# low pass filter

Discussion in 'Homework Help' started by Viktor Vaughn, Apr 10, 2015.

1. ### Viktor Vaughn Thread Starter New Member

Mar 21, 2015
24
0
My homework question wants me to design a low pass filter with the gain of 5 and an adjustable roll-off frequency between 500Hz and 1KHz. I already calculated which resistors to use for the gain of 5. My question is, am I using more than one pot for my adjustable roll-off?

2. ### Papabravo Expert

Feb 24, 2006
11,125
2,172
I don't know if you're using more than 1 pot or not. We can't see your schematic diagram.

3. ### Viktor Vaughn Thread Starter New Member

Mar 21, 2015
24
0
There is no diagram.

4. ### WBahn Moderator

Mar 31, 2012
20,221
5,747
How can we tell what you are and aren't using since you haven't posted a schematic. Show your work.

Feb 24, 2006
11,125
2,172

6. ### WBahn Moderator

Mar 31, 2012
20,221
5,747
Unfortunately, this is Crystal Ball Tune-up Month and all of us have sent our units in for calibration. My back-up mind reading devices haven't been doing too well, lately, either. So you might have to actually resort to communicating your circuit to us by posting a diagram.

7. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,673
1,536
Interesting. What are you going to do with those resistors?

8. ### MrAl Distinguished Member

Jun 17, 2014
3,726
789
Hi,

If there is no diagram then how did you calculate any resistors for the gain of 5?

In other words, we need to see what kind of circuit you intend to use, unless of course you would like to see other circuits that might do this, but then a question is what resistors did you calculate already.

A couple quick examples:
1. Sallen Key
2. Passive with amplification

What we also dont know is the required order of the filter, so we'll assume first order unless otherwise specified.

What we know so far:
1. Gain of 5 in the passband.
2. Adjustable cutoff frequency of 500Hz to 1000Hz.

For a quick example of a first order passive which you can add a pure gain to, we have an RC circuit where the resistor is connected to the input and the output is taken from across the capacitor. The cutoff frequency is multiplicatively related to the RC time constant, so if we have a 500Hz cutoff with a given resistor value we would have 1000Hz cutoff with a resistor that is 1/2 that value. This means a simple pot would have to adjust from some resistance R to some other resistance R/2, or better yet, have a fixed resistor in series with it so one end of the pot gives you R ohms and the other end gives you R/2 ohms. You then could use a pure gain amplifier after that to obtain the gain of 5 in the passband.
The value of R is found from;
w=1/(R*C)

where w=2*pi*f.

So after choosing a value for the capacitor (say 0.033uf for example) calculate R for the lowest frequency, then subtract a fixed resistor of value one half that value, half the value of R for the pot, then add that fixed resistor in series.

Note this is only ONE way to do it.

Last edited: Apr 11, 2015