Frequency Doubler Concept Circuit

Research into using the LM3900 directly as a half-wave rectifier, then doing offset addition for waveform signal processing of a triangle waveform into a 2x harmonic. It does work, but has frequency dependence on waveshape fidelity: higher frequencies tend to have a half-period DC imbalance. This appears to be because the rectified half triangle is not "snappy." The circuit design is imperfect but usable across the musical frequency range.

The input signal was calibrated to swing 2V down from +6V, to allow use of the precision 6V temperature stable LM3900 voltage reference circuit, such as in a VCO. The LM3900 output easily swings 2V at any DC voltage within output compliance on a wideband basis, and with extremely little phase shift.

Various benchtest scope fotos follow.

Basic Rectified Triangle Wave.

Offset-Addition to create 2x Triangle.

Amplitude Characteristic.

Overlay of Key Signals.

Not Quite 1Vpp given Precision 2Vpp Input at 1kHz.

Offset alignment of 2x output into middle of input signal.

Frequency Sweep: 20 Hz.

50 Hz.

100 Hz.

1 kHz, some offset readjustment.

1 kHz.

Checking bottom end at 10 Hz.

200 Hz.

500 Hz.

1 kHz.

2 kHz, waveform assymetry more apparent.

5 kHz.

10 kHz. Still kind of works, but disparity would cause shimmering harmonic.

Way off at 20 kHz, but also beyond hearing range too.

8 kHz.

Response at A7 frequency.

Overlay of signals at A7.

Attempting better DC alignment at A7.

Still gnarly DC offset for half-period at 8 kHz.

16 kHz.

A7.

A6.

A5.

A4.

Measurements of 2x output, but this is on an 8-bit scope.

Measuring 2 periods of output to compare with fundamental input.