Band pass filter and twin-T notch filter

[C.L.low]

Hi,

I've encountered some problem during solved the band pass filter and twin-T notch filter. I don't really understand what the question exactly meant...hope can get some idea in here..thx


1.) A signal conditioning system uses a frequency variation from 6 kHz to 60 kHz to carry measurement information. There is considerable noise at 120 Hz and at 1MHz. Design a band pass filter to reduce the noise by 90%. What is the effect on the desired pass band frequencies?


2.) A frequency of 400 Hz prevails aboard an aircraft. Design a twin-T notch filter to reduce the 400 Hz signal. What the effect would this have on voice signals at 10 to 300 Hz? At what higher frequency is the output down by 3 dB?

 

[C.L.low]

anyone? :(

 

[gneill]

For (1), it looks like a low pass filter will serve, right?; The indicated noise is all above the signal band. So, with a cutoff frequency at the highest signal frequency, 60kHz, what order of low pass filter will provide the required suppression at 120 kHz? In other words, how many dB down does the transfer function magnitude have to be at 120kHz to reduce the noise at that frequency by 90%?

 

[Chopin]

Lowpass filter won't work, because one of the frequencies is below the signal band, and one is above it. You'll need to use a bandpass filter, like the question says.

You're supposed to design a bandpass filter that will attenuate 120Hz by 90%, and 1MHz by 90%. So your first step should be to figure out what points on a bandpass's transfer function gives 90% attenuation. Once you have that, you can figure out where the pass band points need to be in order for that 90% to land on those two frequencies. Then all you have to do is figure out what effect that filter will have on the signal range (6kHz - 60kHz).

For #2, basically all you have to do is look at the transfer function of the Twin-T circuit. If it's centered at 400Hz, what will it do at other frequencies?

 

[gneill]

Lowpass filter won't work, because one of the frequencies is below the signal band, and one is above it. You'll need to use a bandpass filter, like the question says.

Doh! You're right. My eyes saw "120kHz" when I they should have seen "120 Hz". My bad.
So, bandpass it is.

 

[C.L.low]

Lowpass filter won't work, because one of the frequencies is below the signal band, and one is above it. You'll need to use a bandpass filter, like the question says.

You're supposed to design a bandpass filter that will attenuate 120Hz by 90%, and 1MHz by 90%. So your first step should be to figure out what points on a bandpass's transfer function gives 90% attenuation. Once you have that, you can figure out where the pass band points need to be in order for that 90% to land on those two frequencies. Then all you have to do is figure out what effect that filter will have on the signal range (6kHz - 60kHz).

For #2, basically all you have to do is look at the transfer function of the Twin-T circuit. If it's centered at 400Hz, what will it do at other frequencies?

Hi,
you mean I have to find the Vout/Vin first? Or 0.9 (90%) is my Vout/Vin?
So sorry I'm not familiar in this subtopic. :(

 

[Chopin]

If I'm reading the question correctly, they want 90% attenuation, so you want the point where it's 10% of its original value. So Vout/Vin = 0.1. Now figure out where the 3dB points for the bandpass filter need to be in order for 120Hz and 1MHz to be attenuated by that much.

 

[Redbelly98]

Moderator's note:

It's fair to give clarification on what the questions are asking, since the original request was for clarification and nothing more.

Further help with actually solving the problem would require a posted attempt at a solution by the OP.