I use a LoRaWAN transceiver operating in the US915 band (902-928 MHz). In addition, cellular frequency bands operate from 869-894 MHz, very close to the US915 band. How worried should I be about potential interference? I had a consultant tell me that I need a SAW filter on the RX front-end to knock the interferer down, but every design I have ever seen that uses the SX126x transceiver does not include such a filter.
Any responses to this issue would be greatly appreciated.
I’ll be “the guy” who responds to his own post…
Looking through the forum, I noticed a post that was asking the same question about the EU868 band. It referenced a Semtech app note AN1200.04 that said:
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; and
with fixed radio networks at when operating in the 902 - 928 MHz band.
The app note is no longer available on the Semtech website as far as I could tell (it is available here). The date on this document is 2006 (17 years old). The title of the document is “RF Design Guidelines:
PCB Layout and Circuit Optimization,” and it appears to have been superseded by documents such as AN1200.66 and the various application notes with titles such as “Reference Design Explanation.” None of the newer documents mention using a filter to reduce the potential interference of cellular traffic. The SX1276 reference design has a PCB footprint for the filter, but the corresponding application note, AN1200.19, does not reference using a SAW filter at all.
I can only assume that early on, the idea of cellular interference was theoretical and not actually observed. Thus, as the original author noted, a zero ohm resistor replaces the filter.
It could also be that manufacturers (such as Semtech and me, for that matter) do not want to include the filter because it makes the device specific to a certain operating region; by not having a filter, a single part number could cover most of the world instead of having a unit for the US and a different unit for Europe.
If this is a real issue, having an application note that directly addresses the problem would be good.
Most all LoRa modules include the necessary TX antenna matching components that effectivly restrict the module to particular bands such as 433Mhz, 868Mhz, 915Mhz etc.
That is standard RF design practice, most all RF modules, including ones using Semtechs LoRa devices follow it.
I’m certainly no RF expert and I’m sure you would know better. Do you include a SAW filter in your LoRaWAN designs?