ELRS Gemini Xrossband TX
- Ishman Mann
Owner: @Ishman Mann
ToDo:
Come up w/key features and Modify the schematic by @Nolan Haines for groundside TX applications
Create a corresponding layout
Table of Contents
Background
This board will take the best parts of ELRS Gemini TX and ELRS Redundant Diversity RX, and combine them into a groundside TX. Likely this board won’t use Diversity TX (which doesn’t really exist), but Xrossband.
Ideas
something like Xrossband would be pretty cool. It's the next step after Gemini in terms of TX. Basically have two Geminis on the board, with one at 900-910 MHZ and the other at 2.4-2.44GHz. ExpressLRS Gemini Xrossband: The Future of Dual-Band FPV Control - FpvGuru . Hardware wise it could use the same LR221s, main differences would be in firmware
Key references:
LoRa
Needs Assessment
Top goals
Current sense on the tx
Have 2 Geminis (4 LR221s) on the board for Xrossband. Make it compatible with the RX board.
Improve upon the ELRS Gemini TX.
Port over existing features and add new ones
Design to solve/avoid the issue it was having!
Easily programmable
Configurable over UART, WIFI, etc…
Constraints
Size and weight are not limiting factors
Architecture
I probably don’t need the ESP 01F because Xrossband has dual band anyway. But there is a chance I will keep it.
(possibly) Need to have high output power, eg 1W. (But no off the shelf RX can support over 250mW telem). Need to determine appropriate power arch and if the current LR221s are good enough.
→ Seems like the current LR221s on the ELRS Redundant Diversity RX has a TX power of 15dBm max for 900 MHz. Is this enough? DEFINITELY NOT, we need something like 27dBm or 30dBm
^But what about this high gain antenna thing?? → I believe its more directional than a normal antenna, but its worth looking into
Solid option: use an RF amplifier - there are plenty of options and this needs a decision matrix
per https://www.mouser.ca/pdfDocs/LR1121_data_sheet_V1_1.pdf,
Given LoRa’s strict noise requirements, a LNA (low noise amplifier) is needed??? Rx to TX side??
For low frequency amplifier (either 8dBm or 15dBm to achieve 30dBm):
the bigger question is how to use LoRa in a high noise environment, and the solution may lie beyond just power amplification
Another question is if LoRa is already good enough to operate in high noise environments
“operates below noise floor“??
Considering all the laws and technicallity of LoRa, I need to think more deeply about whether to amplify or use some other technique to extend its range in high noise environments
https://chatgpt.com/share/67b8250a-7ba0-800f-bc30-4b8ed33c48f4 ← key discussion
Carefully draft a power architecture. The one in the diagram below is just rough with not much thought yet. Definitely want good isolation where needed, and need to consider power demand
consider dual passband filter of somekind
https://blaauw.engin.umich.edu/wp-content/uploads/sites/342/2017/11/472.pdf
If this can somehow be implemented, it would be a lot more cheap than a tunable filter (which I would never add because they’re so niche and expensive)
Im probably not going to add a ESP-01F backpack. It would require having one on the drone side reciever too, which would add to board size on the drone. Likely it wont happen. Xrossband uses 2 frequencies anyway, so why even change it midflight. Thats overkill.
Note about LR221 modules' pins:
Component Selection
Test/Validation Plan
Layout Considerations
Microstrip vs. Coplanar Waveguides
ENIG has more losses than bare copper for coplanar waveguides
RF Microstrips and Ground Plane Clearance: How Close is Too Close?
Note: the main point of a ground pour is manufacturability and shielding, but it may not be desirable everywhere if we want a microstrip configuration instead of waveguide