I just noticed this in the LM13700 datasheet.
The following accompanying text says Figure 34 but I think they meant 35 (figure 34 uses a single LM13700)
The circuit of Figure 34 employs two LM13700 packages, with three of the amplifiers configured as lo-pass filters and the fourth as a limiter/inverter. The circuit oscillates at the frequency at which the loop phase-shift is 360° or 180° for the inverter and 60° per filter stage. This VCO operates from 5 Hz to 50 kHz with less than 1% THD.