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Download Voltage Controlled Oscillator(VCO) using 555 timer IC.
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Name of the Expt: To Construct and test a Voltage Controlled Oscillator(VCO) using 555 timer IC. Objectives: After completing this experiment we will able to know1. How does voltage is used to control an oscillator. 3. Where can we use this circuit. 4. the designing procedure of a voltage controlled oscillator. Theory: A Voltage Controlled Oscillator (VCO) is the synth module that generates the pitch (frequency) of the notes we hear will have both a coarse and fine frequency adjustment knobs. That will allow the user to precisely "tune" this pitch source. A VCO is akin to a single guitar string. When the open string is plucked, it will generate a specific note. The coarse and fine frequency adjustments on the VCO control panel are similar in function to the tuning knob on a guitar. The VCO also has a 1V/OCT CV input. That CV comes from the keyboard controller. It is that CV that changes the pitch of the VCO much the same way as fretting a guitar string changes its pitch. The circuit is sometimes called a voltage-to-frequency converter because the output frequency can be changed by changing the input voltage. As discussed in previous blog posts, pin 5 terminal is voltage control terminal and its function is to control the threshold and trigger levels. Normally, the control voltage is ++2/3VCC because of the internal voltage divider. However, an external voltage can be applied to this terminal directly or through a pot, as illustrated in figure, and by adjusting the pot, control voltage can be varied. Voltage across the timing capacitor is depicted in figure, which varies between +Vcontrol and ½ Vcontrol. If control voltage is increased, the capacitor takes a longer to charge and discharge; the frequency, therefore, decreases. Thus the frequency can be changed by changing the control voltage. Incidentally, the control voltage may be made available through a pot, or it may be output of a transistor circuit, opamp, or some other device. Like any oscillator, the frequency stays fixed unless something acts on it to change. If the oscillator is set for a fairly low frequency, the corresponding note will be low on the musical scale. Now suppose that the oscillator frequency could be precisely controlled with a voltage and nothing more. What would that do for us? Assuming the voltage was very stable, this voltage could be used to force the oscillator to exact note frequencies. And so it does! Differing, but specific voltages will cause the oscillator to function at frequencies that are musical notes according to the 1V/octave standard already explained. The oscillator has a “main” control port called the 1V/OCT input. The next question usually asked is "well what turns the oscillator on and off to make the notes?". The simple answer is: nothing. The oscillator is on all the time. I'll explain that a bit later, let's just worry about voltage control for a bit longer. For charging So that total T=w+0.693R2C…………………………………(i) F==1/T………………………………………......(ii) time period in given by T=W This circuit is a voltage-controlled oscillator (VCO) that uses the 555 timer IC as the main component. As expected, the 555 timer is configured as an astable multivibrator to be able to serve as an oscillator. An astable multivibrator is just a timing circuit whose output oscillates between 'low' and 'high' continuously, in effect generating a train of pulses. The difference of this circuit with the basic 555 astable circuit is that its 555's pin 5 is tied to an external voltage source. Pin 5 is the 555's control voltage pin, which allows the user to directly adjust the threshold voltages to which the pin 2/pin 6 input voltages are compared by the 555's internal comparators. Since the outputs of these comparators control the internal flip-flop that toggles the output of the 555, adjusting the pin 5 control voltage also adjusts the frequency at which the 555 toggles its output. Increasing the input voltage at pin 5 decreases the output oscillation frequency while decreasing the input voltage increases the output oscillation frequency. A VCO normally just sits there outputting a variety of waveforms, at a frequency determined by the combination of the "coarse" and "fine tune" adjustments on the control panel. The keyboard controller's CV output is added to the internal VCO control voltage to determine the final pitch output by the VCO. Since the coarse and fine tune controls adjust the VCO pitch, in addition to the keyboard CV, one can make the VCO assume any frequency for any note. Just because you press the A1 key on the keyboard and the keyboard controller outputs a very specific voltage for that key, nothing says the output pitch has to be the pitch of A1 (55.000Hz). This flexibility allows for on the fly transposing of notes. When multiple VCOs are used, each VCO will get the keyboard controller CV. By adjusting the individual VCO frequencies, one can get very thick chords and note beatings. With maybe eight VCOs running, one can get a "wall of sound" if so inclined. That is the beauty of analog synthesis ... there is essentially no limits as to how many elements comprise the final sound. (Well, your bank account balance usually is the limiting factor. Analog synthesis modules are rather expensive. A single VCO can easily cost in excess of $400. Even a modest system will cost $5,000 or more. Why then would anybody in their right mind have one of these things? There are a great many digital synths out there that sell for a fraction of that price. Are you nuts? The answer is: the sound. Digital synths can sound very good but going beyond the stock sounds is tedious in the extreme, has very limited capability, and the user interfaces for making changes, sucks.) 1V/OCT Input - A control voltage input that changes VCO frequency according to the applied voltage. Negative or low amplitude positive voltages produce lower pitched notes. More positive voltages produce higher pitched notes. It is not uncommon for a VCO to be able to produce pitches from 1Hz to 30,000Hz with only voltage control. FM Input - A control voltage sent to the FM input permits the VCO to be frequency modulated. Vibrato is an example of frequency modulation. There is nothing preventing the user from using the output pitch of one VCO to FM modulate a second (or third) VCO. There is usually an FM level adjust panel control associated with this input, to adjust how much frequency modulation the VCO "sees". PWM Input - This control voltage input changes the duty cycle of the pulse output. The full name is "Pulse Width Modulation", hence the shortened moniker "PWM". This control input does not change the frequency of the VCO, it only changes the harmonic content of the pulse output by adjusting the duty cycle of the waveform. Rectangular waveforms have varying harmonic content based on the "on" time of the wave with respect to the "off" time. The ratio of "on" time to "off" time is the duty cycle and is usually expressed as a percentage of the "on" time. Pulse Output - This output is a pitch source with a rectangular shaped waveform. The duty cycle of this waveform can be adjust with a control voltage presented to the PWM input. There is also a panel control to adjust the initial duty cycle when no PWM input is present. By changing the duty cycle, the number and strength of harmonics is varied. A 50% duty cycle pulse output sounds much like a clarinet. Saw Output - This output is a pitch source with a waveform that is shaped like the tooth of a saw blade. It is a linear ramp that quickly resets to zero, only to begin a linear ramp once again. The "sawtooth" output is harmonically rich and is often used as the basis for creating many other sounds. By itself, a sawtooth waveform sounds much like a bowed string on a violin. Other Inputs & Outputs - Many additional inputs and outputs may be present on a VCO. Their use will be documented in the user manual that accompanies the VCO. As with many synthesizer modules, there are limitless variations possible. Note: Most pitch outputs of a VCO emit waveforms that measure about 10 volts in amplitude. The most positive peak of the waveform will be about +5V while the most negative peak will be about -5V (for a total amplitude of 10V). The reason for such "hot" signals is that one gets good sonic fidelity. As a waveform passes through perhaps six or eight processing modules, each additional module degrades the signal somewhat. That signal degradation is cumulative. But if you start out with a "hot" signal, you can degrade the signal much more before your ears will hear it. After all, these "hot" signals are reduced in amplitude before recording. When the amplitude is reduced for recording, the degradation is also reduced by the same amount ... it gets very small! Apparatus: 1. 555 timer IC 2. Oscilloscope. 3. Some Resistor(2.2,3.3,4.7,5.6,1.8,10,1,)KΩ 4. Capacitor (.01u, 33n, )F 5. Trainer with bread board. 6. Connecting Wires Practical Circuit: Calculations: Obs-01: Here R1=10KΩ, R2=100KΩ,Vcc=10Volt we know = T=w+0.693R2C F==1/T Obs No Control voltage Frequency Vc(volt) KHz 01 3.00 5.50 02 3.5 4.76 03 04 05 06 07 08 4.05 4.50 5.02 5.51 6.00 6.50 4.16 3.70 3.23 2.78 2.32 1.88 Maximum Frequency Fmax(KHz) Minimum Frequency Fmin(KHz) Bandwidth Bw= (FmaxFmin)KHz 5.5 1.88 3.7 VCO Test Procedures and Results 1. First arrange the circuit on the bread board as like as practical circuit.Testing of the prototype voltage-controlled oscillator was conducted to evaluate several key operating parameters, including tuning range, output drive, and phase noise, and stability of these over temperature. During the tests, the VCO was not connected to the other components of the PLL. This allowed the control voltage to be manually set, and the outputs to be monitored using appropriate test equipment. 2. The tuning range of the VCO was characterized by incrementally sweeping the control voltage from rail to rail (0 V to 3.5 V) in steps of 0.25 V. with the output frequency at each step being measured using a oscilloscope. In addition, the tuning range tests were conducted over the complete range of operating temperatures, from 25ºC to 200ºC, in increments of 25ºC. The set of curves derived from these tests provided some insight into the gain of the VCO as well as its stability over temperature. Calculations: Result and Discussion: From the above data table it is observed that frequency fully depends on the voltage(control). So it is clear that voltage can control frequency hence the name voltage controlled oscillator. But there is little variations these variations normally occurred due to the following reasonsi) Tolerance of the resistor ii) Dielectric loss of the capacitor. iii) Parallax error iv) Ageing problem of the various components v) Miscellaneous. Precautions: a. We have to cheek all the connection as to practical circuit otherwise components may be destroyed. b. Supply voltage is fixed at not more than 15V.Connections are made rigidly. c. Connections are made rigidly. d. Calibrate the oscillator more accurately. e. Readings are taken attentively. Book Ref: i). OP-amp and Linear Integrated circuits byRamakant A Gawkaward ii). www.electronics-tutorial.org iii). www.wikipedia.com iv). www.yahooanswer.com