maximum cutoff frequency possible for high pass filter?

Thread Starter

ztigmax

Joined Sep 12, 2015
2
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?
 

MrAl

Joined Jun 17, 2014
8,510
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:

ScottWang

Joined Aug 23, 2012
7,072
Hi,

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.
...
...
...
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.
 

MrAl

Joined Jun 17, 2014
8,510
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.
 

ScottWang

Joined Aug 23, 2012
7,072
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.
Done
 

GopherT

Joined Nov 23, 2012
8,012
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.

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 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.
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:

MrAl

Joined Jun 17, 2014
8,510
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.
Hi,

Maybe i made a mistake :)
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.
 

GopherT

Joined Nov 23, 2012
8,012
Hi,

Maybe i made a mistake :)
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.
Yup,


thanks
 
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