Powerful but compact utility oscillator

(Copied and pasted from the product's page in the official website)

The 2131 VCO replaces the trusty workhorse Compact VCO – taking very similar lines, but adding some important features.

The VCO is based on the SSI2131 integrated circuit (itself a stripped down version of the SSI2130 used in the 2130 VCO). It is a high performance triangle-core oscillator-on-a-chip which features excellent tracking and temperature stability.

Tuning – a sum of Tune (10 Octave range), Fine (1 Octave range), Octave switch (off / +1 Octave), Frequency Modulation (Linear or Exponential) and precision 1V/Oct input. The Range switch selects Audio (Osc) or sub-audio (LFO) rates.

Ranges (with Fine control centred) – Osc approx 20Hz to 20kHz, LFO 0.2Hz (5 secs) to 200Hz – ranges can be extended far below these with negative offsets (Osc tested down to 0.5Hz and up to 30kHz).

Frequency Modulation – variable depth Frequency Modulation can be set to either Linear or Exponential (with centre off mute). Linear is typically AC-coupled though a rear jumper allows experimentation with DC-coupled. Note, the SSI2131 does not offer Thru-Zero like the SSI2130. Exponential has a convenient polarity switch.

Waveforms – the Shape dial sweeps from Triangle wave to either Saw or Square/Pulse. Typically set as 50/50 duty Square, there is a rear jumper allowing the duty cycle to be set as a narrow pulse. See the diagram for waveform polarity/phase relationships. All waveforms are bipolar +/-5V.

Sync – an important improvement vs the CVCO was ensuring improved phase alignment when sync-ing. With a standard comparator on the input, any voltage over c.+1v will reset the oscillator core. For Tri wave this is -5V, while the Saw and Square waves reset to +5V – perfect for clocking duties.

Rear trimmers – adjust with care and small movements! Ensure the VCO has been powered for 30 minutes before adjusting.
Scale & HFTrm set the tracking and will be adjusted in tandem – first adjust the Scale at low frequencies, then dial in the High-Frequency Tracking accordingly.
Offs(et) – sets the base frequency. Set to 20Hz with Tune at minimum and Fine at mid-point.
Oct(ave) – adjusts the precision 1V source for the octave switch.

https://www.bugbrand.co.uk/product/2131-vco/