LC BPF experiments

Revisiting the RTLSDR CW Skimmer with a dedicated mini pc required some rethinking of the input stages/filtering when it came to thinking about multi-band support.

Other multi band skimmers appear to have either gone with completely wide open front ends , or multiple antennas/cascaded combiners. With 8bit sampling and a single antenna (active loop) I needed to take a more selective approach to protect the front end.

Inspiration came from looking at papers on multi band front end filtering for 4g networks. It turns out you can easily parallel BPFs with relatively minimal downsides, the main one being reduced stop band attenuation between BPF center frequencies, but it wins in simplicity. I had never contemplated this approach, and have yet to find anything in the amateur literature on it.

7MHz & 10.1MHz BPFs. Each 200kHz BW
Response from my simple scalar analyzer

First up, while the above is calibrated, so that 0dB is at 0, the two little bumps at 3.5 and 5MHz indicates that the output amp has minor but noticeable second harmonic output. Adjusting for that, I’m guessing the actual insertion loss is less than 1dB and probably 2-3dB for 10MHz. Know your test equipment and it’s limitations.

The second learning, was that if you’re looking at 100-200kHz bandwidth filters at these frequencies, toroid size is actually critical. FT37 will be marginal at best, FT50 will be easily sufficient. charts are also available online plotting Q vs frequency, turns and core type. So one improvement will be to swap the FT37-6 one of the filters for larger cores.

The other, again related to Q, is that capacitor selection is also important. NP0 or silver mica. Trying generic ceramics resulted in 20dB of insertion loss! Probably not anything new for anyone experienced, but nice to see this play out visually, and impressive the difference.

The other test was to attempt to measure the Q of the inductors with the SNA. Tried two approaches, the working one being lightly coupling (4.7pf) to the LC circuit and measuring 3 points of the frequency response curve. Calculated a Loaded Q of 175 for the FT37-6 at 11Mhz which matches closely with a unloaded Q of 190 as expected from other sources. It may be crude, but the SNA is a workhorse.

So time to rebuild this in the new front-end enclosure for the skimmer…

3 thoughts on “LC BPF experiments

  1. Interesting approach, agree it should work quite well for this application. Curious why the size of toroid matters for a RX application as size is usually only a consideration for power handling (core saturation) which shouldn’t be an issue? Also, I think you mean T50-6 (FT is generally the prefix for ferrite toroids rather than iron powder).

    • Well spotted on the FT. Yes T50-6.

      It’s in relation to loaded vs unloaded Q. A rule of thumb I’ve seen is that the unloaded Q of the components needs to be 3 times that of the loaded Q for reasonable insertion loss (at least for Butterworth). Inductors on T37-6 cores max out at around 190 around 10MHz, T50-6 it appears easy to hit 250 which would be sufficient for a 100kHz wide filter in the mid HF bands.

      If you look on line for “Q-Curves for Iron Powder Cores” from micrometals you’ll find an interesting trove of information for optimising inductors on their materials. My crude measurements seemed to align very closely with their graphs.

      This is the second time I’ve seen size matter for RX. The other was in relation to IMD, 6/51/6.52 in EMRFD – the preselector for the Triad Receiver. Again they emphasise unloaded Q.

      https://martein.home.xs4all.nl/pa3ake/hmode/bpf_intro.html delves into this subject. In this case not even T50 is necessarily sufficient for the most extreme situations.

  2. OK, yes, that makes sense, it’s easy to forget that core losses are just as significant for RX as TX. The Q-Curves document is a great resource that I hadn’t come across before, thanks for the pointer!

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