Use of instruments

Glmoyo May 2015

212.55 Explain the functions of the main controls of an oscilloscope. Controls: channel gain, time base speed, sync/trigger, time base mode

(alternate scan or switching)

The electron beam emitted by the heated cathode at the rear end of the tube is accelerated and focused by one or more anodes, and strikes the front of the tube, producing a bright spot on the phosphorescent screen.

The electron beam is bent, or deflected, by voltages applied to two sets of plates fixed in the tube.

The horizontal deflection plates, or X-plates produce side to side movement. They are linked to the time base. This produces a sawtooth waveform.

During the rising phase of the sawtooth, the spot is driven at a uniform rate from left to right across the front of the screen.

During the falling phase, the electron beam returns rapidly from right to left, but the spot is 'blanked out' so that nothing appears on the screen.

In this way, the time base generates the X-axis of the V/t graph.

The slope of the rising phase varies with the frequency of the sawtooth and can be adjusted, using the TIME/DIV control, to change the scale of the X-axis.

Dividing the oscilloscope screen into squares allows the horizontal scale to be expressed in seconds, milliseconds or microseconds per division (s/DIV, ms/DIV, s/DIV).

The signal to be displayed is connected to the input. The AC/DC switch is usually kept in the DC position (switch closed) so that there is a direct connection to the Y-amplifier.

In the AC position (switch open) a capacitor is placed in the signal path. The capacitor blocks DC signals but allows AC signals to pass.

The Y-amplifier is linked in turn to a pair of Y-plates so that it provides the Y-axis of the V/t graph.

The overall gain of the Y-amplifier can be adjusted, using the VOLTS/DIV control, so that the resulting display is neither too small or too large, but fits the screen and can be seen clearly. The vertical scale is usually given in V/DIV or mV/DIV.

The trigger circuit is used to delay the time base waveform so that the same section of the input signal is displayed on the screen each time the spot moves across. The effect of this is to give a stable picture on the oscilloscope screen, making it easier to measure and interpret the signal.

X-POS and Y-POS controls - to change the positions of the axes. Adjusting Y-POS allows the zero level on the Y-axis to be changed, Adjusting X-POS allows the zero level on the X-axis to be changed.

For a start ensure that: all push button switches are in

the OUT position

all slide switches are in the UP position

all rotating controls are CENTRED

the central TIME/DIV and VOLTS/DIV and the HOLD OFF controls are in the calibrated, or CAL position

Oscilloscope Controls

1. On/Off switches help to control electrical transients which can be harmful to sensitive circuit components.

2. Intensity - Adjust the brightness of the trace

3. Beam finder - The screen will display what quadrant the trace is in; then use the horizontal (#10) and vertical controls (#15) to move the trace to the middle of the screen.

4. Triggering source and mode enables the scope to start the sweep at the same point on the waveform to produce a stable image on the screen. Use "internal" or "auto trigger.

5. Trigger Slope - enables selection of voltage potion to trigger the scope on(up or down)

6. Trigger Level -Triggering allows horizontally alignment of repetitions of the signal waveform.

7. Sweep calibration. It adjusts the horizontal scale.

Triggering

A continuous input waveform and four successive sweeps on the scope screen. The trigger is set for positive slope with the trigger level at the dashed line.

Trigger inorevakuti wave ngaritangire apa

I-Trigger ithi i wave kaliqalisele lapha .

8. Sweep. This determines the horizontal scale for the oscillograph. The scale is read in the upper white window. Its units are seconds/division.

9. Horizontal position. This enables movement of the signal back and forth along the X-axis.

10. Channel select. Most oscilloscopes are dual trace. This means that they can display two signals at once hence have a set of two channel knobs

11. Signal ports. There is one signal port for each channel. It is a BNC connector for this oscilloscope. BNC bayonet connector

12. Sensitivity calibration. This knob is used to change the vertical scale. It must be turned all the way clockwise.

13. Sensitivity. This determines the vertical scale. It is read in the left hand white window. The units are volts/division.

14. Vertical position. This knob controls the vertical position of the trace. 15. AC/DC select. When this is set to "AC" the DC part of the signal is filtered out by

a capacitor placed in series between the signal input and the scope. When the selector is set to "ground", the beam will move to zero volts. When the selector is set to "DC", the entire signal will be displayed on the scope.

212.56 Describe be applications of the oscilloscope. Applications: waveform observation, measurement of amplitude,

time, frequency and phase

Frequency measurement The frequency (f) of the signal is defined as the rate at which a periodic

signal repeats. It is usually measured in units of Hertz (Hz), where 1 Hz = 1 cycle per second.

It can be seen that four(4) cycles occur within one second; therefore, thesignal has a frequency of 4 Hz.

The frequency, f, of a wave is inversely related to its period (T):f = 1/T

The period of the signal is 250 milliseconds, therefore the frequency of that signal is:

f = 1/T

f = 1/250 milliseconds

f = 4 hertz

Amplitude measurement

Assuming the Volts/Div knob reads 2 V/div, the above peak-to-peak voltage would be: Vpp = 2 volts/div * 5.2 div = 10.4 volts

Assuming the Sec/Div control knob read 50ms, the above period would be:T = 50 milliseconds/div * 5.25 div = 2625.0 milliseconds = 2.625 seconds

Frequency =1/2.625sec =

212.57 Describe the use of probes to Improve theperformance of oscilloscopes and electronicinstruments at high frequencies. Types of probe:

Low capacitance impose low capacitance to the load

Multiplier - Scope probes are generally classified according to the level of attenuation of the signal they provide. Types including 1X (giving a 1 : 1 attenuation ratio), 10X (giving a 10 : 1 attenuation ratio) and 100X (giving a 100 : 1 attenuation ratio) are available:

Rectifier this type rectifies ac to dc for measurement in the oscilloscope

1X scope probes or low frequency probe The most basic form of oscilloscope probe, or scope probe, is what is

often termed the 1X probe.

It does not attenuate the incoming voltage as many other probes do.

It consists of a connector to interface to the oscilloscope (generally a BNC connector), and a length of coaxial cable which is connected to the probe itself.

This comprises a mechanical clip arrangement so that the probe can be attached to the appropriate test point, and an earth or ground clip to be attached to the appropriate ground point on the circuit under test.

The 1X probes are suitable for many low frequency applications.

They typically offer the same input impedance of the oscilloscope which is normally 1 M Ohm.

However for applications where better accuracy is needed and as frequencies start to rise, other test probes are needed

Probe

10X scope probes or low capacitance probe To enable better accuracy to be achieved higher levels of impedance are

required.

To achieve this attenuators are built into the end of the probe that connects with the circuit under test.

The most common type of probe with a built in attenuator gives an attenuation of ten, and it is known as a 10X oscilloscope probe.

The attenuation enables the impedance presented to the circuit under test to be increased by a factor of ten, and this enables more accurate measurements to be made.

In particular the level of capacitance seen by the circuit is reduced and this reduces the high frequency loading of the circuit by the probe.

212.58 Define the terms 'resolution' and 'accuracy' of instruments and determine typical values from manufacturer's data.

1) Accuracy: It is defined as the degree of the closeness with which instrument reading approaches the true value of the quantity being measured. Accuracy can be expressed in three ways (a) Point accuracy (b) Accuracy as the percentage of scale of range. (c) Accuracy as percentage of true value.

2) Sensitivity: It is defined as the ratio of the magnitude response of the output signal to the magnitude response of the input signal.

3) Precision /Reproducibility: It is defined as the degree of the closeness with which a given quantity may be repeatedly measured. High value of reproducibility means low value of drift.

4) Resolution measures the capability of an instrument to articulate or detect the smallest difference or change in the quantity being measured. That is what is the smallest change that an instrument can detect and indicate as a different reading?

Accuracy versus Precision

Poor precision, poor accuracy

Good precision (Good repeatability)

Poor accuracy(high deviation from position)

Poor precision (repeatability)

Improved accuracy

Good precision (Good repeatability)

Good accuracy(No deviation from position)

Resolution

That tickmark is a tenth of a second. The best a good eye can do is resolve a reading to 1/10 second, which is therefore the resolution of the stopwatch.

The digital stopwatch has two digits beyond the seconds, so it subdivides time in hundredths of a second. Since it is easy to read to 1/100 of a second, that is its resolution.

212.59 Calculate errors in instrument readings and the tolerance which must be applied arising frompractical limitations.

Limitations:

loading due to instrument impedance

resolution and accuracy of the instrument

Digits Displayed and Over-ranging shows the number of digits displayed the DMM.

It is often specified as a certain number of full digits (i.e. digits that can display values from 0 to 9) and an additional over-range digit referred to as a 1/2 digit.

That 1/2 digit typically shows only the values 0 or 1.

For example, a 6 1/2 digit display has a 7-digit readout, but the most significant digit can read 0 or 1 while the other 6 digits can take any value from 0 to 9.

Hence, the range of counts is 1,999,999. This should not be confused with resolution; a DMM can have many more digits displayed than its effective resolution.

Note: The 1/2 digit has been referred to by DMM manufacturers as any digit that is not a full digit. A full digit can take any value from 0 to 9.

Number of Counts -- the number of divisions into which a given measurement range is divided. For example, a traditional 5 1/2 digit voltmeter has 199,999 counts (from +199,999 to -199,999) or 399,999 total counts.

Error due to resolution limitation The resolution of a measurement system is the smallest yet to distinguish

different in values.

Digital measuring systems

A digital system converts an analog signal to a digital equivalent with an AD converter. The difference between two values, the resolution, is therefore always equal to one bit.

Or in the case of a digital multimeter, this is 1 digit.

It's also possible to express the resolution in other units than bits.

As an example a digital oscilloscope which has an 8 bit AD converter.

If the vertical sensitivity is set to 100 mV/div and the number of divisions is 8, the total range will be 800 mV. The 8 bits represent 28 = 256 different values.

The resolution in volts is then 800 mV / 256 = 3125 mV.

Calculation of resolution In order to determine the resolution of a system in terms of voltage, we

have to make a few calculations.

First, assume a measurement system capable of making measurements across a 10V range (20V span) using a 16-bits A/D converter.

Next, determine the smallest possible increment we can detect at 16 bits.

That is, 216 = 65,536, or 1 part in 65,536, so 20V65536 = 305 microvolt (uV) per A/D count.

Therefore, the smallest theoretical change we can detect is 305 uV.

Error due to accuracy limitation Accuracy -- a measure of the capability of the instrument to faithfully indicate the

value of the measured signal. Accuracy can never be better than the resolution of the instrument because accuracy is:

Accuracy = % of reading + off set

A reading device that has a specified accuracy of 0.015% will actually give a reading that is between 0.99985 and 1.00015 times the actual value. Note here that the 0.015% number is in reality the error.

For example, a 5 1/2 digit voltmeter can have an accuracy of 0.0125% of reading + 24 V on its 2.5 V range which results in an error of 0.00125% x 1V +24uV = 149 V when measuring a 1V signal.

On the other hand, the resolution of this same voltmeter is 12 V, 12 times better than the accuracy.

Precision -- a measure of the stability of the instrument and its capability of resulting in the same measurement over and over again for the same input signal

For instance, if you are monitoring a constant voltage of 1 V, and you notice that your measured value changes by 20 V between measurements, for example, then your measurement precision is Precision = (1-20uV/1V) x 100% = 99.998%

Accuracy: 5 % reading (3 % full scale)Range: 100 V, Reading: 70 V

The total measurement uncertainty is now calculated as follows:

= %

% =

70 = 3.5

= %

% =

70 = 3

a total uncertainty of 7.5 V up and down. The real value should be between 62.5 and 77.5 volts.

Digital A digital multimeters can hold a specification of 2.0 % reading + 4 digits.

This means that 0.04 have to be added to the reading uncertainty of 2 %.

An example based on the 3 digit digital readout:

This will read 5.00 V while the 20 V range is selected.

2 % of the reading would mean an uncertainty of 0.1 V.

Add to this the inaccuracy of the digits (= 0.04 V).

The total uncertainty is therefore 0.14 V. The real value should be between 4.86 and 5.14 volts.

Error due to loading

Typically the 1M resistor is in parallel with the circuit to be measured

Thus total resistance Rp = Rc //Ri

Thus extra current flowing (error current) is I = Vi/Ri

Example A typical specification for an AC power sources voltage meter is given

below: Range: 0.0 - 400 V, Voltage Resolution =0.1 V; Accuracy = (1% of reading + 2 counts)

The user has set the voltage to 277V and the source is indicating an output of 277V on the display.

1) Calculate the % of reading. The accuracy indicates 1% of reading: 277V*0.01 = 2.77V. So at this point the accuracy can be displayed as ( 2.77V + 2 counts)

2) Calculate the multiple of the resolution and add to the percentage: multiple of resolution = 2 counts = 2*0.1V = 0.2V. Thus accuracy = ( 2.77V + 0.2V) = ( 2.97V)

3) Value is first added to the base reading of 277V to get the high end of the range: 277V + 2.97V = 279.97V

4) Value is then subtracted to the base reading of 277V to get the low end of the range: 277 2.97V = 274.03V 5. This means that if the display is registering an output of 277V, it could actually be outputting anywhere from 274.03-279.9V

212.60 Describe the operation and use of a simple logic probe.

10X scope probes or low capacitance probe circuit

As the 10X probe attenuates the signal by a factor of ten, this obviously means that the signal entering the scope itself is reduced.

This has to be taken into account. Some oscilloscopes automatically adjust the scales according to the probe present, although not all are able to do this. It is worth checking before making a reading.

The 10X scope probe uses a series resistor (9 M Ohms) to provide a 10 : 1 attenuation when it is used with the 1 M Ohm input impedance of the scope itself.

A 1 M Ohm impedance is the standard impedance used for oscilloscope inputs and therefore this enables scope probes to be interchanged between oscilloscopes of different manufacturers.

10X oscilloscope probes also allow some compensation for frequency variations present.

A capacitor network is embodied into the probe as shown.

The capacitor connected to ground can then be used to equalize the frequency performance of the probe.

7.12 Describe an r.f. oscillator as a self-driven amplifier with a parallel LC circuit as the load.

Tuned frequency oscillators (LC)

0v

C1L1L2

C2

C4

T1

R1

R3 C3

+VCC

R2Vo

6. (a) conditions for sinewave oscillations are [3] Gain must be infinite

loop gain must be 1and must occur at a single frequency

phase angle of the feedback signal must be = 0o or 360o at the input to ensure positive feedback

(b) Two advantages of using crystal oscillators are [2]

crystal oscillator are highly precise or preset frequency that was set on design i.e. they always give very close to the exact frequency they are designed for

they are highly stable that is they give a constant frequency output

(c) given L1 =1mH and C1 =4nF [3] (i) circuit operation of the LC tuned oscillator

The circuit is a tuned LC oscillator. It has the following parts

Amplifiers: Provided by the transistor R1, R2, and R3 as bias and stability components for class A operation. C1 and L2 acts as the collector load. This circuit provides the required gain to maintain oscillations

Feedback, is provided by the turns ratio of L1 to L2. It determines the amount of Voutthat is fed back.

fosc- the main frequency of oscillation is provided by L1,and C1 tank circuit, which acts as the collector load of the transistor and develops a voltage at a frequency that is its natural resonant frequency given by fosc= 1/2LC. When power is switch ON a voltage surge occurs on this circuit and generates the first ac pulse that is feedback to the transistor and hence oscillations start..

Phase correction Transistor output obtained from the collector is 180o out of phase with the input hence L2 to L1 transformation provides a phase change of 180

o so that the feedback signal is maintained in the same phase or direction as the previous input.

The oscillations are maintained by the operation of all the units each doing its function

(ii) Frequency of oscillation is given by [2]

fosc= 1/2LC = 1/2(1m x4n) =1/24-12) =1/2 x2-6 = 79577.47

=80kHz

Typical circuit