10 Hz to 10 Khz VCO With Square and Triangle Wave:

Transconductance amp gives oscillator reciprocal response: 04/17/03  EDN-Design Ideas / The circuit in Figure 1 is a variation on the familiar integrator/comparator triangle-wave oscillator, which you typically implement with tw1o op amps. An integrator and a comparator connect in a positive-feedback loop; the comparator drives the integrator and vice versa. A fixed amount of hysteresis exists in the comparator, producing a triangle wave at the integrator`s output...

Triangle Wave oscillator:

Voltage Controlled Triangle / Square Generator: