Voltage Controlled Oscillator Circuit (IC 566)

Voltage Controlled Oscillator Circuit (IC 566)

May-05,07,08,10,11,14,15,16, Dec.-03,06,09,10,ll,15,16,17

A voltage controlled oscillator is an oscillator circuit in which the frequency of oscillations can be controlled by an externally applied voltage. The VCO provides the linear relationship between the applied voltage and the oscillation frequency. Applied voltage is called control voltage. The control of frequency with the help of control voltage is also called voltage to frequency conversion. Hence VCO is also called voltage to frequency converter. Practically VCO is used to produce square and triangular waveforms whose frequency is controlled by control voltage.

The Fig. 4.2.1 shows op-amp based voltage controlled oscillator circuit.

The circuit associated with op-amp A₁ is V-I converter i.e. it forces current through capacitor C which is linearity proportional to the input voltage V_i. For getting triangular and square waveforms, capacitor must charge and discharge and thus current direction through capacitor must keep on alternating. The direction of current through capacitor is controlled by the MOSFET switch Mi. It is driven by Schmitt trigger comparator CMP. When the output of CMP is low, M₁ is off and resistance R remains floating while when the output of CMP is high, M1 is on and resistance R gets connected to the ground. The squarewave is obtained at terminal V_SQ and triangular wave is obtained at V_TR. When output transistor Q is on, V_SQ is nothing but V_CE(sat) i-e- almost zero. While when Q is off then V_SQ is decided by voltage divider of R₁ and R₂.

V_SQ = V_CC R₁ / R₁ + R₂  .... when Q is off

As the output V_SQ is connected to the non inverting terminal of comparator CMP, the high and low values of V_SQ are the upper and lower threshold levels of comparator CMP.

Due to voltage divider of R3 and R3, the voltage of node A is,

V_A = V_i R₃ / R₃  R₃ = V_i / 2  …. (4-2.1)

Due to the virtual ground, V_B = V_A

V_B = V_i / 2   .... (4.2.2)

Thus the current I through 2R resistor is,

The equation (4.2.3) shows that op-amp A i acts as voltage to current converter.

Assume that the output V_SQ is low hence M₁ is off and the resistance R is floating.

So entire current I = 4R(V_i) has to pass through the capacitor C and the voltage VTR starts decreasing, due to polarity of voltage across capacitor. So V_TR ramps down.

When V_TR becomes zero then comparator CMP changes its state and output VSq becomes high. Due to this M₁ becomes on and the resistance R gets connected to the ground. The current through resistance R is thus.

I_R = V_B / R = V_i / 2R = 2 V_i / 4R = 2I …. (4.2.4)

Now current I out of 2I is supplied by resistance 2R but remaining current I must be supplied by capacitor, as shown in the Fig. 4.2.2. Thus the direction of current through the capacitor reverses. Hence V_TR starts ramping up i.e. starts increasing. It can be noted that capacitor current I gets maintained at same magnitude only its direction gets reversed hence a triangular waveform is available at V_TR. As soon as V_TR reaches the level of V_SQ, the schmitt trigger comparator changes its state making M₁ off. Thus the current I through capacitor again reverses its direction and V_TR ramps down. Thus the cycle continues.

Period of Oscillation

The period of oscillation is determined by equating the charge expressions,

CΔV = IΔt

During any half cycle,

The equation (4.2.5) shows that the output frequency is linearly proportional to the input voltage V_i and thus circuit works as voltage controlled oscillator.

But the accuracy of such VCO circuit is limited because of speed of response of op-amp A i comparator CMP and MOSFET switch M₁. Similarly error is caused at low frequencies due to input bias current and the offset voltage of op-amp A x. The non-zero channel resistance of M₁ when it is on i.e. r_ds(on) is another factor causing an error. Hence in practice integrated version of VCO is used.

 

Example 4.2.1 In the VCO circuit shown in the Fig. 4.2.1, the various parameters are R = 10 kΩ, C = 1.25 nF, V_UT = 10 V and V_LT = 0 V. If input is changed from 10 mV to 10 V calculate the range over which the output frequency can be varied.

Solution : For the VCO circuit,

Hence the range over which output frequency can be varied is 10 Hz to 10 kHz.

 

1. Voltage Controlled Oscillator (VCO) 1C 566

A Voltage Controlled Oscillator (VCO) is an oscillator circuit in which the frequency of oscillations can be controlled by an externally applied voltage. The VCO provides the linear relationship between the applied voltage and the oscillation frequency. Applied voltage is called control voltage. The control of frequency with the help of control voltage is also called voltage to frequency conversion. Hence VCO is also called voltage to frequency converter. Practically VCO is used to produce square and triangular waveforms whose frequency is controlled by control voltage.

Practically VCO is available in IC form. The commonly used VCO ICs are NE/SE 566, LM 566 etc. The Fig. 4.2.3 shows the pin diagram of NE/SE 566 VCO manufactured by Signetics. It is 8 pin IC, which provides two output pins. Its feature is that simultaneously IC provides square and triangular wave outputs which are the functions of input voltage. The input voltage is also called modulating input voltage.

Operation : The op-amp A₁ is used as a buffer. The op-amp A₂ is used as a Schmitt trigger and the op-amp A₃ is used as an inverter. The voltage V_C is applied to the modulation input pin, which is a control voltage.

The capacitor C₁ is linearly charged or discharged by a constant current source. The charging current can be controlled by controlling the voltage VC at pin 5 or by varying the resistance R₁which is external to the IC. The charging and discharging levels are determined by the Schmitt trigger.

The output voltage of Schmitt trigger is designed to swing between +V and 0.5 V. For R_a = R_b, the voltage at noninverting terminal swings between 0.5(+V) to 0.25(+V). Thus the triangular wave is generated due to alternate charging and discharging of the capacitor C₁, in linear manner. When C₁ voltage increases  beyond 0.5(+V), the Schmitt trigger output goes low, and the capacitor starts discharging. When the voltage becomes less than 0.25 (+V), the output of the Schmitt trigger goes high. Due to similar current sources used for charging and discharging, the time taken by Ci to charge and discharge is same. This produces exact triangular wave. The output of the Schmitt trigger output is step response which is available at the pin 3 as a square wave output. The various waveforms are shown in the Fig. 4.2.5.

The frequency of the output waveform is,

fo = 2(+V – V_C) / C₁R₁(+V)  …. (4.2.7)

R₁ should be in the range of 2 kΩ to 20 kΩ. The equation shows that the frequency of the output oscillations is a frequency of the control voltage V_C. The frequency is also a function of the external resistance R₁ and capacitor C₁.

 

2. Typical Connection Diagram of 566 VCO

Practically the modulating input voltage at the pin 5 is controlled using a potential divider formed by the resistances R₂ and R₃. Such a circuit is shown in the Fig. 4.2.6.

 

The R₂ - R₃ potential divider is used to set the control voltage VC. The initial voltage VC at the pins must be such that,

0.75(+V) ≤ V_C <+V

where +V is the supply voltage.

The modulating input is connected through capacitor coupling to the pin 5 and it must be less than 3V(p-p). For a fixed V_C and Ci the frequency can be varied in the range 10 : 1 by varying R1 between 2 kΩ to 20 kQΩ. For fixed R₁ and C1 the frequency can be varied in the range 10 : 1 by varying control voltage Vc. In both the cases maximum possible output frequency is 1 MHz. The capacitor C₂ connected between pins 5 and 6 is used to elliminate any oscillations present in constant current source.

 

3. Features of 566 VCO

1. Wide supply voltage range 10 V to 24 V.

2. Very linear modulation characteristics.

3. High temperature stability.

4. Excellent power supply rejection.

5. 10 to 1 frequency range with fixed C₁.

6. The frequency can be controlled by means of current, voltage, resistor, or capacitor

 

4. Derivation of Voltage to Frequency Conversion Factor

The voltage to frequency conversion factor is an important factor of IC of 566. It is denoted as K_v, and defined as,

K_v = Δ f_o / ΔV_C  ... (4.2.8)

Here Δ V_C is the change in control voltage producing corresponding change of Δ f_o in the frequency.

f_o = New frequency

f_o = Original frequency   ... (4.2.9)

Δ f_o = f_o - f_o

While V_C is changed by Δ V_C to achieve this,

From the expression of f_o,

This is the required voltage to frequency conversion factor.

Thus output frequency can be expressed as,

f_o = K_v (+V) / 8

 

5. Applications of VCO

The various applications of VCO are

1. FM modulation.

2. Signal generation (Triangular or square wave)

3. Function generation.

4. Frequency shift keying i.e. FSK demodulator.

5. In frequency multipliers.

6. Converting low frequency signals such as EEG and EKG into audio frequency range signals.

7. Tone generation.

 

6. Ramp Generator using 566

The Fig. 4.2.7 shows the ramp generator using LM 566. Here, transistor Q₁ is used as short circuit path for capacitor. During charging the output at pin 3 is low (square wave). This makes Q₁ OFF and hence capacitor charging time is decided by the R 1r C1, +V and V C . When capacitor voltage charges upto the upper threshold voltage of the Schmitt trigger, its output goes high, making output at pin 3 high. As a result transistor Q₁ is made ON. It provides short circuit path for capacitor to discharge in no time. When capacitor voltage goes below lower threshold of schmitt trigger, the voltage at pin is made low, making transistor Q₁ OFF. The capacitor Ci starts charging again and cycle repeats.

For this circuit, capacitor charging time is decided by R₁ C₁, +V and V_C and discharge time is nearly zero. Therefore we get ramp signal at the triangular wave output of the LM 566.

 

Example 4.2.1 For the VCO circuit, assume R2 -2.2 kQ,, R1 = R3 = 15 kQ, and C1 = 0.001 pF. Assume Vcc = 12 V. Determine the output frequency, the change in output frequency if modulating input Vc is varied from 7 V to 8 V.

Dec.-14, Marks 8

Solution :

Review Questions

1. Explain the working of voltage controlled oscillator.

2. Explain the voltage controlled oscillator IC with a neat block diagram. Give its typical connection diagram and its output waveforms.

3. With the help of schematic circuit explain the operation of 566 voltage controlled oscillator. State the expression for the output frequency. Discuss any two applications.

Dec.-06, 09,16, May.-08, 10, 11, 14, 16, Marks 16

4. Define the voltage to frequency converting factor and derive its expression. List the applications of VCO.

May-03,05,06,07,12, Dec.-03,04,08,10,16