Inductors for LC filter

[felis]

Hello,

I'm designing an LC-filter for my project. It is 7pol low-pass Butterworth with corner frequency of 20MHz. After calculating I'm getting inductor values of ~1..3uH.

Looking in my Mouser catalog I can see that for such inductors the test frequency is usually ~7-10MHz. It's often 25MHz for lower values (tens of nanohenries). Do I need to derate inductance given at lower frequency and if so what's the procedure?

Thank you,
Oleg.

 

[Roff]

I haven't checked the Mouser catalog, but the mfr's datasheet should have minimum self-resonant frequency. From this, you can calculate the maximum equivalent parallel capacitance, then add this to a simulation model to see how it performs.
I suspect that you might have better success with a lower impedance filter. Are you trying to make one with unequal termination impedances (unterminated, in the extreme case)? I seem to recall this makes the inductance range across the filter more extreme, and also makes component tolerances more critical.

 

[felis]

Thanks, I'm starting to understand it now .

 

[felis]

I'm playing with this filter in LTSpice. As far as I understand the procedure, I need to calculate the values for inductors and capacitors, then replace them with close real world values, then model and see if the skirt is still where I want it to be. Is this correct or am I missing something?

Also - what is the easiest way to measure SRF of an inductor?

 

[stevez]

Not sure if this is much help but there's quite a bit of information available in amateur radio publications - Radio Amateur's Handbook published annually by ARRL. As might be obvious, the publication is aimed at the amateur radio world. Construction techniques are provided as well as advice on how to deal with the difference between theoretical values and reality. I have constructed a number of the filters - they do work with the limits of my ability to measure and observe performance.