FPV, right? Long range maybe? With a 5.8GHz system, it’s really hard to call it a long-range but what if you put a ridiculous amount of power on transmitter module? 500mW? 800mW? Nope, not enough. Let’s go big and have a practical look at AKK X2-Ultimate which says it does 1200mW of RF output power! 1.2W put into the aether. Wow…
Sometimes you win, sometimes you lose. This time I've lost. Not by much, but always. During testing of my DIY LoRa RC link, radio caught a glitch during a flip few meters above a ground. It was not even a failsafe situation. Link recovered a few milliseconds later, but it was too late and quadcopter crashed into the ground.
During a roll, while being behind a tree, RX antenna got hidden behind a carbon fuselage and both antennas were at 90 deg. That was enough.
Damage is not severe, nothing I can not 3D print in one evening. It's more like a discredited honor or something.
The glitch was so short that is was not even recorded in blackbox log. RSSI was fine, no locked rcData. Quadcopter just kept 90deg attitude for too long.
There is a slight chance it was not faulted in software or hardware. Maybe there was a strong rouge TX polluting the aether. Why? I've caught a failsafe on a different quad (2.4GHz FrSky link) while being only a few meters away and a friend caught a failsafe on a TBS Crossfire. So maybe it's not entirely my fault after all.
Looks like I was able to solve all major known problems with my DIY long range radio system Crossbow. I'm writing known, since no idea what lies beneath… Anyhow… What changed? Quite a lot:
I've extended Arduino-LoRa library with ability to transfer full packet in single SPI transaction. Right now, each read of write to SX1276 uses single transaction. Previously, there were 2 transactions per byte…
The same library now has ability to send packets in async mode. Previously it was blocking code execution until LoRa packet was transmitted. Huge waste on processing time
With OpenTX 2.2.1 on the loose, I was finally able to drop PPM input from Taranis to TX module and replace it with S.BUS. But not without problems. According to specification, S.Bus should be SERIAL_8E2. But my Taranis clearly outputs it as SERIAL_8N2
For now, OLED display is disabled. It was taking too much time to update it using I2C and TX module was loosing S.Bus packets
I've improved RC channels processing time, time required for encoding/decoding went down by 1ms
Next test hopefully this weekend. If weather allows, of course. We have very wet autumn this winter in central Europe this year…
LoRa modulation has some advantages. Like superb receiver sensitivity and immunity to interference. Has some problems, true, but at the end, it's a great way to send small packets of data to long ranges using low power.
Anyhow, today only one picture: how LoRa spectrum compares to FSK signal spectrum? Like this:
Those two peaks are nearby FSK stations, while plateau is 250kHz wide LoRa signal. Difference is at least clearly visible 😉
Finally, much later than I originally expected, Crossbow LRS, my DIY medium range RC radio link was used to control something that flies. Not much, and not far. It was only my experimental 6" GPS Racer quadcopter. And I reached only 350m. Small steps, I had no intention to beat and records after all.
Just as reminder, by RC link is as follows:
Based on LoRa32u4 II dev. boards
Semtech SX1276 868MHz
Programmed with Arduino
Planned range: up to 5km
Next test, this time without twitching, in a few weeks.
CC1101 is another example of modern radio modules. I might not have the receiver sensitivity or LoRa SX1276, but with proper antennas should give more than 1 km of radio transmission. Recently I got a couple of them, so expect some new projects with CC1101 and Arduino.
Now, something that took me some time to find out, so you will not have to: CC1101 pinout:
Only two weeks ago I thought I solved all my major problems with DIY LoRa RC link. I was wrong. I was able to solve one problem (link unstable due to rouge packets messing up with protocol decoding), but an old problem came up again: PPM input from Taranis is no longer stable. At least I know why since this is a second time this is happening.
Current code read bytes from SX1276 buffer inside interrupt callback procedure (ISR). PPM decoding is also done in ISR. How many threads ATmega has? What happens when one ISR is triggered while second is still executed? Problems. The solution is to keep ISRs as simple and fast as possible. My code was not simple and fast enough.
On top of that, it turned out that Arduino LoRa library I’m using is not efficient. It performs 2 SPI transactions to read one byte from SX1276 FIFO buffer. So, 12 bytes of typical data packet equals 24 SPI transaction… Looks like I will have to do some low-level coding I wanted to avoid in the beginning… Oh well…