Hello -

I'm using Qucs to model up some guitar effects and I've created a crude but serviceable model of a guitar pickup to use as input. I've got a voltage source producing a 500mV sinewave at 440Hz in series with an inductor and a resistor and that trio is in parallel with a small cap (see image).

What I don't understand is why the signal I'm pulling off has a harmonic added to the voltage source's signal (I can change its frequency by changing the value of the inductor). If the source is really a sinewave, I can understand the L/R/C network causing a certain attenuation, but I wouldn't expect such a circuit to induce a resonance like this except at the transition to a discontinuous signal like a pulse or sawtooth wave. Have I done something wrong here?

Thanks!

 

Well, 3 H seems kinda huge for the coil in a guitar pickup. Other than that, your harmonic is at exactly the resonant frequency of the LC. My guess is that it was kicked off by the startup transient of the 440 Hz sinewave. If the wave goes from zero to full amplitude instantly, this transient has lots of harmonics in it.

The harmonic shows up in LT Spice, but is gone by the start of the 4th cycle.

ak

 

No, 1-10H is the typical range; see http://www.ozvalveamps.org/pickups.htm.

I realized that even a sinewave starting at zero is still a sharp corner but I wouldn't expect it to keep on going with no die-off. I tried it again with a 1M resistor shunting output to ground (typical input impedance of a tube amp, per the linked page) and that just about entirely removed the ring except for a trace of it in the first cycle.

 

Sounds about right. The longer ring might be an artifact of the simulation software.

ak