WAVEFORM GENERATORS (VCO)

Transcription

1 Drexel University Electrical and Computer Engr. Dept. Electrical Engineering Laboratory III, ECEL 303 E.L. Gerber Object WAVEFORM GENERATORS (VCO) The object of this experiment is to learn some of the basics of electronic waveform generation. Standard IC chips will be used to produce various waveforms as well as a frequency modulation circuit. Introduction One of the old standard integrated circuit voltage-controlled oscillator (VCO) devices is the LM566. It is capable of generating a fixed frequency square wave and triangle wave. The frequency of these waves can also be varied via an external input voltage. This device has a maximum operating frequency of 1 MHz with a 10-to-1 range of frequency variation with a change in modulating input voltage. It is a very basic chip and only requires two external circuit elements to operate. Figure 1 illustrates a basic block diagram of the LM566 VCO chip and the external timing circuit. Theory Voltage-Controlled Oscillator Operation Fig. 1. Voltage Controlled Oscillator Block Diagram 7-1

2 The chip has a basic oscillator circuit built in but requires external timing elements and. The chip provides a constant DC current source which is reversed under internal chip command. The magnitude of the current, however, can be controlled externally via the timing resistor. The fundamental principle of the system is the current flow in a capacitor, i(t) = Cdvdt. Since the current in the capacitor is constant, hence, the voltage across it (pins 7-1) will be a positive or negative ramp as seen in Fig. 2. Referring to Figure 1, the current source/sink circuit provides a constant charging or discharging current to the external timing capacitor. The amount of current is controlled by the timing resistor,. Increasing the value of decreases the capacitor current. Control of this current is also possible by changing the voltage across the resistor via the modulating input. The voltage at pin 6 is normally maintained at the same voltage as pin 5. Thus, if the modulating voltage at pin 5 is increased, the voltage at pin 6 increases, resulting in less voltage across and, therefore, less charging current. All voltages must be positive. Refer to Fig. 1 again, the voltage developed on capacitor is applied to the Schmitt trigger circuit U 2 via the buffer amplifier U 1. The output voltage swing on the Schmitt trigger goes from to 0.5. Resistors R a and R b form a positive feedback loop from the output of U 2 to its non-inverting input. With equal dividing resistors R a and R b, the non-inverting input swing is from 0.5 to If the voltage on the timing capacitor exceeds 0.5 during charging, it will cause the Schmitt trigger output to go low (0.5 ). A low level on the output of U 2 causes the current source to change to a sink (discharging ). When discharges to 0.25, the output of the U 2 will swing high ( ), causing the current sink to return to a source (charging ). Since the source and sink currents are equal, it takes the same amount of time to charge as it does to discharge this capacitor. This results in a triangular voltage waveform (Fig.2) on which is available as a buffered output at pin 4. A square wave appears at the output of the Schmitt trigger and is inverted by inverter U 3 for a second output at pin 3. If the current from the source/sink is increased, the charge/discharge time for the capacitor is reduced and the output frequency is increased (shorter period). See Fig. 2. Determining VCO Output Frequency The output frequency can then be changed by three methods: 1. Changing the value of. 2. Changing the value of. 3. Changing the voltage at the modulating input terminal. We can determine the actual frequency of oscillation from the time it takes to charge and discharge the capacitor. The basic equation for a capacitor is: i(t) = C dv dt or v(t) = 1 C Ú i(t)dt (1) 7-2

3 where dv is the voltage change on the capacitor during the time change dt. The total voltage on the capacitor changes from 0.25 to 0.5 because of the limits of the controlled current source/sink circuit. Thus V = = 0.25 (2) From Eq.1, for a constant current, Dt = 0.25 I (3) The triangular waveform on the capacitor has a period T = 2 t (equal charging and discharging time). The frequency of oscillation is: f = 1 T = 1 2Dt (4) Fig. 2. Voltage Waveform Across. Substituting t from Eq. 3 and Eq. 4, the frequency of oscillation is: Ohm s law gives, f = I 0.5 (5) I = - V 5 (6) Where V 5 is the modulating input voltage at pin 5, then: f = 2( - V 5 ) (7) For best operation, the resistance of should be between 2 to 20 kw. In normal operation, and are selected for the desired center operating frequency. With no modulation signal the output frequency is fixed by Eq. 5. The modulating input voltage can be varied to give a variation in the output frequency i.e., 7-3

4 frequency modulation. The range of allowable variation of the modulating input signal is from 0.75 to, which yields an output frequency variation of about 10 to 1. With no modulating input signal, the voltage at pin 5 should be biased at 7/8. This allows us to simplify Eq. 7 to give the unmodulated frequency f o, f 0 = 2( ) = 1 4 (8) If we wish to determine what input modulation voltage ( V) is required to produce a given output frequency deviation ( f) we can calculate the original frequency is f o and the new frequency f 1 from Eq.7, f = f 1 - f 0 = 2( - V 5 + DV) - 2( - V 5 ) = 2DV (9) Solving for V: DV = Df 2 Substituting from Eq. 8: DV = Df = Df 8 f 0 2 (10) Circuit Applications (8038): 1-Oscillator. The ICL 8038 chip operates on the same principle as the older LM 566. It has a reversible constant current source and it requires external R-C elements to set the oscillation frequency, f o. Unlike the LM 566 the duty-cycle of the output signal can be changed by varying one of the timing resistors, R A or R B. See Fig. 3. The period of the output signal, T o (= 1/f o ) is given in equation 11, where is the timing capacitor. For a symmetrical output signal, 50 % duty-cycle, R A = R B, then equation 11 becomes equation 12. T o = 3 2 R C È 1 + R B A T Î Í 2R A - R B (11) f o = 1/3 (12) 7-4

5 2-DC Sweep Input. The output frequency can also be controlled directly by an external DC voltage connected to pin 8, see Fig. 4. V DC applied across pins 8 and 6 will change the voltage across the timing resistor, see equations 6 and 7, and therefore change the output frequency. With R A, R B and fixed then V DC will control the output frequency of the VCO. The modulation rate of the system is f/ V. 3-AC Modulation. The output frequency of the VCO can be varied as a function of time by an AC input signal, Vin = V sin(2πf m t), see Fig. 5. This is known as frequency modulation or FM. This voltage must be connected to pin 8 through a coupling capacitor, C C. The amplitude of the input signal V will control the frequency change f of the VCO. Whereas the frequency of the input signal, f m, controls the rate of change of the output signal. When f m is very low you can see the changing output signal on an scope. PreLab Design a symmetrical square and triangle wave generator, as shown in Fig. 3, to generate a signal of approximately 620 Hz. Design means calculate the values for and R A, with R B = 10 kω. Repeat the design for 1.2 khz. Laboratory 1 Oscillation: a) Build the 620-Hz symmetrical square and triangle generator (Fig. 3) designed in the PreLab with R B = 10 kω, and = 22-V DC. Use one or two capacitors to obtain the correct value of. Adjust R A (the pot) for 50% duty-cycle which can be measured directly on the scope using the "Time Display". Measure and calculate the VCO s output frequency. Capture both outputs on the 'scope include V PP, Freq, and duty-cycle of the square wave only. Also measure the pot resistance. Compare frequency measurement with your design. b) Now vary the pot (R A ) until the duty-cycle of the square wave only is 25%. Measure and record the resistance values and calculate the total period, T O, and frequency from Eq. 11. Capture the square wave output only on the 'scope include V PP, Freq, and duty-cycle. c) Repeat b for 66% duty-cycle. d) Repeat b for 75% duty-cycle. e) Tabulate these results for a, b, c, and d as: R A, R B, %D.C., f o, and T o. 7-5

6 f) Reset R A for 50% duty-cycle. Replace the capacitor with a substitution box and vary over five orders of magnitude starting at 0.1 nf. Measure the frequency and the duty-cycle. Plot these results on a log-log graph. 2 DC Sweep: Connect the circuit shown in Fig. 4. Apply a second DC power supply between pins 8 and with the polarity as shown. Start with V DC set to zero. Increase the DC sweep voltage from 0V, in one-volt steps, until the output signal shuts down. Measure and record the sweep voltage, V DC, and the frequency. Plot these results. Determine the VCO's modulation rate, k, from this plot. 7-6

7 3 AC Modulation: Connect the circuit in Fig. 5. Let R C = 10 kω and C C = 200 µf. Apply a 1-VPP, 100 Hz sine wave from the HP function generator to the AC input to ground. Measure the AC input at pin 8 not the HP generator. The capacitor will drop a large voltage at low frequencies so you will need to increase the amplitude of the input as you lower the frequency. Observe the VCO s square wave output on the scope. Reduce the input frequency to 10 Hz and observe the VCO s output on the scope. Reduce again to 1 Hz and observe the VCO s output on the scope. Reduce again to 0.1 Hz, and increase the input each time. Has the output frequency range changed? Repeat the last part with a ramp function at 10 Hz and 0.1 Hz. Repeat the last part with a square function at 10 Hz and 0.1 Hz. Now increase the input AC to 2 VPP at low frequency, observe and capture the output. Has the output frequency range changed? Parts List: 1 ICL 8038 circuit board Resistors; 10 kω, 25 kω pot Capacitors; 200 µf, substitution box. 2 Twisted DC leads 2 BNC/clip 7-7

8 ICL