Voltage-controlled-oscillator-Block-Diagram

In most cases, the frequency of an oscillator is determined by the time constant RC. However, in cases or applications such as FM, tone generators, and frequency-shift keying (FSK), the frequency is to be controlled by means of an input voltage, called the control voltage. This can be achieved in a voltage-controlled oscillator (VCO). A VCO is a circuit that provides an oscillating output signal (typically of square-wave or triangular waveform) whose frequency can be adjusted over a range by a dc voltage. An example of a VCO is the 566 IC unit, that provides simultaneously the square-wave and triangular-wave outputs as a function of input voltage. The frequency of oscillation is set by an external resistor R₁ and a capacitor C₁ and the voltage V_c applied to the control terminals. Figure shows that the 566 IC unit contains current sources to charge and discharge an external capacitor C_v at a rate set by an external resistor R₁ and the modulating dc input voltage. A Schmitt trigger circuit is employed to switch the current sources between charging and discharging the capacitor, and the triangular voltage produced across the capacitor and square-wave from the Schmitt trigger are provided as outputs through buffer amplifiers. Both the output waveforms are buffered so that the output impedance of each is 50 f2. The typical magnitude of the triangular wave and the square wave are 2.4 V_peak._to-peak and 5.4V_peak._to._peak.

The frequency of the output waveforms is approximated by

f_out = 2(V⁺ - V_c)/R₁C₁V⁺

Voltage-controlled-oscillator-Circuit-Diagram

Figure shows the pin connection of the 566 unit. The VCO can be programmed over a 10-to-l frequency range by proper selection of an external resistor and capacitor, and then modulated over a 10-to-l frequency range by a control voltage, V_{c The voltage controlled oscillators (VCOs) are commonly used in converting low-frequency signals such as EEG (electro-encephalograms) or ECG (electro-cardiograms) into an audio­frequency (AF range).}

A function generator is a signal source that has the capability of producing different types of waveforms as its output signal. The most common output waveforms are sine-waves, triangular waves, square waves, and sawtooth waves. The frequencies of such waveforms may be adjusted from a fraction of a hertz to several hundred kHz.

Actually the function generators are very versatile instruments as they are capable of producing a wide  variety of waveforms and frequencies. In fact, each of the waveform they generate are particularly suitable for a different group of applications. The uses of sinusoidal outputs and square-wave outputs have already been described in the earlier Arts. The triangular-wave and sawtooth wave outputs of function generators are commonly used for those applications which need a signal that increases (or reduces) at a specific linear rate. They are also used in driving sweep oscillators in oscilloscopes and the X-axis of X-Y recorders.

Many function generators are also capable of generating two different waveforms simultaneously (from different output terminals, of course). This can be a useful feature when two generated signals are required for particular application. For instance, by provid­ing a square wave for linearity measurements in an audio-system, a simultaneous sawtooth output may be used to drive the horizontal deflection amplifier of an oscilloscope, providing a visual display of the measurement result. For another example, a triangular-wave and a sine-wave of equal frequencies can be produced simultaneously. If the zero crossings of both the waves are made to occur at the same time, a linearly varying waveform is available which can be started at the point of zero phase of a sine-wave.

Another important feature of some function generators is their capability of phase-locking to an external signal source. One function generator may be used to phase lock a second function generator, and the two output signals can be displaced in phase by an adjustable amount. In addition, one function generator may be phase locked to a harmonic of the sine-wave of another function generator. By adjustment of the phase and the amplitude of the

harmonics, almost any waveform may be produced by the summation of the fundamental frequency generated by one function generator and the harmonic generated by the other function generator. The function generator can also be phase locked to an accurate fre­quency standard, and all its output waveforms will have the same frequency, stability, and accuracy as the standard.

Function Generator Block Diagram

The block diagram of a function generator is given in figure. In this instrument the frequency is controlled by varying the magnitude of current that drives the integrator. This instrument provides different types of waveforms (such as sinusoidal, triangular and square waves) as its output signal with a frequency range of 0.01 Hz to 100 kHz.

The frequency controlled voltage regulates two current supply sources. Current supply source 1 supplies constant current to the integrator whose output voltage rises linearly with time. An increase or decrease in the current increases or reduces the slope of the output voltage and thus controls the frequency.

The voltage comparator multivibrator changes state at a predetermined maximum level, of the integrator output voltage. This change cuts-off the current supply from supply source 1 and switches to the supply source 2. The current supply source 2 supplies a reverse current to the integrator so that its output drops linearly with time. When the output attains a pre­determined level, the voltage comparator again changes state and switches on to the current supply source. The output of the integrator is a triangular wave whose frequency depends on the current supplied by the constant current supply sources. The comparator output provides a square wave of the same frequency as output. The resistance diode network changes the slope of the triangular wave as its amplitude changes and produces a sinusoidal wave with less than 1% distortion.

PUT CONTROLLED SAWTOOTH WAVE GENERATOR

A PUT controlled sawtooth generator circuit is shown in figure. When power is first applied, the programmable unijunction transistor (PUT) is off. The capacitor C begins to charge up and the output voltage rises. This continues until the output voltage (which is also the PUT anode voltage) is about 0.7 V above the control input (the gate voltage). The PUT gets switched on. The capacitor C is shorted out through PUT and, therefore, capacitor gets immediately dis­charged through the PUT. The output voltage, which is equal to the voltage across the capacitor, falls. When the current through the PUT falls below its holding current I_H, it goes off and the cycle repeats. When the PUT turns off, approximately 1 V is usually left on the capacitor. The output waveform is shown in figure.

PUT CONTROLLED SAWTOOTH WAVE FORM

The time period of the PUT controlled sawtooth generator depends on the charge rate (V/RC) and the control voltage V_control. This is obvious from figure.

Time period, T = Distance / Rate = (V_control + 0.7 V) – 1V / (І-V І/RC)

= V_control RC / І-V І………………. {І-V І = magnitude of –V}

and frequency, f = 1/T = І-V І/V_control RC

The PUT controlled sawtooth generator can be used as a voltage-to-frequency converter.

Precautions

• The cathode of the PUT must be tied to ground or virtual ground and current flows only from anode to cathode. So PUT cannot be used to control a negative ramp generator.
• To turn-off the current through the PUT must drop below its holding current I_H (specified by the manufacturer). When the PUT is on, a current equal to that used to charge the capacitor, in addition to capacitor discharge current, flows through the PUT. This current flows through R to V^- must be below I_H.

That is, I = V/R < I_H

Failing which, once the PUT goes on, it will be held on by this charge current, even when the capacitor has fully discharged. This charge current can be lowered by increasing R or reducing negative voltage V^-.. However, both of these factors affect the change rate and. Therefore, the frequency. For keeping frequency unaffected the changes in either R or V^- will have to be balanced with appropriate changes in C.

How to make a Sawtooth Wave Generator using Op-Amp 741 IC ?

Ramp Generator

Sometimes it is felt necessary to provide a relatively slow linear ramp with a rapid fall (or rise in the case of a negative ramp) at its end. This is a sawtooth wave. Also, in applications such as time base generators and power control circuits, the sawtooth must be triggered by (or be synchronized with) some control signal.

sawtooth waveform

The difference between the triangular and sawtooth waveforms is that in triangular waves the rise time is always equal to its fall time while the sawtooth waveforms have different rise and fall times i.e. sawtooth wave may rise positively many times faster than it falls negatively or vice-versa.

The circuit shown in figure provides the ability of controlling ramp generation with an external signal. In the circuit shown, an NPN BJT has been placed around the charging capacitor C and emitter of the transistor is tied to the inverting (-) terminal of the op-amp, which is at virtual ground. Resistor R_B is for limiting the base current and so for protecting the BJT. However, R_B is to be kept rela­tively small to assure that the transistor can be driven into saturation.

With a zero or negative control input voltage, the transistor is off. The capacitor charges up from the op-amp output, through C, R_in and to V-. The charge rate is given as

Rate = V- / R_in *C

If the control voltage is not changed, the capacitor C will eventually charge up, and hold the output at + V_sat.

However, when a positive control input is applied, the transistor gets turned on. If this voltage is large enough to force transistor into satu­ration the capacitor is effectively short- circuited. The capacitor C rapidly dis­charges.

The output voltage falls to zero (actually about 0.2 V) and stays there as long as positive control voltage keeps the transistor saturated. The expected obtainable waveform is given in figure. For control of negative going ramps, the circuit shown in figure will require several minor changes. First, the charging voltage, connected to R_in, polarity will have to be reversed to V+. This reverses the direction of charging current. It means capacitor will also have to reversed, if it is electrolytic one. The emitter of the transistor must be connected to virtual ground (the inverting input terminal of op-amp). To allow the capacitor to discharge from left to right, NPN transistor would have to be replaced by PNP transistor. In this case, a zero or positive control input would keep the PNP transistor off, while a negative control input would be required to turn the transistor on.

How to make a Triangular waveform using Schmitt Trigger and Integrator ?

triangular waveform

Another triangular-waveform generator that needs fewer components is shown in figure. The arrangement consists of a non-inverting Schmitt trigger A_x and an integrator A₂. The output of a Schmitt trigger is a rectangular wave that drives an integrator. The output of the integrator is a triangular wave, which is fed back and used to drive the Schmitt trigger. Thus first stage drives the second, and the second drives the first. But the question arises on how the circuit gets started in the first place. This is explained below.

When the Schmitt trigger is connected to power supplies for the first time, the output of the Schmitt trigger must be either low or high. When the Schmitt trigger output is low, the output of the integrator will be a rising ramp while for Schmitt trigger high output, the integrator will produce falling ramp. Either way, the triangular waveform has started, and the positive feedback will keep it going.

The transfer characteristic of the Schmitt trigger is shown in figure. When the output is low, the input must increase to the UTP to switch the output to high. Likewise, when the output is high, the input must fall to the LTP to switch the output to low. The triangular-wave produced by the integrator is capable of driving the Schmitt trigger. When the output of Schmitt trigger is low, the integrator develops a rising ramp which increases till it reaches UTP, as illustrated in figure. At this point the output of the Schmitt Trigger switches to the high state and forces the triangular-wave to reverse in direction. The negative or falling ramp produced by the integrator now falls till it reaches LTP, where another Schmitt output change occurs.