hey there...
i wanted to modified my 1st order Butterworth low pass filter with K=1.
i want to increased the roll off rate.. but, after changing the resistor value,
the capacitor value and fo, i the roll of rate still the same..
how can i increased my roll off rate?

 

I believe each reactive element will add another -20 Db/decade. You need to add more capacitors, but keep the total reactance the same to achieve the same cutoff frequency.

 

If it is a passive network, the rate is only 3 dB/octave.

 

If it is a passive network, the rate is only 3 dB/octave.

No.
A single RC rolls off at 6dB/octave which is 20db/decade. Opamps and ordinary audio tone controls use it.

A fairly complicated 3db/octave filter is used to convert white noise into pink noise that needs a roll off of 3dB/octave.

A Sallen and Key Butterworth 2nd-order filter rolls off at 12db/octave and a Sallen and Key 3rd-order Butterworth filter rolls off at 18dB/octave.

Switched capacitor lowpass filter ICs roll off at 6db/octave per order. An 8th-order Butterworth filter rolls off at 48db/octave and will filter a square-wave into a pretty good sine-wave.

 

The -3 dB point stuck in my head - http://en.wikipedia.org/wiki/Roll-off

 

If you want to increase to rolloff while keeping the flatness there are other types of filters. For example, an m-derived filter has a very sharp knee, but not as good rejection.

http://en.wikipedia.org/wiki/M-derived_filter

 

A Bessel filter has good phase response but has a droopy frequency response.
A Butterworth filter has the flattest frequency response and fairly sharp cutoff but rings a little at the cutoff frequency.
A Chebychev filter has a very sharp cutoff but has ripple in the passband and ripple at higher frequencies.