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. https://fpvguru.in/blogs/expresslrs-gemini-xrossband-the-future-of-dual-band-fpv-control/ . Hardware wise it could use the same LR221s, main differences would be in firmware

Key references:

• ELRS Gemini TX

• ELRS Redundant Diversity RX

Needs Assessment

Top goals

• 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 Xcrossband has dual band anyway. But there is a chance I will keep it.

• Need to have high output power, eg 1W. Need to determine appropriate power arch and if the current LR221s are good enough.

• 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)

Test/Validation Plan

Layout Considerations

• https://www.youtube.com/watch?v=SwNb0ndIT5k&ab_channel=Rogers%27AdvancedElectronicsSolutions

  • ENIG has more losses than bare copper for coplanar waveguides

• https://resources.altium.com/p/microstrip-ground-clearance-how-close-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