Questions With RF Modules

B

Bahadur

New Member
Hi All

I am really happy with my DIY F450 and 1200mm build. Its pretty strong and responsive

I work with a small R&D firm where we test various RF products for various purpose and need help with some questions.
FIY, FC Pixhawk 4 mini, T Motor ESC and U7 motors remains as it is in frame. I keep changing Telemetry, Rx & Tx and Vtx for testing purpose.

So far I have tested

Telemetries :-
- RFD 900x Long Range Modules
- Holybro SiK Telemetry 915 MHz

Rx Tx
- FrSky Taranix X7 with X8R Pro Rx, SR10Pro Rx
- Jumper T-Pro 4 in 1 Multi Protocol Tx with Flysky AFHDS Rx, AFHDS2 Rx, Radiomaster R86, RadioLink CM921, Spectrum AR620
- Herelink Video Transmitter
- Bosscam 5.8 Analog Vtx


Now I am procuring more Rx and Tx with different protocols mainly for recording RF signatures from different manufacturers.

The questions I have (Suppose my quad will be strictly within 200 meters from me, nothing more)

1. Why we use RF modules? Like XJT or FrSky R9M? What changes they introduce which a normal transmitter can not provide?
2. Can I purchase "Jumper AION ELRS Nano Tx module" to upgrade my "Jumper T-Pro 4-in1" transmitter so it supports ELRS Receivers?
3. Are ELRS Rx compatible with ELRS Tx irrespective of brands?
4. What makes RFD900x Telemetry (Supports upto 50km range) different from HolyBro Telemetry (1km range) apart from distance?
5. I use telemetry to connect my Quad/Octa with Mission Planner GCS. What different Exotic telemetries like DragonLink or XBEE offers apart from mission planner connection?
6. What are the decision factors in selection of 433 Mhz Telm vs 915 Mhz Telm irrespective of frequency considering distance is 200 meters. Both offers connectivity.
7. Why there are exotic protocols exists like AFHDS3 and ANT (particularly for Flysky Brand) when AFHDS & AFHDS2 already exists?
8. Why to use Long Range ELRS modules when the operator will be using Long range telemetry to command the quad? (In case of 2km+ flight)
9. What makes a Receiver and Telemetry different from each other since both offer Data (RSSI or etc) back & forth.

I request you all to please help me clearing my doubts.

Regards
Bahadur
 
Dugdog47

Dugdog47

Well-Known Member
I'm. no expert but the layman's answer is rx and tx are different from telemetry because the two might cross while simultaneously transmitting and receiving at the same time. You need two different signals.

915mhz gets better penetration though objects for long range and freestyle plus shorter latency for racing.

There are many youtube videos comparing all the popular protocols like elrs, ghost, crossfire etc.

So many exist because the different brands compete for who has the best depending upon what your needs are.
 
LoneRCRanger

LoneRCRanger

Well-Known Member
rx stands for Radio Receiver Frequency. Whereas tx stands for Radio Transmitter Frequency. Anyone with HAM or CB Radios understand that.
 
wafflejock

wafflejock

Well-Known Member
Yah some terms being muddled in the original post that make this all a bit hard to answer...

Like @LoneRCRanger said the Rx/Tx acronyms used generically to mean receiver and transmitter, where basically a receiver is reading data and a transmitter is transmitting (or one device is outputting a data stream and another device is inputting/receiving the data stream). Along with transmitters and receivers there are "transceivers" that both transmit and receive data, "receivers" that include telemetry info being sent back are basically tranceivers so now it's a two way communication really.

There's a few layers to any network communications, there is the physical layer that is the actual signals over the air or on a wire then there are protocols used for transmitting data on top of those physical layers.

With regard to quadcopters specifically the physical layer is radio frequency (RF) electromagnetic signals. Typically bands and channels (ranges of sub-frequencies) are chosen around the 5.8GHz range for video transmitters and receivers and at 2.4GHz (carrier) signal frequency but for longer range 900MHz or other lower frequency/longer bandwidth signals are preferred (depending on RF laws in your region) https://www.expresslrs.org/2.0/hardware/receiver-selection/

On quadcopters a video link is a one way stream coming from the quadcopter down to the goggles, the control link from the controller/transmitter in your hands to the (transceiver)receiver on the quad is a high speed connection and then there's a lower speed/duty connection from the receiver back to your transmitter (telemetry data). As the page above explains a bit if search for "telemetry" can see basically the receivers are always using a fixed data rate for sending back to the transmitter but the transmitter itself can have the power level and data rate adjusted with ELRS transmitter/receiver combos. ELRS is a wireless protocol on top of the physical layer that uses some LoRA (long range radio) tech to help filter out signals from the noise so even a weak signal can often be picked/filtered out and decoded.

---

To address the specific questions:
  1. Why we use RF modules? Like XJT or FrSky R9M? What changes they introduce which a normal transmitter can not provide?
Having swappable RF modules basically keeps the transmitter "future proof". The control sticks and switches and UI for setting things up and saving model specific config is all separated from the actual radio components so if they burn out can easily be replaced (can happen if amplifier circuit overheats causes thermal expansion issues/chips to come free or other unexpected failures, example: gets wet when on). As to what features/capabilities one TX module and or protocol will offer over another you'd have to pick out two very specific modules and protocols to compare, the basic answer is different ones will have different amount of amplificiation and control over the amplification or when comes to ELRS can control the data rate and make trade off in distance for quality signals vs speed of updates sent from TX to RX. There are other features like receivers that can keep a quad in pit mode/low power until TX signals it to give power things up completely or if comparing protocols like FrSky vs DSMX there are differences in the latency and how the signals themselves are sliced up for transmission and decoded (also differences in how binding is done in the case of ELRS using a password in the TX and RX firmware to handshake instead of relying on doing a binding voodoo dance).

2. Can I purchase "Jumper AION ELRS Nano Tx module" to upgrade my "Jumper T-Pro 4-in1" transmitter so it supports ELRS Receivers?​
I think the nano TX modules are made for smaller transmitters (the xbox/ps console style joysticks). Make sure the TX-module is made to fit a "standard" XJT module bay, if so then it will work but you will need to update the TX firmware and add a LUA script onto the flash card to configure the ELRS transmitter settings (power/frequency). Oscar Liang's site covered this well enough for me to get it working here on a Jumper T16. Takes some doing getting OpenTX companion app setup and learning to flash the ELRS receivers/update the config on them but honestly went pretty smooth so long as using same telemetry settings on the TX firmware and on the RX firmware between open TX and the ELRS configurator program everything worked well once had the pieces in place.

3. Are ELRS Rx compatible with ELRS Tx irrespective of brands?​
Yes so long as antennas and all on the same frequency and using the same protocol (ELRS in this case) then are compatible.

4. What makes RFD900x Telemetry (Supports upto 50km range) different from HolyBro Telemetry (1km range) apart from distance?​
Likely on different frequencies, generally speaking lower frequency is better for range but I think higher frequency better in terms of less noise from distant sources (less interference to try and ignore)

5. I use telemetry to connect my Quad/Octa with Mission Planner GCS. What different Exotic telemetries like DragonLink or XBEE offers apart from mission planner connection?​
Sorry not familiar with various telemetry options to give more info on this one.

6. What are the decision factors in selection of 433 Mhz Telm vs 915 Mhz Telm irrespective of frequency considering distance is 200 meters. Both offers connectivity.​
As mentioned above basically if distance not a concern higher frequency probably seeing less "noise" from distance sources.

7. Why there are exotic protocols exists like AFHDS3 and ANT (particularly for Flysky Brand) when AFHDS & AFHDS2 already exists?​
Flysky and other brands that make proprietary protocols are always trying to one up the competition as well as some "planned obsolesence" in some cases it seems or rather just creating something new to have something to market to be honest. Maybe there is more to this but just my understanding from hearing about ACCESS protocol with FrSky (company behind open source openTX but fairly closed down about their protocols and making things backwards compatible)

8. Why to use Long Range ELRS modules when the operator will be using Long range telemetry to command the quad? (In case of 2km+ flight)​
Not sure I understand what being asked here but the "telemetry" is just the info being sent from the receiver back down to the transmitter, so there are no control signals coming in through that the TX still needs to send control signals to the quad (unless you mean it will fly by GPS control alone, but I would still want a RF controller to be able to flip a switch to have it bail assuming I'm in range at least and fail-safes for what it will do if RF signal from the TX lost, probably have it loiter or reduce power till gets low then cut power entirely to let it drop)

9. What makes a Receiver and Telemetry different from each other since both offer Data (RSSI or etc) back & forth.​
These aren't separate things, a receiver either does or does not offer "telemetry" (that is it's really a pure receiver and can't send data back out, or it's a transceiver and it can relay data back to the transmitter just typically at a lower bandwidth/rate as the control signals are coming from the transmitter)

https://oscarliang.com/setup-expresslrs-2-4ghz/ (frequency here doesn't matter the guide applies generically to setting up ELRS components from scratch, can get some things already flashed/ready to go but should cover the bases if doing it yourself entirely too)
 
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