I need to design a band-pass passive filter with pass band 10kHz~150KHz. One of the needed inductor is 2.2mH. I find there are two choice: Axial type or Can type. But it seems they are designed mainly for EMI filter. May I use them in signal chain filter?
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\$\begingroup\$ Look at Stackpole Ferrite Core, with low-loss materials. \$\endgroup\$– analogsystemsrfCommented Sep 19, 2018 at 3:57
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\$\begingroup\$ Are you dealing with tiny signals? The self-shielding (can-type) might reject stray magnetic fields better than axial. A high self-resonant frequency would be preferable. \$\endgroup\$– glen_geekCommented Sep 19, 2018 at 4:26
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\$\begingroup\$ Why 2.2mH? What source/load impedance + AC current? This range tends to have low SRF and high DCR or are too big. Any specs on phase shift? or group delay or bandstop slope? \$\endgroup\$– D.A.S.Commented Sep 19, 2018 at 4:53
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\$\begingroup\$ The input signal is low signal of several mVs to 20V. Source impedance about 50Ohms, input impedance of the back end is about 1 MOhms. Yes, the above two inductor both have relative low SRF, only around 1MHz \$\endgroup\$– divergerCommented Sep 19, 2018 at 5:59
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\$\begingroup\$ The Q is usually poor at 2.2mH, which also tend to have high leakage capacitance, unless it is a huge toroid with heavy gauge wire. But it may be ok with the tiny current your working with. \$\endgroup\$– user105652Commented Sep 19, 2018 at 5:59
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1 Answer
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For a 50 Ohm filter, the 2.2mH choke you linked has 7.5Ohm "may" shift a breakpoint up 1 order of magnitude near 1kHz, resulting in 20dB less attenuation below that. Other chokes are available (at higher current ratings not needed) with 0.25 Ohm that will work better for low-frequency rejection. https://uk.farnell.com/hammond/1537n/rf-choke-500uh-5-3-2a-axial/dp/2759969
BTW you can get a perfectly flat Bessel PB response with a 50 Ohm load, albeit -6dB unless you buffer the input to ~ 0 Ohms Here's a simple 4th order approximation.
You could also make it an 8th order active filter. with a quad OA with no chokes.
