The input frequency is sweeped, the filter
cutoff frequency is determined by the switch on time (50 percent
in this case) and the resistor networks effecitve value,
roughly 16 kOhm, and is roughly 1kHz here (quick computation
seems to match graphical results, wil do more acurate analysis).
The sampling frequency is 100kHz, which makes
for easy anti-aliasing filtering, and when the
filter clock is linked with a sample generator clock, there is
no need for an input anti aliasing filter.
Changing the sampling signals pulse width (on-off ratio) clearly changes the cutuff frequency within the roughly expected decade range.
The digital pule is 100kHz with 80 percent duty cycle, the input signal is a frequency sweeped sine wave.
The sweeped sine can be seen in the the blowup of the peak, and it is clear that after the filter has been exitated, the resonating filter component in the output (a damped oscilation) takes quite some time to damp out, and interferes with the input signal.
This can also be seen in the frequency analysis:
Two parts are easily distinguished (I'd include more views if I had the web space): the sweeped oscilator , which is heavily damped at the ends of the frequency axis, and the damped oscilation, which remains equal in frequency.
Note that the sample time is almost half a second, and that the sample frequency is 100kHz, which led to hours of simulation time, but I guess there is room for improving this.
(Also in switched version, i.e. switch based amplitude feed?)
Finally of course: what does it sound like? This is a downsampled version, for easier listening (the original is 400mS long).