# low pass filter

Discussion in 'Misc Electronics' started by exxos, Jul 9, 2005.

1. ### exxosGuest

Hi all,

I'm trying to design a low pass filter, though no matter what values I use I
seem to flatten the signal line :-( Can someone give me a clue about what
values I need, I want to filter out anything above about 250khz, anything
under say 200khz to be let though.

I found some online LPF calculators though I can't seem to get those values
to work either :-\

Cheers,
Chris

2. ### CWattersGuest

Is this the same problem - noise spikes on the digital signal?

http://www.muzique.com/schem/filter.htm

3. ### CWattersGuest

The corner frequency for an RC filter occurs when the impedence of the C is
the same as the R.

eg when

1/(2*pi*F*C) = R

Pick R or C and rearrange the equation to get the other

or cheat and use the link I posted.

4. ### exxosGuest

thanks again for the link, It looks like I need 5R with 0.1UF, I was using
100R with 0.1UF, even so I couldn't even get a few hz though it, it seems to
short out my signal generator. 100R with 0.005UF I will try shortly...

It is the same problem as I posted before, I've been on google all day
trying to find a possible cure, the only thing which is closest is the
74LS14, Schmitt device with 0.8 Hysteresis , if that could be like 2V
Hysteresis then that might work. A schmitt sure does help cut down the
problem but far from a cure.

The problem is a just like it is here
http://www.ee.ttu.edu/lab/robot/hyster.htm

Its hard to say how large the spike is, but I know its between 1V and 2V, it
could be a rail to rail spike I know ive had that on the fall). Though its a
bit to fast for my CRT scope to see, the spike seems to be 0.1US, its
filtering that out so its not bounceing on and off when its not supposed to.

I've been playing around with duty cycle, now its a bit hard to explain but
I will try, at say 50:50 ratio the spike is on the rise of the cycle, now
if you adjust the duty either + or - about 10% then you can see the spike
move onto the ground level, OR, onto the high level... When the spike is at
12V it really does not matter since the wave is going via some buffer's
which are ON at about 2.5V so the spike there does not matter really.... On
the LOW spike, the spike Can't be more than 2.5V else the problem wouldn't
go away. But when you adjust the duty you can see the spike moving across
the low level, then slowly moving up the rise and across the high level..

Like I say when the spike is on the low or high its not efecting the
circuit, but when its inbetween (on the rise) its causing false turn
on/off's.

I found a artice on adding Hysteresis to a comparitor, though the values I
worked out I doubt would work without overdriving the comparitor, actually I
am not sure if they even will work at 200khz... probably not....

Cheers,
Chris

5. ### Tom MacIntyreGuest

What is the impedance? What components are you using?

Tom

6. ### exxosGuest

My value was a bit off (my bad) I was using 100nf with 100R, After some
testing and using that page which the other chap posted, What actually works
is 33R with 10nf, it starts cutting off about 100khz (you loose about a
volt)..... If I use 100R with 10nf that still works though not as good, and
the wave ends up looking like a triangle wave and not a square, pluging
values for about 500khz filter seems to work out about best.

Chris

7. ### CWattersGuest

What sort of 100nF capacitor? The tollerance on some types is as high as
80%. If using 100R then the source impedence might also be a factor.

8. ### exxosGuest

its a polyester 100nf, I think philips made them, the orange jobbies. Tried
to find them in farnell but they might not to them anymore.

The best value seems to be 33R with 10nF, give me about 0.5V spikes then, A
schmitt gate should clean up the rest of it...

Chris

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