# Trouble understanding an test question about frequency filters

Discussion in 'Homework Help' started by Tine Ogrinc, Aug 29, 2016.

1. ### Tine Ogrinc Thread Starter New Member

Aug 29, 2016
2
0
I have been studying for a test and have come across this question from one of the previous years tests. I think I was absent when we did this topic as I cannot grasp it at all. I turned to my notes for help, then to some notes I found online, then to google and still no success in understanding how to solve such problems.
I even asked on reddit where I might get a response but the test is in 9h so I would preffer to ask here as well and double my chances of getting help.

I dont really care much for the specific answer to this assignment, more just what this assignment is and how to handle it.

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2. ### MrAl Distinguished Member

Jun 17, 2014
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Hi,

First, you would probably get more responses if you post the picture right side up not sideways as that is harder to view sideways. Also, the writing is a little hard to read.
Also, is there an input to this system or not?

Did you ever analyze a block diagram system before? If not you'll have to look into that first. The approach would be to first simplify the blocks and then calculate the output, then see what gain A it takes to produce either a zero real part or some positive real part. As A changes one way or the other the real part will move toward zero, and when it reaches zero that means the system oscillates assuming that is a dominant real part, and if it moves to a point greater than zero then the output moves toward plus or minus infinity. It's only when the real part is negative is the system considered stable.

3. ### Tine Ogrinc Thread Starter New Member

Aug 29, 2016
2
0
Yeah I know about the sideways thing, didnt think of it late last night, was tired.

There is no input, only what is depicted.

Luckily this wasnt on the test, and I also found a classmate to explain it to me.
The p both the blocks is meant as derivative. You are supposed to multiple the two blocks together to get Z, then do some maths, and replace every p with d/dt. Then swap Z with e^(something) (the standard form for any impulse), and then you can calculate A for a stabile circuit.
Thanks anyways