I need a high pass filter for my measuring setup and I'am a total newbie to analog filters. I have weak hf pulses (0.5-1.2 MHz, 10 mV) superimposed on lf sine wave (8-40 kHz, up to 5 V), I'm interested in the pulses only and need the best resolution possible.
My first attempt is a 2nd order active Butterworth filter, cutoff 100 kHz, using NE5534 opamp, circuit below. I am not in the stage of using the signals intended or checking the stop band attenuation, just trying.
When using sweep as input signal (blue), I am measuring this on output (green). It looks like a resonance peak at approx. 90 kHz. When I ommit the filter parts (input on +), the amplification is the same for all frequencies; when I ommit the 10k resistors on the right side, I get nice filter with no peak. What causes this behaviour?
I am aware that the opamp is not suitable for MHz signals, but I am still not sure what bandwidth and V/us rate is sufficient for my application.