is clear that the low frequency signal, fL is applied to channel 1. The high frequency signal fH is given to channel 2. The channel 1 preamplifier's output reaches as period trigger to the gate control flip-flop. The channel 2 preamplifier's output reaches the A input of the Main gate. The gate control flip-flop is triggered by the input from channel 1. The output of this flip-flop is applied at the B input of the main gate.

The number of cycles of the high frequency signal fH, that occur during the period of the low frequency signal fL are counted and displayed by the instrument. In case of multiple ratio measurement the period of the low frequency signal can be enhanced by the use of a chain of decimal dividers.

## Friday, 30 August 2019 ## Universal Counter Block Diagram

UNIVERSAL COUNTER:

Universal Counter can be defined as: A counter counts the number of cycles over a second. With the advent digital integrated circuits the design of electronic counters has become easy. An electronic counter can be used to count or measure frequency, time period, time interval between two actions and to determine the ratio between two frequencies. It can be used as count totalizer. The advantage of a digital counter is that it displays its count, frequency or time period etc., on a numeric display, offering all the advantages of digital displays.

A universal Counter is a one piece combined unit that has in it all the circuits for measurement of frequency, time period, frequency ratio etc. Desired functions can be selected using the function switch. Block diagram of the universal counter is shown in Figure.

BLOCK DIAGRAM OF UNIVERSAL COUNTER:

There are two channels for the input named as chl and ch2. The input signal reaches the preamplifiers through the attenuators. Two Schmitt trigger circuits are fed from the output of the two preamplifiers. The two Schmitt triggers present their outputs to the Logic Control Circuitry. The second channel is meant for frequency ratio and multiple ratio measurement.

The block Logic Control Circuitry contains logic gates that are enabled or disabled depending on the selection of the function by the function switch and on the status of the inputs to the L.C.C. The output from the Logic Control Circuitry directly reaches one input of the Main Gate. The Main Gate output drives a decimal counter. The output of the decimal counter drives a decoder driver which drives the display unit.

The second input to the Main Gate is derived from Gate Control Flip Flop. The gate control flip-flop is controlled by the output of the logic control circuit.

The crystal oscillator (also called the clock oscillator) generates an output voltage at 1 MHz, and is sinusoidal in its nature. The crystal oscillator offers excellent frequency stability. In order to improve on the long term frequency stability, the crystal oscillator is kept in a constant temperature oven.

The output of the crystal oscillator is given to a Schmitt trigger circuit. This converts the sinusoidal voltage of the crystal oscillator in to train of pulses. The rate of the pulse train offered by the Schmitt trigger will be equal to the frequency of the clock oscillator.

The output from the Schmitt trigger is presented to a chain of decade dividers. The output of the Schmitt trigger, as well as the outputs of the decade dividers is brought over the ways of a selector switch. The selector switch has seven ways w1 to w7, and we can select pulses from 1 μs to 1 s. These are the time base pulses to the logic control circuitry, again through another Schmitt trigger.

## Thursday, 29 August 2019 ## Testing of Regulated Power Supply

Testing of Regulated Power Supply:

For testing the specifications of a regulated power supply the following procedure is adopted.

(a) Output:

The output of the power supply can be measured using a Digital Volt Meter (D.V.M.). This voltage should tally with the specified voltage when the line voltage is within the tolerance expressed by the manufacturer. A dimmer stat (variac) of the same power rating as that of the power supply is  taken in order to check whether the output voltage is constant within the tolerance values of the mains voltage., The line voltage input to the power supply is to be given from the output of the variac. The output voltage from the variac is first adjusted to the lower limit of mains voltage measuring the output of the variac using a D.V.M. in its AC. volts range. Now the D.C. voltage of the power supply is to be measured. Then the AC. from the variac is to be adjusted to the normal voltage and the D.C. output is to be measured. Finally the output of the variac is to be increased to the upper limit specified by the manufacturer and the D.C. output is to be measured. In all the three measurements the output must be constant.

(c) Ripple:

A capacitor of 0.1 μ F is to be connected in series with test terminals of the D.V.M. The D.V.M. is to be kept in the AC. volts range. Now the voltmeter is to be connected across the D.C. output terminals of the regulated power supply. The voltage indicated by the AC. voltmeter is the ripple output of the power supply. It should be very low of the order of 1 mV R.M.S. or as is specified.

## Tuesday, 27 August 2019 ## Q Meter Block Diagram and Working

Q-meter is an instrument to measure some of the electrical properties of inductors and capacitors. The basic principle of resonance is used in the measurement of 'Q'. We know from the concept of resonant circuits that the voltage across the tank circuit is 'Q' times the applied voltage. Therefore by applying a fixed voltage to the circuit, the voltage across the capacitor can be calibrated in terms of 'Q'. The magnification factor 'Q' of the circuit is defined as

Q = XL/R = XC/R = EC/R

(a) Description of the Block Diagram:

The block schematic diagram of a 'Q' meter is shown in Figure. It consists of an oscillator which works in the frequency range of 50 kHz to 50 MHz. The oscillator drives a current through a low value of resistance Rsh. The value of the shunt resistance may be typically 0.2 Ω. The resistor represents a voltage source that can supply a voltage of magnitude E volts. A thermocouple meter is connected across the resistance to measure the voltage across it.
This meter is marked as multiply Q by. A variable capacitor is arranged in series with the test terminals as shown in the schematic diagram. An electronic voltmeter is connected across this variable capacitor also called the resonating capacitor. The electronic voltmeter's dial will be calibrated directly in 'Q' value.

(b) Measurement of ‘Q’:

To measure the value of the Q of a coil the coil is connected to the test terminals of the Q meter. The oscillator may be tuned to resonate or the capacitor may be tuned to the frequency of the oscillator. The Q of the coil will be obtained by multiplying the value of the reading obtained on the output meter with the multiply Q by meter.

The indicated 'Q' is called the circuit 'Q' because of the loss of the resonating capacitor, voltmeter and insertion resistor will include in measuring circuit. The effective 'Q' will be greater than the indicated 'Q'. This difference can be neglected where the resistance of the coil is relatively small compared to the value of the insertion resistors. The inductance of the coil can be calculated from the knowledge of frequency and resonating capacitor value. This is because we know that XL = XC and therefore
L = 1 /(2πf)2C henry

## Monday, 26 August 2019 ## Digital LCR Meter Block Diagram and Working

The principle involved in the measurement of the component values is the measurement of voltage across, or current through the device under test. The voltage can be directly measured with a D.V.M if the voltage developed is made proportional to the value of component under test. If the current through the component under test represents the value of component then the current can be converted in to voltage. That voltage can be measured with the D.V.M. In either case the voltage measured with the D.V.M. can be made to represent the value in precise and proper units of the value of the component under test.

The best example of this type of measurement is the resistance measurement that is done in a digital multimeter. Actually a constant current source supplies the current through the internal range resistor, and the unknown resistor forming a potential divider circuit. The part of the voltage across the unknown resistor is given to the D.V.M., which intern directly gives a readout of the value of resistance in ohms. Selection of proper values of range resistors, and switching them results in the range selection.

### (a) Description of Block Diagram:

The block diagram of a digital L.C.R. bridge or L.C.R. meter is shown in Figure. It consists of a 1 kHz oscillator and a current to voltage converter, it is nothing but an operational amplifier. The oscillator output reaches the current to voltage converter through a selectable source resistance Rs and the component under test (DUT). The output of the current to voltage converter and the differential amplifier along with the associated feedback circuit goes to the control switch. From the control switch the output is fed to the average voltage detector and the phase sensitive detector. These two outputs of the A.V.D. and P.S.D. act as reference and input signals respectively to the digital voltmeter module. The digital voltmeter, displays the value of inductance or capacitance depending on the component connected to the test terminals.

In addition to the above blocks we find a constant current source, a range selector and two terminals for the unknown resistance. They are connected appropriately to the D.V.M. module. This arrangement is exclusively for the measurement of resistance. The measurement of resistance is done as is done in the case of Digital Multimeters.

### (b) Working Principle Involved in Measuring L and C:

In the block diagram illustrated in Figure, DUT represents device under test that is the component under test. A capacitor, or an inductance can be connected across these terminals to estimate its value. An oscillator working at 1 kHz frequency is used to apply a test signal to the component under test through a selectable source resistance Rs. An operational amplifier works as a current to voltage convertor. It has a range selector resistor Rr , incorporated in its feedback path. The operational amplifier drives the junction of component under test and Rr. to a virtual ground. Therefore Rr, will not change the current through the unknown component (or the component under test or device under test DUT).

Therefore the voltage across the unknown component is E1 , as is marked in the block diagram. The signal current will flow through Rr . It produces a voltage across Rr , which is proportional to the current through the unknown component.

The voltages E1 and E2, are vector quantities. Therefore they define the characteristics of the device at a given test frequency and signal level.

Mathematically, E1 α V and E2 α I
The capacitance C α 1/V α E2/E1
The inductance L α V/I α E1/E2

These ratios are adopted in the measurement modes and are displayed using dual slope converter module.

Working of the digital L.C.R. meter: The values of Rs and Rr will be selected depending on the impedance of the unknown component.

Inductance is measured in the series equivalent mode. The impedance of the unknown inductance is usually low at the test frequency. The value of Rs , is selected to be much higher than the impedance of unknown inductance's impedance. This results in a constant current drive through the unknown inductance. The magnitude of current will be given by the value of Rs.

Capacitance will be measured in the parallel equivalent mode of operation. The impedance will be high, Hence Rs, will be a much lower value, than the impedance offered by the capacitor at the test frequency. This results in the constant voltage drive.

The values of Rs and Rr, in any selected range are equal. Therefore equal voltage drops are obtained across Rs and Rr, with same signal current flowing through them.

The signal voltage E1, is allowed through the differential amplifier. Then it is given to a control switch SWA. This signal voltage E2, is also given to the control switch SWA, supplies the greater of E1 or E2 to the average voltage detector. The lesser one is given to the phase sensitive detector P.S.D.

The outputs of A.V.D. and P.S.D. are steady voltages (D.C. voltages). They are given to the D.V.M. module as reference and input signals. Hence the D.V.M displays the value.

### (c) Resistance Measurement:

The resistance measurement is effected using the constant current source and range resistors as is done in a digital multimeter. Separate terminals are provided for measuring the resistance values.

This is a simple block diagram and illustrates only the basic principle of digital LCR meter. Commercial models have different circuit configurations. However in principle the digital component testers use the principle in measuring the voltage caused by the component or measuring the voltage caused by the current flow in the component, to estimate the value of the component under measurement.

(d) Specifications of Digital L.C.R. Meter:

The following are the specifications of a typical Digital L.C.R. meter:

 Designation Specifications 1. Measuring Range:Inductance :Capacitance :Resistance : 0-2 Henries in 5 decades.0-2 Micro Farads in 5 decades.0-2 Mega Ohms in 5 decades. 2. Resolution : 0.1 mH/mF/ohm 3. Accuracy : + 1% + I digit. 4. Source : 1 kHz for L & C and D.C. for Ω 5. Compliance stress : 1 Vpp AC/100 m App. AC. 6. Guard : Terminals provided 7. External Polarization : External Polarization is applicable for capacitance measurement. 8. Effect of `Q' : Within the accuracy of Q > 1 9. Power supply : 230 V + 15% 50 Hz

## Sunday, 25 August 2019 ## Wagner Ground Connection

SHIELDING AND GROUNDING OF BRIDGES:

Stray capacitance exists between the various bridge elements and ground. It also exists between the bridge arms. These stray capacitances shunt the bridge arms and cause errors in the measurements. This is more particularly at the higher frequencies, while measuring small capacitances and large inductors. To eliminate the stray capacitance, shields are used. Shields are made of copper or aluminium and are grounded. The use of shields will not eliminate stray capacitances completely, but marks them constant in value such that they can be compensated.

WAGNER GROUND CONNECTION:

This is an arrangement that eliminates the capacitance (stray) between the detector terminals and ground. The schematic diagram is shown for a capacitance bridge in Figure. The capacitors C1 and C2 represent the stray capacitances. The oscillator is removed from its usual ground connection and bridged by a series combination of resistor Rw and capacitor Cw. The junction of Rw and Cw is called Wagner ground connection and is connected to ground. Initially the detector is connected to point 1 and R1 is adjusted for null or minimum balance (minimum sound in headphones). This is obtained by throwing the switch to position 1. Later the switch is set to position 2. It connects the detector to the Wagner ground point. Resistor Rw is now adjusted for minimum sound.

When the switch is set to position one again; there will be some unbalance. Resistances R1 and R3 are then adjusted for minimum detector response. Again the switch is set to position 2. When a null is obtained finally the points 1 and 2 are at the same potential which is ground potential. When this condition is achieved the stray capacitances C1 and C2 are effectively shorted and have no effect on normal bridge balance. There are also capacitances from points C and D to ground. These capacitances are eliminated by Wagner ground. With Wagner ground the capacitances of the bridge arms are not eliminated. Therefore care is to be taken to shield the bridge arms.

## Saturday, 24 August 2019 ## Errors and Precautions in Using Bridge Circuits

ERRORS IN BRIDGES:

Study of the different bridge circuits in this topic was made under idealized conditions. Assumption was made that the bridge consists of lumped impedance units connected only by the wires used in the circuit connecting the elements. This is far from truth. The reason is that idealized conditions can be valid if the frequency is low, the impedances under consideration are low. and that the expected accuracy is not high. Practically some factors like stray couplings between bridge arms etc, modify the balance conditions making the balance impracticable or wrong balance condition. The following are the factors causing errors in bridges:

1. Stray conductance effects, due to imperfect insulation.

2. Mutual inductance effects due to magnetic coupling between various components of the bridge.

3. Stray capacitance effects, due to electrostatic fields between conductors at different potentials.

4. Residuals in components for example the presence of small magnitudes of series inductance or shunt capacitance in non-reactive resistors.

PRECAUTIONS FOR REDUCING ERRORS:

The following precautions may be taken to avoid errors :

1. High quality components must be used for the elements of the bridge.

2. The layout of the bridge must be made to avoid interaction of the bridge arms.

3. The sensitivity of the bridge must be more.

4. The bridge components and other pieces must be mounted on insulation stands to prevent stray conductance effects.

5. Presence of large conducting masses near the bridge arms must be avoided to prevent eddy current effects.

6. Residual error can be avoided by identifying the nature, evaluating them and compensating them.

7. Wave filters that eliminate the unwanted harmonics from the source or tuned detectors in place of headphones may be used to avoid the difficulty of frequency and wave form errors.

## Friday, 23 August 2019 ## Universal Impedance Bridge

The universal impedance bridge can be found in any communication laboratory. It is a very useful laboratory instrument. Universal Impedance Bridge is used for the measurement of resistance, capacitance, and inductance. It uses different bridge configurations to measure the values of D.C resistance, A.C resistance, inductance, capacitance, Q factor of coils, dissipation factor of capacitors. Dials and multiplier switches are provided to directly read the value of the component under measurement.

It consists of suitable impedance standards, A.C. and D.C. sources, along with A.C. and D.C. null detectors. The appropriate bridge circuit will be utilised for measurement of the particular component by the switching arrangement provided in the bridge.

The universal impedance bridge uses four bridge configurations for its operation. The Wheatstone bridge circuit is used for measurement of both the A.C. and D.C. resistance. A standard capacitor and a resistor of the precision type is used in a four arm network to measure capacitance. This determines the losses in the unknown capacitor. Maxwell bridge circuit is used for low Q inductors. Hay bridge circuit is used for measurements of inductors with Q factor above 10.

A suspension galvanometer with a current sensitivity of 0.5 per division is used for D.C. resistance measurements. A selective amplifier operating an electron ray indicator tube is used as null detector for all the A.C. measurements. Provision is made to connect external A.C. and D.C. null detectors. High impedance headphones can also be connected to this bridge as null detector. The D.C. source is a small power supply. The alternating voltage source consists of an oscillator with plug in RC networks that can be used for required frequency. The standard frequency used in the bridge is 10 kHz.

## Thursday, 22 August 2019 ## Wien Bridge and Resonance Bridge

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### Wien Bridge:

Wien bridge is primarily popular as a frequency determining bridge. It has several applications. Some of the applications are in harmonic distortion analyser, in audio frequency generation. It has a series RC combination in the adjoining arm. The other two arms are provided with resistances. The impedance of arm 1 is Z1 = R1 – j/ωC1. The admittance of arm 3, is Y3 = 1/R3 + j ωC3. The arrangement is shown in figure.

The balance equation can be obtained as follows:

R2 = (R1 – j/ωC1) R4(1/R3+jωC3) ------------------------- 1

Expanding the expression we have:

R2 = R1R4/R3 + (jωC3R1R4) – jR4/ ωC1R3 + R4C3/C1------------------------- 2

Equating the real terms:

R2 = R1R4/R3 + R4C3/C1 ------------------------- 3

It can be reduced to:

R2/R4 = R1/R3 +C3/C1 ------------------------- 4

Equating the imaginary terms:

ωC3R1R4 = R4/ ωC1R3 ------------------------- 5

where, ω = 2πf

Solving for f, we get

f = 1/2π ( C1C3 R1R3) ------------------------- 6

From the above we observe that the two conditions for balance result in an expression determining the required resistance ratio, R2/R4 and another expression determining the frequency of the applied voltage. So we have to satisfy the above equation (4) and (6), to balance the bridge.

The arrangement in most of the wien bridge circuits is such that the values of R1, R2 and C1,C3 are made equal. Hence the equation (4) reduces to R2/R4 = 2. It also reduces equation (6) to

f = 1/2 πCR ------------------------ 7

Thus Equation (7) is the general expression for the frequency of the Wien Bridge.
Practically Capacitors C1 and C3 are fixed values. R1 and R3 are variable resistors controlled by a common shaft. Now providing R2 = 2R4, the bridge can be used as a frequency determining device with single balance control, which can be calibrated directly in terms of frequency. The source supplying this bridge must be free from harmonics. If not, the balancing will be difficult. Hence it is clear that the bridge is frequency sensitive.

### Resonance Bridge:

Resonance bridge consists of reactance concentrated in one arm. They are adjusted to give series resonance so that this arm offers resistance impedance. The resonance bridge is shown in figure. From the schematic diagram of the bridge, we find that the ratio arms are formed by R1 and R2. Resistance R3 is connected in the standard arm. The fourth arm consists of an inductance Lx, capacitance Cx and resistance Rx.

This bridge can be used to measure frequency in terms of inductance and capacitance. It is also used to measure capacitance in terms of frequency and a variable inductance. It can also be used to measure inductance in terms of frequency and a variable capacitance. The balance equation can be obtained as follows:

R1(Rx + jωLx – j/ωCx) = R2 R3

At resonance

XL = Xc and fx = 1/2π (LC)

Zx = Rx

Therefore, Rx = R2 R3/R1

As can be seen from the above the bridge is balanced by resistance alone. Resistance R3 is used for this purpose.