Function Generator ICL 8038

Function Generator ICL 8038

Function generators are designed to provide the basic waveforms such as square wave, triangular wave and sine wave. They are also called as waveform generators. The monolithic function generators provide these basic waveforms with a minimum number of external components reducing complexity, but increasing the reliability of the circuit. They find application in communication, telemetry, electronic music and testing and calibration in the laboratory.

In function generators, VCO (voltage controlled oscillator) generates the triangular and square waves. The triangular wave is passed through the on chip wave shaper to generate a sine wave. The sawtooth and pulse waveforms are generated by configuring the oscillator for a highly asymmetric duty cycle. In this section we will discuss the ICL 8038 precision function generator from Intersil.

1. Basic Principle

It is very easy to understand working principle of ICL 8038 function generator using its simplified block diagram, as shown in the Fig. 5.16.1.

The operation of ICL 8038 is based on charging and discharging of a grounded capacitor C, whose charging discharging rates are controlled by programmable current generators I_A and I_B, respectively. When switch is at position A, the capacitor charges at a rate determined by current source I_A . Once the capacitor voltage reaches VOT/ the upper comparator (CMP1) triggers and resets the flip-flop output. This causes the switch position to change from position A to B. Now, capacitor starts discharging at the rate determined by the current sink I_B.

Once the capacitor reaches V_LT, the lower comparator (CMP2) triggers and sets the flip-flop output. This causes the switch position to change from position B to A. And this cycle repeats. As a result, we get square wave at the output of flip-flop and triangular wave across capacitor. The triangular wave is then passed through the on chip wave shaper to generate sine wave.

To allow automatic frequency control, currents IA and IB are made programmable through an external control voltage V_i. For equal magnitudes of I_A and I_B, output waveforms are symmetrical conversely, when two currents are unequal, output waveforms are asymmetrical. By making one of the currents much larger than other we can get sawtooth waveform across capacitor and rectangular wave at the output of flip-flop.

2. Circuit Diagram

The Fig. 5.16.2 shows the most simplified form of circuit diagram for ICL 8038.

As shown in the Fig. 5.16.2, transistors Q₁ and Q₂ form programmable current sources whose magnitudes are set by external resistors R_A and R_B. These current sources are driven by the emitter follower (transistor Q₃). It also compensates base-emitter voltage drops for Q₁ and Q₂ to ensure V_RA - V_RB - V_i . Thus, I_A = V_i/R_A and I_B = V_i/R_B. The current I_A controls the charging rate of capacitor C. The current I_B is diverted to current mirror (Q₄-Q₅-Q₆)/ where it undergoes polarity reversal as well as amplification by 2 due to the combined action of Q₅ and Q₆. The result is a current sink of magnitude of 2 I_B, as shown in the Fig. 5.16.2.

The voltage across capacitor is applied to the schmitt trigger. The schmitt trigger shown in Fig. 5.16.2 is similar to that of the IC555, with V_UT = 2/3 V_CC and V_LT = 1/3V_CC. Transistor Q₇ acts as a switch. When output of flip-flop is high, Q₇ saturates and pulls the bases of Q₅ and Q₆ low, thus shutting off the current sink. As a result capacitor C starts charging at a rate set by current I_A. Once the capacitor voltage reaches 2/3 V_CC(V_UT), CMP1 triggers and clears the flip-flop, thus turning Q₇ off. This enables current mirror to sink current equal to 2 I_B So that net current flowing out of the capacitor is I_L = 2 I_B - I_A  This causes capacitor to discharge. Once capacitor voltage reaches 1/3 V_CC(V_LT)/ CMP2 triggers and sets the flip-flop and action repeats. It is important to note that net current flowing out of capacitor C should be positive i.e. 2 I_B - I_A > 0 discharging capacitor and hence 2 I_B > I_A.

3. Frequency of Output Waveform

The frequency of the output waveform can be determined as follows :

f_out= 1/ T where T = T_c + T_d

The charging time T_c can be given as

Multiplying R_A by numerator and denominator we get,

With R_A = R_B, duty cycle is 50 % and we get symmetric waveforms. For symmetric waveforms i.e. R_A = R_B, the frequency of the waveform is given as,

4. Pin Configuration of ICL 8038

The Fig. 5.16.3 shows the pin configuration for ICL 8038 function generator, which is available in 14 pin DIP.

Pin 1 and Pin 12 : Sine wave adjust

The external resistor connections to these pins decides the accuracy of the sine wave. For distortion less than 1 % we have to connect 100 kΩ potentiometer between pin 12 and ground or –V_EE. To achieve distortion less than 0.5 % we have to connect two 100 kΩ potentiometers between V_CC and ground with wiper of the one potentiometer connected to pin 1 and other to pin 12, as shown in the Fig. 5.16.4. 

Pin 2 : Sine wave out

The sine wave output is available at this pin. The amplitude of sine wave is 0.22 V_CC, where ± 5V ≤ V_CC ± 15 V.

Pin 3 : Traingular wave output

The triangular wave output is available at this pin. The amplitude of triangular wave is also function of input voltage V_CC. It is 0.33 V_CC

Pin 4 and 5 : Duty cycle/Frequency adjust

We know that frequency of output is proportional to the charging and discharging currents and duty cycle can be adjusted by selecting proper values of R_A and R_B. The external resistors R_A and R_B are connected to pin 4 and pin 5, respectively, as shown in the Fig. 5.16.5.

The values of R_A, R_B and external capacitor connected at pin 10 decides the frequency of the output waveform. The recommended range for values of R_A and R_B is from 1 kΩ to 1 MΩ.

Pin 6 : +V_CC

It is a positive supply. Its voltage should be kept between 10 V to 30 V, for a single supply operation, and ± 5 V to ± 15 V for dual supply operation.

Pin 7 : FM bias

Refer Fig. 5.16.2 (circuit diagram). Pin 7 is a junction of two resistors (R1 = 10 kΩ and R₂ = 40 kΩ ) that form a potential divider with the supply voltage V_CC- Looking at the same figure we can see that voltage Vj is a voltage between VCC and pin 8. The output frequency is proportional to this voltage. By connecting pin 7 to pin 8 the voltage across R₁ appears as a V_i. This increases the output frequency for same values of R_A, R_B and C when pm 8 is connected to V_CC (i.e.Vj = 0).

Pin 8 : FM sweep input

As we know, the voltage between V_CC and pin 8 decides the output frequency. The output frequency can be controlled by applying external voltage usually referred to as sweep voltage to pin 8. For proper operation sweep voltage is kept between (2/3 V_CC + 2) and V_CC

Pin 9 : Square wave output

The square wave output is available at this pin. As this output is open collector, external resistor is required to be connected between V_CC and pin 9 to get the square wave at pin 9.

Pin 10 : Timing capacitor

The external timing capacitor C is connected at this pin.

Pin 11 : V_EE/Ground

If dual supply is used V_EE is connected to this pin. If a single is used, this pin is connected to the ground.

Pin 12 and 14 : Not connected

5. Typical Connection

The Fig. 5.16.6 shows typical connection for ICL 8038 with fixed frequency and 50 % duty cycle.

Key Point The amplitudes of square, triangular and sine waves are V_CC, 0.33 V_CC and respectively. 0.22 V_CC

Review Questions

1. Describe the features and one application of function generator IC.

May-05, Dec.-05, Marks 16

2. Draw the block diagram of the function generator ICL 8038 or any other equivalent and explain its operation.

May-04,11,15, Marks 16, Dec.-08,09,ll, Marks 16

3. Write short note on ICL 8038 function generator.

Dec.-16, 17, May-16, 17, Marks 8