My RTL-SDR V3 is being overloaded by an AM broadcast so I'd like to build a high-pass filter to block the AM broadcast band. I thought I'd start simple and try making the one shown in Fig 3 of "Dealing with AM Broadcast Band Interference to Your Receiver":


Concerning the inductors... The document talks about using T50-2 or T50-15 but I only have T37-2 and T37-6 on hand. I can use https://toroids.info to figure out how many turns I'd need on the T37s to make 3.3 uH, but is there any particular reason to choose one or the other of the T37s I have? The fact that the author of the document seems fine with T50-2 or T50-15 makes me think that the "optimum resonant circuit range" of the toroid isn't very important in this application.

Does this filter design resonate on lower frequencies, meaning it would be better to use the T37-2 which has an optimal range of 250 kHz to 10 MHz?

Thanks for any insight you can provide!

  • $\begingroup$ What frequency are you listening to with the RTL-SDR? The reason I ask is that filter you show might be overkill. It would be useful to separate say a 1 MHz AM transmitter from the 7 MHz ham band, but if you're happy to filter out everything up to 50 MHz or so, then a second order filter (just one L and C) will probably be plenty. Also a filter made to pass HF might start looking funny by 100 MHz and will certainly be awful at 500 MHz. Toroids aren't appropriate there at all. $\endgroup$
    – tomnexus
    Dec 11, 2021 at 5:16
  • $\begingroup$ Thanks for your comment, @tomnexus! Having only recently discovered that RTL-SDR V3 can tune HF, that's the frequency range I'm trying to listen to. $\endgroup$
    – Bezewy
    Dec 11, 2021 at 17:45
  • $\begingroup$ I would recommend you to go with yellow/clear. You want to have less losses at higher frequencies for efficient filter. $\endgroup$
    – k1zmt
    Dec 25, 2021 at 2:05
  • $\begingroup$ @k1zmt What did you mean by "yellow/clear"? The color of the cores? $\endgroup$ Dec 26, 2021 at 22:58
  • $\begingroup$ @MikeWaters, yes. I meant T37-6. $\endgroup$
    – k1zmt
    Dec 27, 2021 at 20:14

1 Answer 1


Simulating the circuit with 50$\Omega$ load and source impedance yields no resonant peaking behaviour.

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Also a filter made to pass HF might start looking funny by 100 MHz and will certainly be awful at 500 MHz. -- @tomnexus

Yeah, that will definitely be a problem with this topology. If you are trying to operate >100MHz things don't look too good (Simulated with capacitor ESL = 1nH, inductor parallel capacitance = 10pF, inductor parallel resistance = 10k$\Omega$. I've wound some inductors in the past, and find I typically got a few picofarads of inter-winding capacitance, but this will vary drastically based on turns and the turn spacing. For caps I'm assuming ceramic SMD, 1nH might be a slight overestimate based on package size/ how compact the filter routing is.

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