12-11-2008 12:53 PM
12-11-2008 01:45 PM
Using the bode analyzer is pretty easy. Make sure that:
1 - You have an AC_POWER source on the input node of your filter
AC_POWER is in the Master Database under the Sources group and POWER_SOURCES family
2 - You are powering, biasing and loading your amplifier properly
3 - You connected the IN + terminal of the bode analyzer to the input node of your filter
4 - You connected the OUT + terminal of the bode analyzer to the output node of your amplifier
5 - You connect the IN - and OUT - terminal of the bode analyzer to ground (unless you want to measure differential voltages)
6 - You are using the interactive simulator (i.e. hitting the green play button or flipping the simulation switch or pressing F5)
7 - Double click on the bode analyzer to view results
Open the RFAmplifier example to see an example of the bode analyzer in action.
12-11-2008 02:00 PM
12-11-2008 02:11 PM
12-11-2008 03:31 PM
It looks like to me that you connected the analyzer correctly.
You may need to zoom out vertically in the bode plot. Try changing the upper limit of the vertical axis to 60dB instead of 0dB (what it is by default). See attached screen shot.
12-11-2008 03:42 PM
12-11-2008 03:49 PM
Unfortunately, I won't be able to help you with that as active filter design is outside my realm of expertise, but other people on this forum would probably know better and could help you.
I hope the bode analyzer is working fine for you. Let us know if you have any other questions.
12-14-2008 07:59 PM
I'm assuming you're trying to build a Sallen-Key 4-pole bandpass, right? I didn't look too closely at the values in your circuit, but this app note should help you: http://focus.ti.com/lit/an/sloa024b/sloa024b.pdf
Without knowing your specifications, it's unclear what frequencies you're aiming for. Dissect the circuit by stage until you get something reasonable.
BTW, the input on the bode plotter should be connected to the output of V5, not after R2. That doesn't seem to be your problem though. I also switched out the 741s with virtual opamps, but your cutoff frequencies seem to be off. You're seeing some strange effects from device limitations <1Hz and >1GHz.