# How to test a low pass filter?

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#### abcyin

##### Full Member level 4
Hi, all

could you please give me some advice about how to test a LPF? the parameters I'm interest in include the insertion loss, the AC responce, and the group delay.

In simulation, I attach a res about 10K to the ouptut node, but while measuring the LPF, how can I connect the output with the input of a VSA or VNA.

If someone gave me some literature about LPF measurement, I will appreciate it.

BR!

i'm gonna ask you something :
your LPF is active or passive ?

if you are using a passive filter ( resistor & capacitor) you of course know the resistance and capacitance.

1-calculate the multiplication of R & C
ω = RC than you have f = ω / 2Π

2- know you 're going to use a Function generator :

3-adjust the switch for sinusoidal .

4-adjust the frequency switch begining from DC ( ω = f = 0) and increasing .

5-while increasing if you notice the gain of the output you will find it decreasing.

6- at the moment the frequency equals the ω calculated in the above the gain must equal the miband gain divided by √2 .

if this is what happened to you than your LPF is working perfectly.:|

if not send me.

mezo said:
i'm gonna ask you something :
your LPF is active or passive ?

if you are using a passive filter ( resistor & capacitor) you of course know the resistance and capacitance.

1-calculate the multiplication of R & C
ω = RC than you have f = ω / 2Π

2- know you 're going to use a Function generator :

3-adjust the switch for sinusoidal .

4-adjust the frequency switch begining from DC ( ω = f = 0) and increasing .

5-while increasing if you notice the gain of the output you will find it decreasing.

6- at the moment the frequency equals the ω calculated in the above the gain must equal the miband gain divided by √2 .

if this is what happened to you than your LPF is working perfectly.:|

if not send me.

it's active.
As your fifth term above, what I want to know is that how can you detect the gain of it, which equiment will you use, maybe you will the spectrum analyzer, but I can't connet the output to the input of VSA directly, because the input resistance will pull down the gain of the LPF.

may be you can add a sinual signal in the input port ,and test the dft of output signal , then you can get the frequency response

maybe you can run S parameters

Transient analysis then use dft function.

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