Low-frequency, voltage-controlled oscillator has adjustable duty cycle

Low-frequency voltage-controlled oscillators (VCOs) can be found in clocks, timers, audio synthesizers, and a variety of other applications. The VCO shown in Figure 1 is based on a simple astable multivibrator, for which the key op-amp characteristics are bandwidth and slew rate.

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Figure 1: This low-cost, voltage-controlled oscillator (VCO) has a wide tuning range and requires little pc-board area.

To analyze the circuit, first disconnect Node 1 from both diodes and connect the capacitor to ground. When VOUT is positive the positive terminal voltage is αVOUT+ , where &#945 = R1 /(R1 +R2 ). When voltage on the negative terminal charges to &#945VOUT+ , the output changes to VOUT- , changing the positive-terminal voltage to &#945VOUT- .

Similarly, when voltage on the negative terminal charges to &#945VOUT- , the output changes to VOUT+ , so the process repeats and the output oscillates. The output for this simplified circuit is a square wave whose period equals:

The complete circuit (obtained by restoring the connection at Node 1) converts the multivibrator to a VCO. Changing the voltage on VTUNE varies the reference point of the charging capacitor, thereby changing the frequency of oscillation. The Schottky diodes and corresponding op amps keep the duty cycle at 50% by maintaining at Node 1 a positive or negative voltage level (depending on the polarity of VOUT ), equal to VTUNE minus one diode drop. Increasing VTUNE increases the frequency of oscillation:

VDIODE depends on current through the diode. For the circuit shown, we set VDIODE = 0.1V. The range of tuning voltage depends on the diodes used and the switching-voltage threshold, which is set by &#945. Figure 2a and 2b show typical results.

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Figure 2a and 2b: Output waveforms (a) and nonlinear frequency-vs.-VTUNE characteristic (b) for the Figure 1 circuit depend on the component values shown in that schematic. The low-cost quad op amp operates on ±3V while minimizing board space and power consumption.

To adjust the VCO’s duty cycle, you can provide asymmetric adjustment of the positive and negative thresholds by replacing the ground connection on R1 with a DC supply.

About the Author
Ken Yang is an application engineer at Maxim Integrated Products ,

1 comment on “Low-frequency, voltage-controlled oscillator has adjustable duty cycle

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    April 17, 2014

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