# Designing filter circuit consideration requirement.

#### Wompalamba

Joined Dec 28, 2020
19
I am wondering if i need to take into account the input and output impedance when finding a filter circuit frequency from it's transfer function. The previous LC filter i designed took into account the input and output impedance which result in -6dB+ even at low frequency when i simulated it. I suspect i might be missing a concept to understand why it is -6db even before the -3db drop at the cutoff frequency. Thank you for your time.

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#### ericgibbs

Joined Jan 29, 2010
11,629
hi W,

Consider removing the Cap, so you have a potential divider, 54.5R and 50R.

For explanation consider that they are both 50R... that will be a 6dB attenuation of any signal on the input, measured at the junction of the two resistors OK.?

E

Update:
If the Cap was connected in parallel with the 50R, its Impedance would decrease as the frequency of the AC input increased.

One image shows the Cap Impedance and the other Impedance of the parallel Cap and 50R, versus Frequency
Also the total Impedance of the network...

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Last edited:

#### Wompalamba

Joined Dec 28, 2020
19
Yes i was refering to my previous thread. Ah i see what you mean, the two 50ohm resistor will form a voltage divider which causes the -6db attenuation even without the RC filter. However, am i right to include the input and output impedance during the calculation of the transfer function to find the cut off frequency of the RC filter? Or must i exclude the impedance during the calculation?

#### ericgibbs

Joined Jan 29, 2010
11,629
hi W,
If you were given the value of the 'actual' Input and Output Impedances, you would include them in your calculations.
They have not been given so you should just analyse the circuit as is given to you.

Don't forget the Zsource and Zload can be complex, not just resistive components.

E