Oscillator Manual

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MODULAR ANALOG ELECTRONICS TRAINER 1 CONTENTS PAGE COMPONENTS LIST 2 LC OSCILLATORS 3 EXPERIMENT 1A 6 EXPERIMENT 1B 9 PHASE-SHIFT OSCILLATOR 12 EXPERIMENT 2 18

Transcript of Oscillator Manual

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CONTENTS

PAGE COMPONENTS LIST 2 LC OSCILLATORS 3 EXPERIMENT 1A 6 EXPERIMENT 1B 9 PHASE-SHIFT OSCILLATOR 12 EXPERIMENT 2 18

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COMPONENTS LIST FOR OSCILLATOR-1 MODULE BOARD CIRCUIT #1 Variable Resistor VR1 500Ω CIRCUIT #2 Resistor R1 10kΩ R2 1kΩ R3 10kΩ R4 1kΩ Variable Resistor VR2 5kΩ Capacitor C1 1µF, 50V C2 10nF C3 10nF C4 2.2nF C5 1µF, 50V C6 1µF, 50V Transistor TR1 2N3904 Inductor L1 1mH L2 10mH CIRCUIT #3 Variable Resistor VR3 1kΩ Resistor R5 1.2kΩ R6 4.7kΩ R7 68kΩ R8 4.7kΩ Capacitor C7 47nF C8 47nF C9 47nF C10 22nF Transistor TR2 2N3906

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LC OSCILLATORS OBJECTIVES : • Describe how the required phasing for positive feedback is achieved in Hartley and

Colpitts Oscillators. • Calculate the resonant frequency of a Hartley tuned circuit. • Calculate the resonant frequency of a Colpitts tuned circuit. • Discuss the Class of bias used in an efficient oscillator circuit. • State the range of frequencies over which the Hartley and Colpitts Oscillators will

operate. INTRODUCTION :

Oscillations in a Tuned Circuit The resonant frequency of a tuned (oscillatory) circuit is determined by the inductive reactance being the same as the capacitive reactance. XL = XC ∴ 2πfL = 1 / 2πfC ∴f2 = 1/4π2LC ∴fo = 1/ [ 2π (LC)1/2 ] In this condition the two currents have the same magnitudes and “cancel out” (as far as external circuit is concerned), resulting in large internal currents, which circulate around the (parallel) tuned circuit. The energy is transfer between the magnetic field of the coil (maximum current) and the electrical field of the capacitor (maximum voltage).

Fig. 1.1 The only losses in the circuit will be largely due to the resistance of the wire of the coil. A small signal injection of energy at the peak of the cycle is all that is necessary to maintain the oscillation. In electronic terms this corresponds to the highly efficient Class C mode of operation of the amplifier.

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This is similar to the effect when a weight is suspended from a pivot and set in swinging motion. Suspended Weight in Motion

Fig. 1.2 Discrete Amplifier Oscillators Most forms of oscillator circuits were derived for the thermionic tubes (valves), the normal form of which provided an inverting amplifier. These were readily adapted for transistors (BJT) when they came along, the common emitter amplifier mode being naturally adapted. The exercises in the Chapter are designed to investigate two of the most common of these, the Hartley and the Colpitts Oscillators. Each of these employs an inverting amplifier and a feedback (β) network with a built-in inversion to give the positive feedback which is the primary requirement of an oscillator. Hartley and Colpitts Tank Circuits The basic difference between them is in the method used to provide a reference ground point for the inversion of the signal.

Fig. 1.3

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In the case of the Hartley Oscillator a center-tapped inductors is used in the oscillatory circuit (Fig. 1.3 (a)). The Colpitts Oscillator uses center-tapped capacitors (Fig. 1.3 (b)). When calculating the resonant frequency, the Hartley tuned circuit contains an inductor consisting of two coils in series. L = L1 + L2. In the case of the Colpitts Oscillator the series components are capacitors. C = [ C1 x C2 ] / [ C1 + C2 ] In either case the alternating signal voltage at the ends of the inductor A & B will be opposite polarity. The amount of the feedback will be determined by the ratio of the two inductors in the case of the Hartley or the capacitors (inversely) in the Colpitts. The necessary timing of the injection of energy to maintain oscillations can be achieved by Class C bias of the amplifier, which is very efficient. Class A bias can be used, and has been for the circuits investigated in earlier chapters, but it is unnecessary.

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MATERIALS & APPARATUS 1. EDUTECH Oscillator-1 Module Board Circuit #1 & #2. 2. EDUTECH EDU-9100 Analog Base Unit. 3. Oscilloscope. 4. Digital Multimeters (DMM). 5. Signal Generator. Experiment 1A : Hartley Oscillator The feedback ratio is that of the inductors provided, which is 10:1 (10mH, 1mH). This gives an inductance of 11mH. A range of capacitors are provided to satisfy the needs of the Colpitts Oscillator. With a little ingenuity and multipatching using jumper wires, as many as twelve different values of capacitance are practically possible, each with its own resonant frequency. This circuit would normally be chosen for ratio frequencies between about 30kHz – 30MHz. Some of the frequencies obtainable with the components provided will be lower for experimental convenience. The small signal gain of the amplifier is adjustable by the degree of decoupling of the emitter resistor. This allows the bias to be varied all the way from Class A through Class C. Objectives : • Describe how the required phasing for positive feedback is achieved in Hartley and

Colpitts Oscillators. • Calculate the resonant frequency of a Hartley tuned circuit. • Calculate the resonant frequency of a Colpitts tuned circuit. • Discuss the Class of bias used in an efficient oscillator circuit. • State the range of frequencies over which the Hartley and Colpitts Oscillators will

operate. Procedures : 1. Place the EDUTECH Oscillator-1 Module Board on the EDU-9100 Analog Base

Unit. 2. Connect the Circuit #1 & #2 as shown in fig. 1.4. Connecting sockets 2.10 & 2.11 for

Hartley circuit. Connecting sockets 2.12 & 2.15 to select C3 = 10nF. Connecting sockets 2.7 & 2.9 to to short out the 100nF series capacitor and simplify the resonant frequency calculations. Switch on the base unit. Adjust the variable 0 – 15V DC power supply on the base unit to maximum to get 15V.

3. Adjust the VR1 on the Module Board to get V1 = 12V. Set the signal gain control VR2 fully clockwise for minimum gain. (maximum resistance of VR2).

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4. Connect socket 2.19 to Ch.1 of oscilloscope and socket 2.20 to Ch.2 of oscilloscope to monitor the emitter current.

Note that the oscilloscope is displaying the quiescent DC bias conditions of the transistor amplifier by the position of the traces.

5. Increase the gain by turning VR2 counterclockwise. Oscillations should commence with VR2 set just below half travel. When first established some instability (“hunting”) of amplitude will occur while the output is less than about 5Vp-p.

Note that a small signal voltage appears at the emitter (Ch.2).

6. Continue to increase gain. The output signal will increase in magnitude until the negative tip meets the positive excursion of the emitter waveform, at which time the transistor will saturate (“bottom”) and distort the output waveform. Back off the setting of VR2 for maximum undistorted output waveform. The tips of the two waveforms should be just about touching.

Note that the gain setting is easy to achieved and that stability is good. This is an excellent practical oscillator circuit.

7. Note the maximum undistorted peak-to-peak value of the output signal =

___________ Vp-p. 8. Change the Ch.2 Y amplifier sensitivity to 0.2V/div and move the Ch.1 trace up the

screen out of the way above the Ch.2 trace.

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9. Turn the gain setting (VR2) fully clockwise again to stop the oscillation and then reduce again, watching the oscilloscope display as you do.

Initially, as oscillations recommence, you will see that the emitter waveform contains a full sinewave AC component as the transistor operates in Class A. As the gain is further increased the transistor current bottoms out into Class AB, then to Class B and finally to Class C as condition occurs for less than 180°.

10. For each of the values of capacitor given in Table 1.1, calculate the theoretical value of frequency of oscillation from the formula and record in Table 1.1

L = 11mH C = 10nF C = 2.2nF C = 12.2nF Theoretical Frequency

Measured Frequency

Table 1.1

11. With the circuit as connected, ensure that the oscilloscope timebase is in the

calibrated setting and measure the time for one cycle. Take the reciprocal to obtain the frequency and record the result in Table 1.1.

12. Transfer the jumper wires from between sockets 2.12 & 2.15 to 2.13 & 2.16, and repeat the measurement and calculation for C4 = 22nF and record in Table 1.1.

13. Add a further jumper wires between sockets 2.12 & 2.15 and repeat for C = 12.2nF. 14. Switch off the base unit.

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Experiment 1B : Colpitts Oscillator The center tap is transfer from the inductors to the capacitors. The available components again give a range of possible frequencies, but not quite so many this time. Only a single inductor is necessary but both will be used initially to compare the operation of the Colpitts and Hartley Oscillators at similar frequencies. The series capacitors necessary for the Colpitts Oscillator result in lower tuned circuit capacitance and therefore higher frequency. Also the Hartley Oscillator requires series inductors which increase the total impedance and reduce frequency. The Colpitts Oscillator will therefore operate at higher frequencies than the Hartley, being practically used at up to 300MHz, although at the very high end capacitances are made up of the stray and inter-electrode capacitances of the transistor. Procedures : 1. Connect the Circuit #1 & #2 as shown in fig. 1.5. Connecting sockets 2.9 & 2.10 for

Colpitts circuit. Connecting sockets 2.11 & 2.14 to short out L3. Connecting sockets 2.12 & 2.15 to select C3 = 10nF.Switch on the base unit.

2. Connect socket 2.19 to Ch.1 of oscilloscope and socket 2.20 to Ch.2 of oscilloscope.

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3. Adjust the gain (VR2) to obtain maximum undistorted output waveform. Measure the frequency of oscillation.

Oscillation frequency = _____________ kHz.

4. Keeping the output signal (Ch.1) as phase reference, sketch the circuit waveforms on the graticule (waveform sketch 1.1) provided. Change the Ch.2 Y amplifier gain as required so that the small signal components can be observed. Label each waveform with its peak-to-peak voltage and the component with respect to 0V (ground).

5. Use the digital multimeter to measure the transistor voltages and record in Table 1.2

with the oscillator running normally. Transistors Voltages Collector

Socket 2.19 Base

Socket 2.4 Emitter

Socket 2.20 Oscillating Quiescent

Table 1.2

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6. Remove the jumper wire from between sockets 2.5 & 2.6 to stop the oscillator. Measure the quiescent DC bias conditions and record those also in Table 1.2.

Questions : 1. The necessary inversion/phase shift for a Hartley Oscillator to operate with an

inverting amplifier is obtained from where ? 2. The necessary inversion/phase shift for a Colpitts Oscillator to operate with an

inverting amplifier is obtained from where ?

Fig. 1.6 3. Calculate the resonant frequency of the Hartley circuit shown in fig. 1.6. 4. Calculate the resonant frequency of the Colpitts circuit shown in fig. 1.6. 5. Why the discrete transistor LC oscillator can use Class C bias ? 6. What are the frequencies range covered by Hartley and Colpitts Oscillators ?

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CHAPTER 2 : PHASE-SHIFT OSCILLATOR OBJECTIVES : • To determine the range of frequency variation of an RC phase-shift oscillator. • To compare the phase of output and feedback voltages in the oscillator. INTRODUCTION :

Phase-Shift Feedback In the Hartley oscillator, the L and C of the tank circuit determine the frequency of oscillation. The phase-shift oscillator uses resistors and capacitors (R & C) as the frequency determining constants. Recall that the requirements for oscillation include (1) an amplifier with (2) feedback from the output to the input circuit in proper phase to overcome the circuit losses and sustain oscillation. The manner in which feedback is accomplished is unimportant, as long as it is in proper phase and of sufficient amplitude to overcome the energy losses in the circuit. The feedback path simply distinguishes one type of oscillator from another.

Fig. 2.1 Oscillator feedback network.

Fig. 2.2 RC phase-shift network

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In fig 2.1 the feedback network is shown in block form. Q1 is an amplifier with a resistive collector load. There is a 180° phase shift between base and collector under normal operating conditions. The feedback network must introduce another 180° phase shift from the collector back to the base, in order to accomplish oscillation. A triple-section RC network, as in fig. 2.2, can do this. Point A is connected to the collector of Q1 in fig. 2.1, and B is connected to the base of Q1. Consider a single section, C1R1 of this feedback network, and assume that the signal vC which is coupled to C1R1 is a sine wave. C1R1 is a capacitive circuit, and the current leads the voltage by an angle which is defined as the “phase angle” of the circuit. The phase angle θ depends on the frequency of vC and on the values R and C and is given by the equation tan θ = XC / R = 1 / ( 2π f CR) (2-1) θ = arctan (XC / R) (2-2) Now C1 and R1 may be chosen so that for a desired frequency f, θ = 60°. Fig. 2.3 shows this 60° phase shift. The voltage vR1 across R1 leads vC , the input voltage, by 60°

Fig. 2.3 Phase shift in RC network.

C2 and R2 can now be chosen to introduce an additional 60° phase shift between vR1 and vR2, so that vR2 leads vC by 120°. Similarly, C3 and R3 are selected to introduce another 60° phase shift, and as a result, vR3 leads vC by 180°. Note that there will be 180° phase shift for just one frequency, as determined by the value of C and R in the feedback network. If any of the selected values of C and R, say R3 in fig. 2.2, is varied, the frequency for which there is a 180° phase shift will change. The phase angles introduced by each section of the feedback network will change because of the new frequency. Oscillation takes place at the frequency for which the total phase shift equals 180°.

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Transistor Phase-Shift Oscillator

Fig. 2.4 Transistor phase-shift oscillator.

A transistor phase-shift oscillator must introduce in-phase feedback from the output to the input to sustain oscillation. If a common-emitter amplifier is used, with a resistive collector load, there is a 180° phase shift between the base and collector. Hence the phase-shift feedback network between collector and base must introduce an additional 180° phase shift, at some frequency, if oscillation is to take place. Here also a three-section RC network may be employed. A transistor connected as phase-shift oscillator is shown in fig. 2.4. In this common-emitter amplifier, feedback is from the collector to the base, that is, from the output to the input. The three-section phase-shift network consists of C1, R1, C2, R2, C3, and R3 in series with Rin, the input resistance. So that each section may introduce a 60° phase shift (approx) at the resonant frequency, the values C1 = C2 = C3 and R1 = R2 = R3 + Rin. By analysis it can be shown that the frequency of oscillation for these conditions may be expressed by the equation f = 1 / [ 2πC * ( 6R1

2 + 4R1RL )1/2 ] (2-3) A necessary condition for sustained oscillation in the RC phase-shift oscillator is given by the equation hfe = 23 + ( 29R1 / RL ) + ( 4RL / R1 ) (2-4) where hfe is the small-signal forward-current transfer ratio of the transistor. Example : A numerical example will illustrate the use of Eqs. (2-3) and (2-4). What is the predicted frequency of oscillation of Fig. 2-4, if R1 = 2200 Ω, RL = 12,000 Ω, and C1 =

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0.1 ? Will a transistor with the given values of R1, RL, and C1 whose hfe = 40 provide sufficient feedback for oscillation ? Solution : (a)

hfe = 23 + ( 29R1 / RL ) + ( 4RL / R1 ) = 23 + [ (29)(2200) / 12000 ] + [ (4)(12000) / 2200] = 23 + 5.32 + 21.8 = 50.12

A transistor whose hfe = 40 will not permit oscillation in this circuit. The hfe of the required transistor must be greater than 50.12. (b)

f = 1 / [ 2πC1 * ( 6R12 + 4R1RL )1/2 ]

= 1 / [ (6.28)(0.1 x 10-6) * [(6)(2200)2 + 4(2200)(12000)]1/2 ] = 104 / [ (6.28) * [(6)(2.2)2 + 4(2.2)(12) ]1/2 ] = 104 / [ 6.28 * (134.6)1/2 ] = 137 Hz

If a proper transistor is chosen, the frequency of oscillation for the values

given in the example will therefore be approximately 137 Hz. Fig. 2.5 is a practical variation of the circuit of a phase-shift oscillator. Note the changed position of the frequency control R4, and the change in the bias circuit. Bias resistor R3 is connected to the collector for bias stabilization. The purpose of C4 is to bypass the base to eliminate parasitic oscillations. Since this is not a perfectly balanced phase-shift circuit, Eq. (2-3) is not directly applicable. However, if rheostat R4 were set for zero resistance and if R3 were made variable, R3 could be adjustable for phase-shift balance and Eq. (2-3) would then apply.

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Fig. 2.5 Experimental transistor phase-shift oscillator.

Measuring Phase with an Oscilloscope To measure the phase relationship between two sine-wave signals, start by connecting the vertical and external horizontal trigger inputs of an oscilloscope to the reference waveform. Adjust the horizontal level so that the zero-degree crossing of the sine-wave starts at the leftmost position on the horizontal axis as illustrate in fig. 2.6. Measure the total time period tp of a cycle of the waveform as shown in fig. 2.6. Move the vertical probe to the test signal point and measure the time distance tc from the leftmost horizontal position to the zero-degree crossing of the test waveform as shown in fig. 2.7. The phase difference between the two waveforms is calculated using this formula : φ = [ tc / tp ] * 360° (2-5) As an example, a 35 kHz sine-wave has a zero-degree crossing 5 µs after the zero-degree crossing of a reference signal of the same frequency. What is the phase difference between these two signals ? The solution is found by first calculating the time period of the 35 kHz sine waves. Inverting 35 kHz produces a time period tp of 28.57 µs. Dividing the zero-degree crossing time tc of 5 µs by the tp and multiplying the quotient by 360° results in a 63° phase angle.

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Fig. 2.6 Referance waveform.

Fig. 2.7 Zero-crossing measurement.

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MATERIALS & APPARATUS 1. EDUTECH Oscillator-1 Module Board Circuit #1 & #3. 2. EDUTECH EDU-9100 Analog Base Unit. 3. Oscilloscope. 4. Digital Multimeters (DMM). 5. Frequency Counter. Experiment 2 Objectives : • To determine the range of frequency variation of an RC phase-shift oscillator. • To compare the phase of output and feedback voltages in the oscillator. Procedures : 1. Place the EDUTECH Oscillator-1 Module Board on the EDU-9100 Analog Base

Unit.. Adjust the base unit variable -15V DC power supply to maximum. Switch on the base unit , adjust VR1 until V1 read -10V. Switch off the base unit.

2. Connect the Circuit #1 & #3 as shown in fig. 2.8. Set VR3, the frequency control, to maximum resistance.

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3. Switch on the base unit. Connect socket 3.9 to Ch.1 and socket 3.6 to Ch.2. Observe and measure the waveform and its peak-to-peak voltage. Record the data in Table 2.1. Measure and record the DC voltage at the collector, emitter and base.

4. Using a frequency counter, determine the oscillator frequency with VR3 set at maximum resistance. Record the data in Table 2.1.

5. Adjust VR3 for minimum resistance. Measure the new oscillator frequency. Record this frequency in Table 2.1.

Test Point

Waveform

Vp-p

DC, V Frequency

Minimum Maximum Collector Emitter X X

Base

Table 2.1 RC Oscillator Frequency Range 6. Set VR3 approximately midway. Observe and measure the peak-to-peak amplitude of

the output waveform at socket 3.9 & 3.10 and the feedback voltage waveform at socket 3.3, 3.5 & 3.6. Measure and compute the phase angle between each socket and Vout. Record this data in Table 2.2.

Test Point Waveform Vp-p Phase Shift, Degrees Collector Socket 3.3 Socket 3.5 Socket 3.6

Table 2.2 RC Oscillator Phase-Shift Relationships

Questions : 1. How is the feedback accomplished in the RC oscillator of fig. 2.4 ? 2. How much phase shift is introduced by C1R1. 3. Compare and measure phase-shift signal between Vout and socket 3.6 with what it

should be theoretically and account for any discrepancy. 4. Why is there a difference in amplitude between the feedback voltage at sockets 3.3,

3.5 & 3.6 ? 5. Refer to fig. 2.5. How is bias stabilization achieved ?

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Note :

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Note :

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Note :