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I'm attempting to design a basic FSK transceiver (architecture shown below) using only discrete components (transistors, diodes, resistors, etc). My idea was to separate the two binary frequency components via band-pass filters but I'm having trouble figuring out how to get such a high-Q (i.e. narrow-band) effect. Most filter calculators suggest capacitor values in the femto-farad range, which is not possible for SMT MLCC's. Any thoughts would be appreciated.

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  • \$\begingroup\$ This is the very reason why people do not use the direct architecture, but use the quadrature demodulation method instead. \$\endgroup\$ – Ale..chenski Oct 18 '16 at 5:07
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    \$\begingroup\$ More practically, you do not want to demodulate signals at 900 MHz unless they have absurd bandwidth. Downconvert your 900 MHz signal to an appropriate intermediate frequency (with today's IQ techniques that might even be 0 Hz) and only then demodulate. You still might not chose a filter method, but if you do, at least the Q requirement will be reasonable. \$\endgroup\$ – Chris Stratton Oct 18 '16 at 5:09
  • \$\begingroup\$ I'm assuming downconverting after the LNA? The reason I ask is because the BFP181 LNA I'm using has better gain at lower frequencies. \$\endgroup\$ – Biff Oct 18 '16 at 5:13
  • \$\begingroup\$ I'm assuming downconverting after the LNA? If you need to ask that then you really need to study radio architectures a bit more. Also, how much experience do you have at designing discrete circuits working at 900 MHz or even 100 MHz ? If the answer is "not much" then this system will be a huge challenge for you to get working. This is RF, it requires experience. Do you know how much inductance 1mm of wire will have ? If you have to look that up then you do not have enough RF experience in my opinion. \$\endgroup\$ – Bimpelrekkie Oct 18 '16 at 6:16
  • \$\begingroup\$ Why use only discrete components - that seems an unreasonable constraint and if you want help you should justify this constraint. \$\endgroup\$ – Andy aka Oct 18 '16 at 9:42

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