I'm newbie in need of some help. I have a simple low pass filter in spice and I also bread bordered the circuit. Getting some significant differences. The cutoff frequency is roughly 110 HZ. Spice shows about 280 mvpp signal at 110 hz. On my scope i'm getting 100 mvpp. Thats about 1/3 the difference. And when I crank up the frequency on the generator 150, 200 and 300 hz. It's at 300 hz that I notice a signficant decrease.

There are two more screen shots below showing bode plots and multiple sine waves for multiple varistor settings in increments of 20k ohms. I'm not seeing this as well on the scope.

What am I missing?

 

Scope is probably affecting the output.
What model scope. Do you know the input impedance and capacitance of this scope?

 

hi anives,
Checked your circuit with LTSpice, this is what I see, almost identical to your sim.
I agree with'sgh' , suspect your scope???
E

 

Have you accounted for the value tolerance of the resistor and capacitor you are using.

 

Thank you all for replying. sghioto is correct. I have a test fixture that I use for designing guitar effects pedals. The return lead to the scope that I created is 1x low impedance connection. It was definitely loading down the circuit. I connected a 10x scope probe and now its behaving like a low pass filter. Being a newbie it's going to be a long journey but that's what makes it fun. Nice to have a forum like this to get answers to perplexing problems.

 

Have you accounted for the value tolerance of the resistor and capacitor you are using.

No I haven't but guessing I would say +/- 5% for both.