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VCO produces positive and negative output frequencies

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The circuit in Figure 1 is a quadrature-output VCO that provides both positive and negative output frequencies, depending on the polarity of the control-voltage input. The circuit provides a function that designers traditionally implement in analog music-effects units, such as Bode/Moog frequency shifters. Bode/Moog shifters use fixed-beat-frequency oscillators at 20 kHz and variable sine oscillators that go higher and lower than 20 kHz. Both oscillators feed into mixers. The circuit in this design reduces the number of oscillators to one and uses no mixers (Figure 1). As a result, the output has fewer spurious harmonics and unharmonically related frequency products, resulting in a cleaner output overall. The two lower transconductance amplifiers, IC4A and IC4B , form a standard, double-integrator bandpass/lowpass filter. The lower amplifiers are active for positive control voltages, and the upper amplifiers, IC3A and IC3B , are effectively turned off for lack of bias current. The upper amplifiers thus play no part in the circuit when the control voltage is positive. The TL072 op amps, IC2A and IC2B , are merely buffers to enable an easier choice of resistor values for IC4B 's input and to avoid excessive loading of the integrator capacitors. The resistor values set the Q (quality factor) to exceed unity. Positive feedback from the bandpass output to the noninverting input makes the filter oscillate. Adjusting the R3 trimmer allows you to adjust the output amplitude and lets you set the drive to the diodes at a level that ensures oscillation but minimizes distortion. This fairly standard configuration yields quadrature outputs from the two buffers with the highest distortion product approximately 40 dB down from the fundamental.

For negative control voltages, the upper transconductance amplifiers, IC3A and IC3B , receive bias current, and the lower amplifiers shut off. The upper amplifiers work in exactly the same way as the lower ones, but they cross-connect to the inputs and integrator capacitors. In this way, the in-phase and quadrature outputs are reversed for negative control voltages, thus creating a smooth transition to what you can consider a “negative frequency.” In operation, when viewing the outputs on an X-Y trace, the circular rotation of the dot becomes slower as you adjust the control voltage near zero and then perfectly reverses direction as the control voltage goes negative. This transition occurs without any unwanted crossing of the circle or drifting off beyond the circle.

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The two current sources, IC1A and IC1B , are fairly self-explanatory; the upper source operates for negative control voltages and vice versa. The R1 gain trimmer on the upper source allows you to adjust the oscillator such that a given negative control voltage yields the same frequency as does the equal-magnitude positive control voltage. This trim compensates for differences in transconductance of the two separate dual-amplifier packages. The diodes across the current-source op amps avoid heavy saturation when the respective source turns off. You trim the R2 potentiometer to obtain equal amplitudes from the upper and the lower sections. The circuit in Figure 1 uses standard, inexpensive, multiple-sourced components. It requires only minimal (and easy) trimming, with no interacting trims. A 0V control input results in a guaranteed 0-Hz output, dependent only on well-controlled op-amp offsets. With the Bode/Moog system, you must make a front-panel adjustment to zero-beat two oscillators running at 20 kHz.

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