Quality RF front end


(Mirtcho) #1

The question that biz me these days is – Do we need decent RF receive frontend?
I would like to share my opinion. Remarks tips and suggestions are welcome.
The main objective of Lora transceiver is to be cheap and brodaly available. The specs says that Lora has tremendous Link budget – unfortunately in practice the link budget is much lower. I think the main reason for this is the cheap receive frontend.

  1. Input filters have very wide bandwidth.
  2. Huge power levels near the 868MHz band – mostly from GSM base stations and UHF/DVB-T – channels 55-69. Some are not evident - e.g. my neighbour has wireless home cinema system at 864MHz
  3. Oversimplified Receiver – LNA, Mixer, and digital IF processing . It looks that it uses direct conversion or very low IF – Am I right?
    The questions are : Will I get better performance with decent 868 MHz frontend? Does somebody try this route? Does it worth it?
    With decent I mean
  • High Q input Filter
  • Narrow band LNA
  • Reasonably high IF frequency to reject any mirror channels.

Best Regards,


#2

don’t forget the tolerance of the used cristal.


(Mirtcho) #3

Thanks for the feedback.
The statics errors like frequency shift scares me less since it is easy to calibrate. SX127x has offset frequency registers.
The phase jitter of crystal can be some issues but I assume that Lora chirp modulation can easily mitigate this problem too.


#4

the problem is imho, not many of us have the equipment to measure this, it’s very specialised and sure nothing for hobbyist like me.
another thing is price … we want pricelevels below 10 euro and at the same time tuned high end filters… :wink:


(Nsornin) #5

The SX1272 has very good rejection at >1MHz offset for an ISM band chip. ref datasheet. However, if you try to receive a weak signal close (<50m) to some GSM BTS you may encounter an interference blocking issue. We have seen a few situation like this. In that case the only solution is to insert a SAW filter (863 - 870 MHz) between your antenna and the chip input. The most common problem we encounter are poor RF matching on the end device module (typically hope RF modules have poor matching) , poor antenna radiation patern (no secret , a small antenna has a squashed spider like radiation pattern), and some time totally inapropriate gateway setup. The problem is that LoRa tends to work at a fair distance even when antennas are not connected and i have seen this several time already. You are right the XTAL offset and jitter are taken care of by on chip calibration and PLL.