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Why Use Oversampling when Undersampling Can do the Job? - Texas Instruments  :    “..If we use the sampling frequency less than twice the maximum frequency component in the signal, then it is called undersampling. Undersampling is also known as band pass sampling, harmonic sampling or super-Nyquist sampling. Nyquist-Shannon Sampling theorem, which is the modified version of the Nyquist sampling theorem, says that the sampling frequency needs to be twice the signal bandwidth and not twice the maximum frequency component, in order to be able to reconstruct the original signal perfectly from the sampled version.“ Justin Peng tweeted the result of his RX888 undersampling the FM band on  https://twitter.com/fei666888/status/1363859596784267265 At my location the FM band stations are very strong and internal HF LPF of RX888 is not enough to cancel the FM band alias. I need to add an external HF Low Pass Filter with higher attenuation of FM band. So I decided to bypass the ...

 In 2018 I read this paper  Band Pass Filter with Low Insertion Loss for 2m   by Dipl ing Tasić Siniša –Tasa YU1LM/QRP, I made my  PCB version for SMD components with a ground plane dimension of 50 x 20 mm so that  a  PCB of 10 x 10 cm  houses 10 filter boards . I mounted my first prototype with tin shield box and verified that the filter was fine in 2018. The point was that the  spacing of the inductors' turns requires some instrumentation to do easily the tuning. The tin shielding soldered box I made was quite laborious for me.  So the pcbs has been unused for two years. I placed the PCB gerber files and scheme in  repository   https://github.com/ik1xpv/ExtIO_sddc/tree/master/hardware/Filters Now with the very nice  SATSAGEN  by  Alberto IU1KVL the tuning of filter is fast and easy,  see  https://youtu.be/yjobY6vs4z0 .  SATSAGEN  requires  a   ADALM-Pluto    or...