RC Lowpass cut off, single pole. - Electronic Circuits Projects Diagrams Free

PhillDubya · 5th May 2009, 05:48 AM

Fundamental question.

A single pole low pass filter, lets say: RC lowpass, will have a VERY shallow cut off.

A 5 pole filter will have a VERY STEEP cut off.

The question:
If I were to design a single pole low pass for 100Hz, due to the shallow cut off, it probably wouldn't actually cut off right at 100Hz correct?

It may actually pass 110, and even 120, etc..., correct?

Basically, the higher order, the higher accuracy...if I am understanding correctly?

Thanks

Sceadwian · 5th May 2009, 06:31 AM

The "cut-off" frequency is where the amplitude decreases by 3db, AKA 50% power for a single pole filter. For a single pole filter the drop in frequency will be very shallow compared to a higher order filter. So yes your understanding is correct.

Roff · 6th May 2009, 02:02 AM

Wikipedia has normalized Bode plots for 1st through 5th order Butterworth lowpass filters. See the plot under The transfer function.

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode

Similar
Title	Starter	Forum	Replies	Latest
Where to buy a Pole Pig ?	Frosty_47	General Electronics Chat	30	23rd August 2009 08:26 AM
help in lowpass filter curcuit.	♪Musicl☺♥☻r	General Electronics Chat	3	19th February 2009 12:41 PM
Lowpass filter for ADC	tuanvoi	General Electronics Chat	1	11th February 2009 03:54 PM
Lowpass active filter failing at the lower range	atferrari	Electronic Projects Design/Ideas/Reviews	11	21st July 2007 01:55 AM
Lowpass Filters	Typhoon	Electronic Projects Design/Ideas/Reviews	1	21st September 2003 04:42 AM

5th May 2009, 05:48 AM	#1
PhillDubya	RC Lowpass cut off, single pole. Fundamental question. A single pole low pass filter, lets say: RC lowpass, will have a VERY shallow cut off. A 5 pole filter will have a VERY STEEP cut off. The question: If I were to design a single pole low pass for 100Hz, due to the shallow cut off, it probably wouldn't actually cut off right at 100Hz correct? It may actually pass 110, and even 120, etc..., correct? Basically, the higher order, the higher accuracy...if I am understanding correctly? Thanks Last edited by PhillDubya; 5th May 2009 at 05:49 AM.

5th May 2009, 06:31 AM	#2
Sceadwian	The "cut-off" frequency is where the amplitude decreases by 3db, AKA 50% power for a single pole filter. For a single pole filter the drop in frequency will be very shallow compared to a higher order filter. So yes your understanding is correct. __________________ "Because I be what I be. I would tell you what you want to know if I could, mum, but I be a cat, and no cat anywhere ever gave anyone a straight answer, har har."

6th May 2009, 02:02 AM	#3
Roff	Wikipedia has normalized Bode plots for 1st through 5th order Butterworth lowpass filters. See the plot under The transfer function. __________________ Ron