| Electronics > Projects, Designs, and Technical Stuff |
| Bandpass filter - ltspice vs real life |
| (1/2) > >> |
| injb:
I'm learning LTSpice, and I am confused about why this simulation gives different results from real life. The circuit is an opamp bandpass filter for an engine knock sensor. I believe that knocking on this engine would be in the 5.7Khz range. I hooked up a function generator to the input and swept the frequency from a few Khz up to around 12Khz, and what I found was that at around 5.5 the response suddenly goes up quickly, and keeps increasing until around 7Khz, and then drops off very quickly above that. So, it seems to be centered around 5.5 - 7 Khz, which is a tad higher than I expected, but close enough. But the spice simulation of the circuit shows that the filter should be most sensitive around 4Khz. So I'm trying to understand what other factors affect this that I haven't taken into account. The capacitors are marked only as "0.01", but I think it has to be uF because otherwise I'd be off by a factor of 1000 which could not work. The resistor values, I measured. I'm attaching screenshots of both decibel and linear scales (I find the linear one clearer) Any ideas where I've gone wrong? EDIT: changed the screenshots to correct a mistake (which didn't make any difference to where the filter is centered) |
| T3sl4co1l:
Huh, what type of "0.01" capacitors? What supply? Also, what supply and GBW is the amp in the simulation? Don't tell me you used the three-terminal ideal amp! :palm: Tim |
| Mechatrommer:
every wire is an inductance, no perfect capacitor without ESR, and no exact value of 0.01uF and 9K ohm in real life. |
| injb:
--- Quote from: T3sl4co1l on April 20, 2020, 03:06:09 am ---Huh, what type of "0.01" capacitors? What supply? Also, what supply and GBW is the amp in the simulation? Don't tell me you used the three-terminal ideal amp! :palm: Tim --- End quote --- --- Quote from: Mechatrommer on April 20, 2020, 03:08:46 am ---every wire is an inductance, no perfect capacitor without ESR, and no exact value of 0.01uF and 9K ohm in real life. --- End quote --- Yes I did use the 3-pin amp - is that a no-no? The actual one wasn't in the list (LM2902). The caps are marked as "0.01 100-WIMA VN" and "5%". The resistor - I checked using a color code calculator (https://www.allaboutcircuits.com/tools/resistor-color-code-calculator/). It's white-black-white-brown-brown which comes out at 9.09k. I also measured it and got approximately that value. |
| JimRemington:
The LM2902 is not even audio quality; performance starts to degrade abruptly at 5 kHz. |
| Navigation |
| Message Index |
| Next page |