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I am looking for ways to improve reception between two of my devices. The communicate on the 868 MHz ISM band with a 2-GFSK modulation.

The problem is that one of them is heavily shielded by metal (and I cannot change that) so reception is pretty bad.

I am using the CC1310 from TI and they recommend their CC1190 as a front-end to improve performance. In the reference schematic of the LAUNCHXL-CC13-90EU which uses a CC1310 and a CC1190 they added a SAW filter (B3725 from Qualcomm) between the CC1310 and the CC1190 RF front-end. I suppose to improve selectivity.

My question is: When do you use SAW filters? I can see that they improve selectivity a lot, but they also add insertion loss. So the overall signal is even weaker. Does it only help when there is a lot of noise on nearby frequencies or does a filter like that improve always performance?

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You've got it right already:

Does it only help when there is a lot of noise on nearby frequencies or does a filter like that improve always performance?

It helps by attenuating strong out-of-band signals. In your case, cellphone signals from 880 MHz upwards are the problem, and in an office or factory these could be much stronger than the signal of interest.

The filter is designed specifically for this, according to its data sheet:
filter transmisison

It doesn't always help. In a quiet environment adding these filters will cost 5 dB of link margin and make things worse. But in a noisy environment where the receiver is being overloaded, they could make a big difference.

Before building a new board just to test this, have a look at the environment, 800-960 MHz, with a spectrum analyser set to max hold, and see how noisy it is. Turn off the one you're testing, leave the other end of the link turned on. If the unwanted signals are more than ?20 dB? stronger than the remote end, then the filters might help.

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