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High-Quality Function Generator System with the XR-2206
INTRODUCTION Waveform or function generators capable of producing AM/FM modulated sine wave outputs find a wide range of applications in electrical measurement and laboratory instrumentation. This application note describes the design, construction and performance of such a completefunction generator system suitable for laboratory usage or hobbyist applications. The entire function generator is comprised of a single XR-2206 monolithic IC and a limited number of passive circuit components. It provides the engineer, student, or hobbyist with a highly versatile laboratory instrument for waveform generation, at a very small fraction of the cost of conventional function generatorsavailable today. However, they represent the typical performance characteristics measured by EXAR’s application engineers during the laboratory evaluation of the function generator system shown in Figure 1. The typical performance specifications listed below apply only when all of the recommended assembly instructions and adjustment procedures are followed: (a) Frequency Ranges: The functiongenerator system is designed to operate over four overlapping frequency ranges: 1Hz 10Hz 100Hz 1KHz to to to to 100Hz 1KHz 10KHz 100KHz
GENERAL DESCRIPTION The basic circuit configuration and the external components necessary for the high-quality function generator system is shown in Figure 1. The circuit shown is designed to operate with either a 12V single power supply, or with 16V splitsupplies. For most applications, split-supply operation is preferred since it results in an output dc level which is nearly at ground potential. The circuit configuration of Figure 1 provides three basic waveforms: sine, triangle and square wave. There are four overlapping frequency ranges which give an overall frequency range of 1Hz to 100KHz. In each range, the frequency may be varied over a 100:1tuning range. The sine or triangle output can be varied from 0 to over 6V (peak to peak) from a 600W source at the output terminal. A squarewave output is available at the sync output terminal for oscilloscope synchronizing or driving logic circuits. TYPICAL PERFORMANCE CHARACTERISTICS The performance characteristics listed are not guaranteed or warranted by EXAR Corporation.
The range selection ismade by switching in different timing capacitors. (b) Frequency Setting: At any range setting, frequency can be varied over a 100:1 tuning range with a potentiometer (see R13 of Figure 1.) (c) Frequency Accuracy: Frequency accuracy of the XR-2206 is set by the timing resistor R and the timing capacitor C, and is given as: f = 1/RC The above expression is accurate to within 15% at any rangesetting. The timing resistor R is the series combination of resistors R4 and R13 of Figure 1. The timing capacitor C is any one of the capacitors C3 through C6, shown in the figure. (d) Sine and Triangle Output: The sine and triangle output amplitudes are variable from 0V to 6Vpp. The amplitude is set by an external potentiometer, R12 of Figure 1. At any given amplitude setting, the triangle outputamplitude is approximately twice as high as the sinewave output. The internal impedance of the output is 600W.
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(e) Sinewave Distortion: The total harmonic distortion of sinewave is less than 1% from 10Hz to 10KHz and less than 3% over the entire frequency range. The selectionof a waveform is made by the triangle/sine selector switch, S2. (f) Sync Output: The sync output provides a 50% duty cycle pulse output with either full swing or upper half swing of the supply voltage depending on the choice of sync output terminals on the printed circuit board (see Figure 1.)
R1 30K Q AM INPUT D S1 Range Select Switch E F G AMPLITUDE 50K R12 H C3 C4...