Dear community,
I am using the MM8108 USB dongle on RPi4. BCF bcf_mf15457.bin
As discussed previously in the forum, I designed an external RF Front-end for this dongle. Sucessfully extracting the TXRX signals from the module MF15457, now I have 2 seperate channels as a photo below.
By adding a LNA to the RX channels, I found out that that noise is increased by the gain of the external LNA which is about ~20dB. This is unexpected. The RX signal remains unchanged, even to reduce slightly.
My theory is that having 2 LNAs cascading: 1 externally and 1 internally inside the MM8108 would adding the noise figure causing SNR reduced significantly.
I wonder if there is a way to disable the internal LNA or bypass the internal LNA. As far as I know, this can be done in the MM6108. Is it feasible in the MM8108?
Thanks and regards,
V.
Heya @bigboy061293,
Unfortunately there’s not a way to force bypass the internal LNA. The chip’s gain control should handle this for you. The MM6108 lacked an internal PA and LNA so there wasn’t anything to disable or bypass.
Since you’ve got cascaded LNA’s you could be looking at some looking at some multipath since the signal will propagate at the module’s connection as well as from the external LNA.
Also are you using any shielding? We’ve noticed from your pictures that your module’s can is missing, which is definitely a non starter for the receiver’s signal integrity.
Hi @michael.mccandless
Thank you for pointing these things out.
About the shield, I removed it to extract the TXRX signal. I will put it back for another test.
Assume this it the multipath problem, can you suggest some position to shield?
Thanks,
V.
No worries! Even for testing on our end we do all we can to keep the cans on our modules, definitely leads to some… less than desirable test results, especially when we’re chasing noise 
You’ll pretty much want to shield your whole rx/tx path from the module to the antennae, a few layers of tin foil should do the trick for testing, especially at the interconnects. Be sure you use some non conductive antistatic layer, eg kapton tape, on first so you don’t risk shorting anything.
I’m definitely interested to see where this is headed, if the datasheet I found for that LNA module is correct at 0.23dB NF the results would be pretty impressive. Definitely too much carefully applied RF magic getting it balanced correctly for my tastes though!
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