But what is it called when the first cycle is smaller amplitude than the second cycle is bigger amplitude?

Amplitude Modulation (AM).

One way this could be accomplished is to multiply a sine wave with a DC-offset square wave of 1/2 the frequency of the sine wave. For instance, the square wave would vary between 3 Volts and 5 Volts. It would be most graceful sounding for a guitar effect if the 0-crossings of the square and sine match so that each cycle of the sine wave is smooth and uninterrupted when it is not near 0.

You could accomplish this in analog land by making a sine-wave generator. You feed the sinewave into a VCA (like and OTA or analog multiplier), and also into a high gain amp/ hard limiter to convert the sine to a square. The converted square would go into a Toggle flip-flop to divide the frequency by 2. You would then give it a DC offset and feed it into the VCA gain control.

If you have two function generators, some of them have an external input and sync function so you can do what I'm describing.

In DSP, this is real easy. You can get a sine function (usually source code is supplied for these types of functions by the DSP manufacturer) with inputs where you can pass frequency, phase, amplitude, and duration. You write a little segement of code to generate the sine wave for the duration of a cycle at one amplitude, then the next cycle at a different amplitude.

You could also generate two sine waves, one at

*f*_{1} and another at 2*

*f*_{1}, with the same phase. Multiply the two in the MAC register and feed to your modulated FX program, or to the D/A converter (depending on how you want to use the LFO).