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Lowpass Filters

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Typhoon

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HI,

Can anyone help me with a circuit that can be used as a lowpass filter.

I have been looking at simple RC lowpass filters, but are these accurate enough to detect a bass audio frequency. Or are Op-amp circuits more accurate to detect a sub 250Hz frequency.

Also if i am using a direct line from an audio source or a microphone to detect the sound, what will the output voltage? Will i need to amplify this using an op-amp so that it can pass through the filter.

My Last question is, can i use a PIC to detect a certain group of frequencies from a large range.

Thanks for your help.

Any info will be appreciated.
 

2XS

New Member
You can use a first order low pass filter with op amp:
**broken link removed**
Where the DC gain (Ho) is calculated R1/R2= Ho
The general form of the transfer function is:
**broken link removed** which is the general form for first-order (one reactive element) low-pass filters.

At high frequencies (w>>wo) the capacitor acts as a short, so the gain of the amplifier goes to zero. At very low frequencies (w<<wo) the capacitor is an open and the gain of the circuit is Ho.

When the magnitude of the capacitor impedance is much less than that of R1. In other words, we have high frequency when 1/wC<<R1, or w>>1/R1C=wo. Since R1 now has little effect on the circuit, it should act as an integrator. Likewise low frequency occurs when w<<1/R1C, and the circuit will act as an amplifier with gain -R1/R2= Ho.
 
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