Analogue Pitch Shifter

Posted on Nov 21, 2012

This is two three pole bandpass filters with the HPF Fo about 50Hz and the LPF Fo about 15kHz. This limits the input bandwidth to the phase sequence array, which can produce 90° phase shifts only over a limited bandwidth. The array is driven with a true and inverted signal, and the array outputs are buffered to feed the following circuitry. The phase sequence array itself has a gradual -4dB dip mid-band and the input filters partly compensate for this. The exact values of the capacitors are not critical but each value set should be matched to 1%.The S_0 and S_90 output buffers provide convenient input points for the carrier feedthrough trims. +/-5V 100mA regulators provide local voltage references.

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The two lower transconductance amplifiers form a standard double integrator bandpass-lowpass filter as shown in the Nat Semi data sheet for the LM13700. These lower TAs are active for positive control voltages, and the upper TAs are effectively turned off by being fed no bias current; These play no part when the CV is positive. The TL074 op-amps are merely buffers to enable easier choice of resistor values on the TA inputs and to make sure that the integrator capacitors are not loaded excessively. The resistor values set Q to be >1. Feedback from the bandpass output to the non-inverting input via the diode limiter provides feedback to make the filter oscillate. Adjusting the trimmer gives output amplitude trimming and allows the drive to the diodes to be set to a level that ensures oscillation but minimises distortion. This fairly standard configuration gives quadrature outputs from the two buffers with the highest distortion product about 40dB down on the fundamental. For negative control voltages the upper TAs are fed bias current and the lower TAs are shut off. The upper TAs work in exactly the same way as the lower ones but they are cross connected to the inputs and integrator caps. In this way the in-phase and quadrature outputs are reversed for negative CV, thus creating an inherently smooth transition to what can be considered to be a negative frequency. In operation, viewing the outputs on an X-Y trace the...

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