is there a way I can get a "sharper" cut-off/slope from a passive Low Pass fil.

[RoGeorge]

The one posted by T3sl4co1l is a 3rd order filter.  Three 1st order filters chained as shown in the above schematic will make a 3rd order filter.  Also the slope being 60dB/decade tells that's a 3rd order filter indeed.

Never heard of a triple first-order filter before.

[T3sl4co1l]

The confusion seems to be on terminology:

Order refers to the number of poles.  It doesn't matter where they are (as long as they are finite).  A filter with a pole each at 1kHz and 1GHz, is still a two-pole filter.

Sharpness is due to the distribution of poles.  In particular, poles should cluster together, typically zero or one real poles and the rest as complex pairs, and typically in some convenient geometric distribution (IIRC, Bessel lie on an ellipse, Butterworth on a circle centered on the origin, Chebyshev on a circle offset right of the origin?), but there is no necessity for this to be the case.  Indeed, practical filters might have a multitude of intentional and parasitic poles and zeroes, and perhaps additional poles added to smooth them out (for example, a filter that needs good attenuation over many decades, might have "clean up" stages with poles deep in the stopband to anticipate those zeroes).

"Homework" is... perhaps insensitive, granted.  But these definitions are readily found in the textbook, and easily seen when working equations (in simplified rational form, the order of the denominator polynomial is the order of the filter, and its roots are the filter's poles).  It's an invitation to refresh rusty knowledge and practice -- I'm half tempted to do the homework myself, honestly, I could probably use it.  I've done the 2nd order (RC) case before, and that was many years ago.  The 3rd order case is sure to be a mess, but in there, shall be found a cubic polynomial all the same.

Tim

[The Electrician]

[ Specified attachment is not available ]Here is a plot of the frequency response using the R and C values in Reply #19 (blue curve) and with all the R and C values equal to the middle values in Reply  #19 (red curve).  Scaling the R and C values helps, but the blue curve is about as sharp as you can get with an RC low pass, no matter how many stages are cascaded, because cascading more stages only makes it less sharp.

[Benta]

Teslacoil, you're cheating a bit here.

Your filter is three RC stages with impedance stepping. A real circuit would be be three RC dividers with opamp buffers in between.

But you're right. It's a terminology question.

Is stacking three 1st order filter responses on top of each other a 3rd order response? Put on your professor hat, raise your index finger and chastise me.

[The Electrician]

Teslacoil, you're cheating a bit here.

Your filter is three RC stages with impedance stepping. A real circuit would be be three RC dividers with opamp buffers in between.

But you're right. It's a terminology question.

Is stacking three 1st order filter responses on top of each other a 3rd order response? Put on your professor hat, raise your index finger and chastise me.

Why is the circuit shown in reply #19 not "real"?

Terminology is important in the hard sciences.  I don't understand what is meant by "stacking" filter responses, but I do understand what is meant by "cascading" filter responses; is that what you mean?

Here is the response of 3 RC lowpasses with (ideal) opamp buffers in between (red curve) and the circuit in reply #19 (blue curve).  It's hardly worth adding buffers versus scaling impedance levels:

[T3sl4co1l]

IIRC, the pole splitting, for a factor of 10 impedance scaling, is around 20%.  So, roughly speaking, the cutoff will be 10% softer than for the buffered case (which approaches a repeated pole; ideally equals, but the hedging language allows for considering nonideal opamps).  The all-equal case I think is close to 250% worse, at least for two poles (as you can see above, it's... not very good!).  Dunno for 3+.

Hrm, I'm not sure that you'd ever use the buffered case, actually?  As long as you're putting gain into the system, you might as well make some complex poles.  And make it odd order, because you get the real pole for free (passive).   Might be some special applications where repeated real poles are desirable, in which case of course that'd be the way to go.

Tim