FM band undersampling

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 internal filter always and to add externally LP or BP filters

RX888 internal HF LPF

I modified the scheme as follows:

Here the picture of the modified pcb area. You need a SMT microscope and good SMT skills  to do the job. You can destroy your RX888! You make it at your own risk.

I removed C52, L7, C55, L5  and made a wire jumper between C52-L7  and C55 - L5.
I soldered the removed component to an unused or ground pad so that can be used in future to reverse the modification.

Note: when opening the RX888 case pay attention to the thermal pad under the PCB. It transfers the heat of the ADC to the box. Slide the PCB very carefully and move the pad with a plastic or wood stick very gently. 

I placed a LNA of 20dB after the band pass filter https://sdr-prototypes.blogspot.com/2021/03/filter-in-box.html.

HDSDR does not know the Nyquist's harmonic  band we are using and the "calculate" button for the correction cannot be used.  I select the RF front-end frequency options dialog and

set SDR hardware in Down/Up converter and set the offset with the correction you want to apply.

Please remember to select the SDR hardware connected to antenna (default) in RF front-end frequency options dialog to return to use HF or VHF in the original way.

Update 27/03/21

Using Pluto+ as generator I measured that at 110 MHz without LNA and filter my RX888 modified gain has a loss of -16 dB referred to HF band, while at 145 MHz the loss increases to - 23 dB.

A good LNA allows undersampling with RX888 till 144 MHz band.