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ELRS Gemini Xrossband TX

ELRS Gemini Xrossband TX

Owned by Ishman Mann
Last updated: 2025-05-04

Owner: @Ishman Mann

ToDo:

  • Curve RF traces (if higher fab cost is acceptable)

  • consider a generic config that supports existing firmware, is there one that uses only 2 LR221s

  •  

  • Come up w/key features and Modify the schematic by @Nolan Haines for groundside TX applications

  • Create a corresponding layout

  • I should rotate U11 90 degrees clockwise

    • image-20250418-050102.png

       

 

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

 

 

 

 

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.

 

 

 

 

 

 

 

  • 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

 

 

 

 

 

  • 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:

image-20250309-083237.png

 

Component Selection

 

 

Xrossband TX component research:

 

https://www.mouser.ca/pdfDocs/LR1121_data_sheet_V1_1.pdf

image-20250321-060240.png

 

Filters:

Following a dual passband filter system, we could implement LPFs on each transmission line – however, for the lf transmission line, it appears acceptable to use a DC blocking capacitor arrangement instead of a filter (for the selected amplifier below).

Based on the confluence, I am making the assumption that the hf transmission is operating at 13dBm and the lf at 22dBm (option for 15dBm). 

Notes I’ll follow when making a selection:

  • Passband frequencies: 2.4-2.44GHz, 900-910MHz

  • Power handling requirements:

  • 22dBm for lf

  • 15dBm for hf

  • Insertion loss: must note when choosing amplifiers

  • Impedance 50 Ω

 

Examples which meet these considerations:

 

For HF: look for a mid frequency of 2.44Ghz:

https://www.lcsc.com/product-detail/SAW-Filters_Taiyo-Yuden-F6HG2G441EG65-J_C2651012.html  NOTE: Find one with better stock/ read amplifier datasheets more clearly.

Picked for it’s low insertion loss (1.0dBm), large operating temperature range and adequate power rating (+24dBm) NOTE: THIS MUST BE PLACED IN FRONT OF THE AMPLIFIER

Will require initial tuning

 

 

For LF: look for a mid frequency somewhere in the low-mid 900s:

https://www.lcsc.com/product-detail/SAW-Filters_Murata-Electronics-SAFFB942MFL0F0AR1X_C91653.html Will no longer be needed with use of selected amplifier.

Picked for it’s low insertion loss (1.5dBm), large operating temperature range and adequate power rating (+15dBm) NOTE: THIS MUST BE PLACED IN FRONT OF THE AMPLIFIER

Will require initial tuning

 

Amplifiers:

Concerns: I believe the legal limit on these frequencies is 30dBm? I haven’t done a whole lot of research on this but the variance in insertion loss on the filters paired with an amplifier pushing the power right to the 30dBm line… could cause a higher than legal power output? I’m not sure if it’s significant enough to be of concern.

^^because of this lets aim for 27-29dBm

 

Considerations: 

  • Impedance 50 Ω

  • Calculate gain separately for HF and LF

  • Note linear power output vs saturated power output–maintain linearity. Passing p1dB results in nonlinearity, which produces distortion

 

Gain Calculation must follow: 28dBm = LoRa Transmission Power - filter insertion loss + gain + Antenna Gain

 

Gain for HF lines:

Gain = 30 - 22 + 1 = 9dBm

https://www.lcsc.com/product-detail/RF-Amplifiers_Skyworks-Solutions-SKY65162-70LF_C2654169.html?s_z=n_sky65162-11  

Note: Requires 5V power supply? → can take 3.3V or 5V

 

Gain for LF lines:

Gain = 30 - 15 + 1.5 = 16.5dBm

https://www.lcsc.com/product-detail/RF-Power-Amplifiers_GPowerTek-GDM0928_C41410372.html?s_z=n_gdm0928

Note: No info on supply voltage?

 

Ishman:

 

 

Test/Validation Plan

 

Layout Considerations

 

Manufacturer options:

  • JLC

  • PCBWay

  • bittele (7pcb)

 

 

 

image-20250412-063828.png

spi on L6, RF on L1, controls on L4

Possible concerns

  • is 5.5mils too thin for current?

  • should I use curved traces?

  • spi as daisy chain better than star?