Owner: Ishman Mann

This will be a groundside TX version of the ELRS Redundant Diversity RX

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

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Key references:

• ELRS Gemini TX

• ELRS Redundant Diversity RX

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.

Image Added

• 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

    Image Added

  • ^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

• 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)

  • 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

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