Low pass filters & Notch Filter

[maidmarion]

Hi just a few questions if anyone can help me on -


1) Can anyone please advise me on what the vocational uses are for a Notch Filter?

2) When setting up an investigation for a Notch filter what sort of components would be used?

3) When 2 low pass filters are in cascade, why does the slope become steeper?

4) What is the relationship between the -3dB and cut-off frequency in a low pass filter circuit?

Any help would be so appreciated!

Thanks
Laura

 

[audioguru]

1) Can anyone please advise me on what the vocational uses are for a Notch Filter?

It removes a narrow range of frequencies. I use a notch filter in my harmonic distortion analyser circuit to remove the fundamental frequency of a sine-wave so that measurement can be made of only the harmonics.

2) When setting up an investigation for a Notch filter what sort of components would be used?

Look at Notch Filter in Google. An opamp, some resistors and some capacitors are usually used to make a notch filter. I made one with a switched-capacitor Butterworth filter IC.

3) When 2 low pass filters are in cascade, why does the slope become steeper?

The slope is also cascaded. The attenuation is doubled at a frequency past the cutoff frequency.

4) What is the relationship between the -3dB and cut-off frequency in a low pass filter circuit?

The -3dB frequency is called the cutoff frequency. The cutoff frequency of a single-order Butterworth filter is calculated by 1/(2 pi RC). The calculation is more complicated when the filter has more orders.

 

[I²R]

A more common and familiar use for notch filters is in the analog cable industry. Cable distributors use notch filters to prevent a particular installation from receiving certain--usually premium--channels. When you subscribe to one of these channels a technician removes the appropriate filter from a special pole-mounted enclosure and you're then able to receive it.

 

[Nigel Goodwin]

A more common and familiar use for notch filters is in the analog cable industry. Cable distributors use notch filters to prevent a particular installation from receiving certain--usually premium--channels. When you subscribe to one of these channels a technician removes the appropriate filter from a special pole-mounted enclosure and you're then able to receive it.

But that really old crude technology, surely no one uses analogue cable these days?.

 

[KMoffett]

A common use for notch filters is in medical equipment used to record ECG/EEG/...signals. All have notch filters for 60Hz (or 50Hz) that filter out the interference radiated from the buildings' power wiring.

Ken

 

[ecerfoglio]

Also, if you want to receive some radio / TV station that is close (in frecuency) to a powerfull local transmiter, then a notch filter between the antena and receiver often helps to reduce interference.

 

[audioguru]

Cable TV in Canada is a combination of old analog and new digital.
All stations are available as digital with better picture and sound than analog.
Old broadcasted stations are available as both. New cable-only stations and pay TV are digital-only. A converter is needed to select and decode them.

Years ago before digital, I saw the cable TV box down the street over-stuffed with notch filters for everybody who did not pay for extra channels.
Then they added new notch filters at each home where the cable enters.
I had one and it messed up my reception of ordinary channels so I complained and they removed it.