low pass "PI" reconstruction filter ?

percy · 2nd May 2006, 01:36 AM

Has anyone used a pi filter as the final low pass reconstruction filter after the dac ? Is it feasible to do so ?
I am also looking for information on the phase response of such a filter. Where can I find design, operation and application information of such a filter for this purpose ?
Thanks!

Bricolo · 2nd May 2006, 07:18 PM

a pi filter is just a 3rd (sometimes 2nd when a resistor is used instead of a coil) order filter

the name pi comes from the shape of the character pi, that matches the schematic of "C1 to ground, L1 between C1 and C2, and C2 to ground"

so it's a classical filter, it has no specific phase or amplitude response, it depends of the components values and the "alignment" (butterworth, bessel, something else...)

percy · 2nd May 2006, 11:29 PM

By phase response I meant the phase shift, or distortion, or linearity, or whatever of the filter - the same effect because of which very higher order or steep analog filters are not desirable for this application or purpose.

I found this thread - Low pass filter design for TDA1541

as shown in post# 18 and 19 the simulated response is indeed very steep so I wanted to find out what the phase linearity of a real pi filter looked like.

macboy · 3rd May 2006, 03:14 PM

Quote:

...as shown in post# 18 and 19 the simulated response is indeed very steep ...

No, it isn't. Check out the vertical scale on the graph... a total of 5 dB are being displayed. So even though it appears steep, that is just a visual artifact of the very small vertical scale. The graph probably is zoomed in like that to show the small amount of pass-band ripple, which would be impossible to see with a 100 dB scale. Likewise it is impossible to see stop-band response with a 5 dB scale. It is a 3rd-order filter so it will have a 3rd-order slope: 18 dB/octave. You can count on that. How it behaves (in amplitude and phase) around the crossover frequency will be determined by the alignment of the different reactive components, as mentioned earlier.

percy · 4th May 2006, 01:13 PM

ok. thanks for pointing that out.

in general then does a pi filter offer any advantages over any 3rd order filter ?

Bricolo · 6th May 2006, 08:45 AM

it is a 3rd order filter

2nd May 2006, 01:36 AM	#1
percy diyAudio Member Join Date: Apr 2004 Location: MN	low pass "PI" reconstruction filter ? Has anyone used a pi filter as the final low pass reconstruction filter after the dac ? Is it feasible to do so ? I am also looking for information on the phase response of such a filter. Where can I find design, operation and application information of such a filter for this purpose ? Thanks!

2nd May 2006, 07:18 PM	#2
Bricolo diyAudio Member Join Date: Nov 2002 Location: Grenoble, FR	a pi filter is just a 3rd (sometimes 2nd when a resistor is used instead of a coil) order filter the name pi comes from the shape of the character pi, that matches the schematic of "C1 to ground, L1 between C1 and C2, and C2 to ground" so it's a classical filter, it has no specific phase or amplitude response, it depends of the components values and the "alignment" (butterworth, bessel, something else...) __________________ Just remember: in theory there's no difference between theory and practice. But in practice it usually is quite a bit difference... Bob Pease

6th May 2006, 08:45 AM	#6
Bricolo diyAudio Member Join Date: Nov 2002 Location: Grenoble, FR	it is a 3rd order filter __________________ Just remember: in theory there's no difference between theory and practice. But in practice it usually is quite a bit difference... Bob Pease

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2nd May 2006, 11:29 PM	#3
percy diyAudio Member Join Date: Apr 2004 Location: MN	By phase response I meant the phase shift, or distortion, or linearity, or whatever of the filter - the same effect because of which very higher order or steep analog filters are not desirable for this application or purpose. I found this thread - Low pass filter design for TDA1541 as shown in post# 18 and 19 the simulated response is indeed very steep so I wanted to find out what the phase linearity of a real pi filter looked like.

4th May 2006, 01:13 PM	#5
percy diyAudio Member Join Date: Apr 2004 Location: MN	ok. thanks for pointing that out. in general then does a pi filter offer any advantages over any 3rd order filter ?

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