VCO produces output frequencies

Posted on Jun 3, 2012

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 tra

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