low pass filter help please

[fran1942]

Hello, I have been asked to:
- design an LPF with a cut off frequency of 17kHz and a roll-off of 40dB Decade. I have attached a picture of the circuit diagram I came up with.
Can someone please confirm if I have this correct ?

The second part of the question was:
- prove both time domain and frequency domain plots for the output waveform for the above filter, if a 5kHz square wave was applied to it.
For this question I am rather lost. Can someone please give me some guidance on how I would display these 2 graphs. I cant find anything in my notes directly explaining how to do this.

Thanks kindly for any help.

 

[goldsmith]

Dear fran
Hi
See below , please :


These are the result of simulation ( of your circuit ) .
So what do you thing about them ?
And about the out put , function , i think with a simple node and voltage method at frequency domain you can do it ( 2 node ) .
Best Wishes
Goldsmith

 

[goldsmith]

The red shape is the out put of first section

 

[klystron]

Get yourself a copy of Sapwin at **broken link removed**

 

[goldsmith]

Dear klystron
Hi
What is Sapwin ?
Thanks
Goldsmith

 

[klystron]

Sapwin is a free very user friendly analogue circuit simulation program. Features include :
Symbolic transfer function from a user defined circuit
Numeric transfer function
Pole zero digrams
Bode plots
Gain and phase response
Step and impluse response

 

[LvW]

Hello, I have been asked to:
- design an LPF with a cut off frequency of 17kHz and a roll-off of 40dB Decade. I have attached a picture of the circuit diagram I came up with.
Can someone please confirm if I have this correct ?

Hi fran1942, there is a (catastrophic) error in your lowpass design.
You have calculated a first order RC stage for the correct cut-off at 17 kHz - and than you have added the same stage to realize a second-order roll-off.
However, you completely have ignored the fact that the second stage (a) loads the first stage and (b) provides additional damping.
That means: Even with a buffer amplifier between both stages (with the aim to avoid the loading effect) there would be an attenuation og -6 dB at 17 kHz.
Thus, the -3dB corner would be much lower than 17 kHz.
My suggestion: Try to find the correct transfer function for the above circuit (for example with SAPWIN, which is a good tool for symbolic analysis, however very limited for classical circuit analysis purposes). Or use another filter topology - the simplest case is an RLC filter.

---------- Post added at 11:41 ---------- Previous post was at 10:45 ----------

The second part of the question was:
- prove both time domain and frequency domain plots for the output waveform for the above filter, if a 5kHz square wave was applied to it.
For this question I am rather lost. Can someone please give me some guidance on how I would display these 2 graphs. I cant find anything in my notes directly explaining how to do this.

1.) To test the filter in the frequency domain is quite logical: Perform an ac analysis using a suitable simulation program, display the magnitude response and verify the correct cut-off at 17 kHz.
2.) What do you expect in the time domain for a 5 kHz input squarewave? Another simulation run (tran analysis) will show a distorted sinusoidal signal at the output.
Then, you can perform a FOURIER analysis to demonstrate that the amplitudes for the fundamental component and for the 3rd and 5th multiple are in accordance with the attenuation values as given in the magnitude response (frequency domain).