I have an mBed Lora 915Mhz shield I attached to a Raspberry Pi. I had some questions about the feeder network. In the attached schematic. Why isn't a simple cap/inductor pair sufficient for an impedance match in this case? Also, they use a capacitor and inductor in series. Is this a resonant band pass filter? Then why is the resonant frequency 1.4Ghz (22pF/5.6nH) and not 915Mhz? I checked the BOM and half the components are DNP. I attached a spice simulation with the actual component values.

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Here is the simulation with the resistor removed:

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1 Answer 1


They needed a 7th order filter to achieve > 50dB /octave LPF, perhaps with linear phase.

Each pole is staggered to achieve the desired Q and phase response overall and not simply a 2nd order resonant BPF to achieve a flat group delay in the area of interest.

You can see each pole by removing the 50R load. These will be spaced over the phase control region possibly from fo/2 to 3fo for this filter.


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