How many octaves is a Sallen Key filter good for?

[Gurner]

Let's say I want to use a Sallen Key LPF...



....& referencing that diagram, I then make the two "R" resistors 'in play' to be a ganged potentiometer.

How many octaves would the filter's useable range likely to be wrt the ganged pot's end to end travel? (I realise the actual frequencies in scope are related to the values chosen, but just for example would say 500hz-2,000Khz 2 octave or say 500Hz-8,000Hz  4 octaves etc be feasible?).

I'm looking to be able clamp the lowpass right down to as low as 200Hz...but have it 'liftable' as high as 5Khz - now I'm new to filters, so I'm just wondering how do-able that requirement actually is & using as few 'chained' filter modules as possible!

Edit: Actually...just after posting, I found a funky sallen key calculator... http://sim.okawa-denshi.jp/en/OPstool.php    & entering a capactance value of C1=100pf, C2=200pf & a resistance value of 200R for both resistors, yields a cutoff of 5,7Khz ....then changing the resistance element to 10,000 ohms (in other words a ganged 10k pot), the cutoff comes right down to 112Hz ....so that's about 6 octaves.

[PRR]

F is exact proportion to R.

So how wide a range can your pot cover?

A common Linear pot may go up to 100K, but not to zero. There's some end-resistance. Usually the end is a metal pad with low resistance, but when you hop-off that you may be at 1K already. And just 2.7 degrees turn gives 2K, an octave down. It is unwise to expect more than 10:1 maybe 20:1 range on a linear pot.

Common "Audio" pots are made with two sections. Half is 80K and the other half is 20K. Each half is linear, so it gets awful touchy to dial a 1K difference. 50:1 range may be reasonable, depending how exact you need to be and how careful you can be.

We usually want frequency to increase clockwise. A common audio pot connected for a good spread will turn counter-clock for increased freq. You want Reverse Audio, which limits your options.

Note that while the math works fine for 200 ohms, your opamps will be straining. And this is the high end of the range where they are already a little gaspy. You generally scale your low end for 1K or 2K.

Do you need huge bandwidth in one turn? 10:1 pot range and a 3-way caps-switch will do 20-200, 200-2K, 2K-20K.

[PRR]

A 100K (reverse-)Audio pot with 4K stopper gives 100K max - 20K center - 4K min. This will cover 25:1 or 30Hz-750Hz. Then switch caps to cover 750Hz-18KHz.

[Gurner]

Thanks, but to get decent resolution I'm going to  to use a 256 position dual digipot (which compared to a mechanical pot with 270 degrees of rotation equates to about 1 degree resolution)  - I only have 10k digipots, so each 'control step' works out at approx 40 ohms (I obviously wont go as low as 40 ohms in that sallen key config ...but will instead make the bottom count say 8-10 steps away from the pot end, therefore say 400-500 ohm). But at least digipots allow pretty dependable step accuracy, & 'wiper' either up or down (the pic doesn't care!)...I'm hoping 256 steps will avoid any zippering too.

I'm intending having a dabble with filters under control of a PIC (I already have an envelope detector going on with my PIC, so want to experiment with say for example "when new note is plucked start the filter way low & ramp it up @ rate 'x'" etc)

BTW: I linked incorrectly in that sallen key calculator ....   http://sim.okawa-denshi.jp/en/OPseikiLowkeisan.htm   (& in my earlier related pot I meant 100nf & 200nf for C1 & C2 respectively!)