I have designed a 7th-order elliptic filter using this online tool.
Cut off frequency - 15 MHz
Pass band ripple - 0.1 dB
Stop band attenuation - 70 dB
I've asked it to use tge standard inductor value of 2.2 μH.
The designed filter is shown below:
This gives a stop band attenuation of around -65 dB which is good enough for my purpose.
I've also simulated this in LTspice which showed similar performance.
I implemented this filter on a homemade PCB. The main requirement of this filter is to cut a 50 MHz noise from an analog signal. In addition, I expected it to cut 2nd and 3rd harmonics of a signal which has a maximum fundamental frequency of around 10 MHz.
When the filter is tested however, the results I got were very different. The 50 MHz noise was still present considerably. So, I tested this filter using a function generator and oscilloscope. Simply, I connected the filter input to my AWG. Two channels of the oscilloscope were connected to the input and output of the filter. Then I changed the frequency from 1 MHz to 100 MHz gradually in the AWG. It showed good attenuation in 15 MHz range. However, after around 30 MHz the attenuation gradually reduced and stayed almost constant. I measured the peak value of the input and output sine from the filter and calculated the attenuation and it was only around -17 dB instead of -65 dB which is shown in the design.
I would like to understand possible causes for this discrepancy. A few that come to my mind are:
Deviation of actual component values (bought from AliExpress), wrong approach in actual measurement/impact of oscilloscope probes/etc.
Problems in PCB layout causes input to leak to output at high frequencies, issue in the design itself which is not evident in the simulation.