# maximum cutoff frequency possible for high pass filter?

Discussion in 'Homework Help' started by ztigmax, Sep 12, 2015.

1. ### ztigmax Thread Starter New Member

Sep 12, 2015
2
0
An engineer needs to make an RC high pass filter.
He has one 100 PF capacitor, one 300 PF capacitor,one 18 Kohm resistor and one 33 Kohm resistor available.
The greatest cut off frequency possible is?

2. ### theinvisible New Member

Sep 12, 2015
1
0
i think it will use the formulla and the answer will be 25

3. ### MrAl Distinguished Member

Jun 17, 2014
2,562
518
Hi,

Show some of your own work so we can take it from there.

You didnt say if it had to be first order or second order. If it needs to be second order then it will be a little harder to answer.
Do you understand how the time constant relates to the frequency?

We can do the second order filter too if you like, which means we'll have to figure out what resistor goes with what cap, and also see what difference it makes.

If this was a low pass filter the first order comes out close to 34kHz and second order comes out close to 18kHz, but you'll need to figure out how to do this to get more accurate figures for the high pass filter for the final answer(s).

Second order requires making a choice for which resistor value appears in the first stage and which in the second stage, and same for the two caps. See if you can figure out which resistor goes in the first stage and which in the second and same for the two caps.

Last edited: Sep 12, 2015
4. ### ScottWang Moderator

Aug 23, 2012
4,933
777
On this homework forum, the members can't do or given the answer to the TS directly, you only can guiding or asking the TS what to do.

5. ### MrAl Distinguished Member

Jun 17, 2014
2,562
518
Hi there Scott,

Oh yeah you're right, at first i went by post #2 which gave an answer.

I modified my original post to query for some OP attempts and just drop a few hints now rather than give complete results

You may wish to modify your reply too to get rid of the complete answers in the quote.

Aug 23, 2012
4,933
777
Done

7. ### GopherT AAC Fanatic!

Nov 23, 2012
6,319
4,043
I get a value closer to 1.1MHz, why is your value so low for a first order? (34k)

The goal was the highest possible band pass with the four components. Shortest possible time constant (t=RC) means lowest possible R and lowest possible C. To get the lowest possible R, put your two resistors in parallel. To get lowest possible C, put the two caps in series. Put those two sub-assemblies together to make a high pass filter.

Last edited: Sep 12, 2015
8. ### MrAl Distinguished Member

Jun 17, 2014
2,562
518
Hi,

That is good that you brought this question up because now if the OP had read the previous posts they may not provide the right answers so he'll have to figure it out for himself.
I'll update my post too.
1.1MHz isnt quite right either BTW, and according to the Homework Section the answers are not supposed to be directly given anyway.

Nov 23, 2012
6,319
4,043
Yup,

thanks