Yellowjacket's filter is based upon the CD4069 logic chip-based filter of the original. It's a "state-variable" (like Maine, or Iowa) filter with a 12dB per-octave attenuation slope. The state-variable part refers to its curves - it can function as a lowpass, bandpass, notch, or highpass filter.
Freq.- Sets the frequency where frequency attenuation begins, i.e. which frequencies are allowed to pass, dependent on the currently selected filter type.
About Yellowjacket Filter Tracking: Many analog synths include "filter tracking" controls or switches. The idea behind filter tracking is that filter cutoff frequency increases as higher notes are played in order to ensure equal brightness (or dullness) regardless of pitch. Like the original Wasp, Yellowjacket does not have any filter tracking controls; filter tracking is "built in" and is actually somewhat greater than the typically seen volt-per-octave standard. This means that Yellowjacket gets brighter at a pretty steep rate as higher keys are played. It's likely you won't notice it, but because of the sharp curve, the filter Freq. knob may not seem to do much when playing high notes (or low notes in highpass mode). You may also notice it when applying Control Envelope or Keyboard Pressure mod. If it seems like you're not getting enough filter mod in the range you're playing in, just tweak the main Filter Freq. knob to compensate.
'Q'- Also know as "resonance," this emphasizes sound energy at and around the current cutoff frequency by adding feedback from the filter's output back to its input. At lower settings, this can be used to create mild resonances such as those heard in acoustic instruments. At extreme settings, it can be used as a sine wave generator, but be careful as high resonance settings can result in loud, screamy, dog-spooking (and speaker blowing) occurrences.
Type- Selects the overall filter curves.
Lo (Lowpass)- Allows frequencies below the cutoff frequency to pass, but blocks frequencies above the cutoff frequency.
Band (Bandpass)- Allows a band of frequencies in the vicinity of the cutoff frequency to pass.
Notch- Removes a band of frequencies close to the cutoff frequency and allows all other frequencies to pass. Notch filters are useful for pseudo-phaser effects when their cutoff frequency is swept, and are notch-yer-problem (We regret to inform you that Cherry Audio manuals are written by your dad).
Hi (Highpass)- Allows frequencies above the cutoff frequency to pass, but blocks frequencies below the cutoff frequency. Because it dramatically reduces low frequencies, the highpass setting is useful for nasally tones with exaggerated high frequencies.