In my circuit:
C1=C2=47nF
R1=R2= 100K (100K||39K for engine balancing)
R3= 200K
R4=100K
If R4 is infinite (disconnected) then A = 1 + 200K / infinite = 1, which is what I measure.
Q = 1/(3 - A) = 1/2.
With R4 connected (100K), A= 1 + 200K/100K = 3.
Then Q = 1/(3 - 3) = infinite, which is probably why it oscillates.
If R3=R4=100K, A = 1 + 100K/100K = 2 and Q becomes 1 / (3 -2) = 1 and probably your 12db/octave.

With Steve's confirmation about using two channels, I'm now hopeful I can make a cheap prop balancer using the ADXL335 (or similar 1G accelerometer), two op-amps (LM358 or better), Arduino Nano and a 2x16 LCD character display.

If I get it working and there's interest, I'll show the rest of the circuit and post the program. Second half of the LM358 is used as a 100x amplifier with 2.5V DC offset (the Nano has a 0-5V A/D input range). Need to feed the LM358 6.5V or better (I use 12 to 14V) in order to get 0 to 5 V voltage swing.

(Why am using the LM358? Bought 5 or 10 from MPJA.com to make pulses for my tail and wing LED strobes.)

Finn

On 3/19/2021 3:09 PM, Charlie England ceengland7@gmail.com wrote: