# Pie Filter (3rd order) low pass issue

#### Damian P

Joined Mar 23, 2020
10
Good afternoon all,
I'm currently working on a Pie - low pass - filter, and I can't seem to figure out few bits about it.

1) In the past when I've done the low pass filter 1rd order, the power loss was -3dB at fc; however I'm not sure what it supposed to be as this circuit is 3rd order ? (My circuit starts from + 20dB and not 0 dB ??)

2) fc = 1/2*pie*sqrt(LC) ---> fc = 15Khz
I tried calculating the value for the capacitor, however they are in parallel ?
c = 1/(900*10^6)*(pie^2)*L

If I follow my formula, do I still have to divide the capacitance by 2 ?

I have attached simulation of two build circuits, the one with the load resistance is the one I would like to use for the project.

I look forward to hear from someone, with adequate knowledge.

Best

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#### LvW

Joined Jun 13, 2013
1,299
It is not clear to me how you start your design.
The "normal" (classical) procedure is first to formulate the requirements to the filter transfer function:
* order of the transfer function (damping requirements)
* Approximation (Butterworth, Chebyshev, Bessel...)
* Gain and damping requirements (passband edge, corner frequency,)

#### Damian P

Joined Mar 23, 2020
10
Hi LvW,
The only requirements are for the filter to have fc - 15Khz and It needs to be a Pie Filter.
I'm not sure how to handle this, hence why I'm asking.

#### crutschow

Joined Mar 14, 2008
27,696
A nit, but it's "Pi" (filter schematic looks like the Greek letter π) not Pie.

#### Damian P

Joined Mar 23, 2020
10
A nit, but it's "Pi" (filter schematic looks like the Greek letter π) not Pie.
Yes, it's Pi filter

#### LvW

Joined Jun 13, 2013
1,299
Hi LvW,
The only requirements are for the filter to have fc - 15Khz and It needs to be a Pie Filter.
I'm not sure how to handle this, hence why I'm asking.
Hi LvW,
The only requirements are for the filter to have fc - 15Khz and It needs to be a Pie Filter.
I'm not sure how to handle this, hence why I'm asking.
What about the resistor values (R1, R2)? Are you free to select other values?