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SAW Filter in RX path

Hi,

We are developing a new version of 868MHz RF module based on SX1276 IC. I asking myself if I need to mount a SAW BPF on the RX path. On SX1276 semtech reference design there is a footprint for a SAW filter but is always replace by 0 Ohm jumper. However I read in AN 1200.04 :

“Semtech recommend that for most applications, provision be made for a SAW filter since ISM band
devices share frequency spectrum with GSM/GPRS which operates close to the 863 - 870 MHz;”

In EU this is true for GSM900 UL (876 to 915MHz) and also for LTE800 UL (832 to 862MHz).

The major advantage of SAW filter is that it will improve OoB (Out-of-band rejection) and mains drawbacks is price and additional insertion loss.

What is your opinion ?

Is there a recommended SAW filter model ?

Best regards

Hi,

Ultimately the selection or otherwise of a SAW filter will be application dependent.From analysis and in the presence of an LTE signal, the blocking immunity of the SX127x is consistent with published datasheet specifications and is limited due to compression in the RX chain rather than intermodulation products. It should be noted that an important consideration when considering multi-system co-existence and blocking immunity is the distance (m) between the interferer and victim receiver as well as the absolute blocking level (dBm).

With reference to LTE Band 8 the lower UL band-edge is 880 MHz (EARFCN 21450), although it is noted that ERC 70-03 references NB ER-GSM in the 873-876 MHz band. However, the concern with LTE Band 20 UL will be the noise skirt of the upper EARFCN that will bleed into LTE channel guard-band and the 863-870 MHz band and which will be difficult to filter.

In general a SAW filter will provide additional blocking immunity, especially should a LTE channel occur at +/-32 MHz from the wanted channel should any digital or reference oscillator noise be observed.

Currently Semtech uses the RF360 B39871B4377P810 SAW filter in its European LoRaWAN GW reference designs. However, other suitable SAW filters are available from multiple vendors and it is recommended that you work with your local manufacturers’ representatives or distributors in selecting the optimum filter for your application.

Regards,
Semtech Support