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- 12th February 2007, 05:36 #1

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## calculate cut off frequency of 2 filter

anybody can give me an equation about how to calculate the cutoff frequency of the filter circuit.

how to determine the value of R and C.

I want to have the cutoff frequency of 200Mhz.

- 12th February 2007, 05:36

- 12th February 2007, 10:57 #2

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## how calculate cutoff frequencies for a circuit

Take a look at the pi low pass filter at http://www.electronics-tutorials.com...ss-filters.htm

- 12th February 2007, 13:53 #3

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## Re: how to calculate the cutoff frequency of the filter circ

Hi Dear ilikebbs!

Remember! To solve such problems in a exact way; you should calculate "Transfer Function" of the circuit. After that you may find the cut-off frequency(s) as below:

Thanks!

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- 12th February 2007, 13:53

- 12th February 2007, 22:43 #4

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## Re: how to calculate the cutoff frequency of the filter circ

ilikebbs,

If the filter is driven by a voltage source, then C1 is irrelevant, and the cutoff frequency is fc = 1/(2PiRC2) (if the load is an open circuit). Do you know the source impedance and load impedance? They both affect the cutoff frequency.

Regards,

Kral

- 13th February 2007, 04:31 #5

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## how to calculate the cutoff frequency of the filter circuit

thanks for your reply.

the circuit is used for VGA interface RGB signal filter. The PP voltage is 0.7v. The impedance should be 75 ohm.

- 13th February 2007, 04:31

- 13th February 2007, 17:59 #6

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## Re: how to calculate the cutoff frequency of the filter circ

To calculate the cut off frequency of the filter you need to define both source and load impedances

- 13th February 2007, 18:43 #7

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## Re: how to calculate the cutoff frequency of the filter circ

ilikebbs,

Four questions:

Is the source resistance 75 Ohms?

Is the load resistance 75 Ohms? If not, what is the load resistance?

Must the filter plus load present a 75 Ohm impedance to the source?

Must the filter plus source present a 75 Ohm impedance to the load?

.

These factors all affect the design. Once these questions are answered it is possible to proceed with a design.

Regards,

Kral

- 14th February 2007, 07:01 #8

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## how to calculate the cutoff frequency of the filter circuit

Thanks Kral.

Source resistance is 75 ohm.

Load resistance is 75 ohm.

The filter plus source presents 75 ohm to the load.

The filter plus load presents 75 ohm to the source.

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