At first I'll describe something about it's features and after that I will go more into the technical aspects of the device and how I made the device itself.
Features (at the current hardware and software revision)
It's a Diversity 8 channel Receiver that features Telemetry and a high precision Variometer. With high precision Vario I mean <=10cm (delta) altitude recognition, wich is head to head with the current top of the line versions by some name Brands like Futaba. Also the Receiver has a small form factor (only 45mm x 25mm) furthermore it's relatively easy to assemble and use. Just set your Transmitter as per normal to D8 Protocol and hold the Bind button on the Receiver while switching it on. Then start the Bind sequence on the TX. After a few seconds the green Led on the Receiver will turn off. That's the binding done. The Telemetry is currently returning the current supply Voltage on the Servo Connectors, the current altitude and the current RSSI value. You can input everything from about 3.3 to 9 volts DC. The builtin protection against reverse polarity protects the Receiver from stupid mistakes, as they often happen on the field... The antennas are the same design as used by the official FrSky receivers. So if you cut one by accident you can just buy a new one, clip it on and continue flying. I also considered to enable the so called "SmartPort" feature that some official Receivers feature. That way you can connect any telemetry Item that you want. But it's not jet implemented. Additionally it features "Full-Range" capabilities by using a low noise amplifier. The practical Distance should be around 1.5 to 3km (for Telemetry a little less I think). The Receiver is now almost ready for some extensive testing in the Lab and on the field.
Many thanks at this point to the Institute for integrated Analog and RF Circuit Design (IAS) at the RWTH-Aachen for making it possible for me to use some of their Rhode & Schwarz testing Equipment in their Laboratory. (Further Posts coming on this topic for sure!)
Design
The design was pretty straight forward, as the Protocol and Hardware used by FrSky have been discussed in many forums. Huge thanks to all of these people that have successfully reverse engineered most of the parts. Also huge thanks to fishpepper, who has made his implementation open Source. I based my Software around his one, as it is well written and easy to adjust for new hardware.
But to get started, I've read a lot for the most part of this project in order to build up some knowledge about RF-Circuit design and the Protocol and Hardware used by FrSky. The design is a 4-Layer PCB with the inner Layers dedicated to VCC and GND. That makes routing of RF-Signals a lot easier. Especially if you don't have massive amount of knowledge and practice. But the board could be routed on a 2 Layer Board with some effort. The Design consists of a STM32F1 32Bit ARM Cortex M3 MCU and a CC2500 RF IC by Texas Instruments. Additionally I implemented a SE2431L to have a LNA (low noise amp) and PA (Power amp) on board to gain the Range. The other Components are more or less for convenience. A onboard EEprom is used to keep the current Bind and frequency offset correction information. An MS5611 is used to get the current altitude (quite expensive by the way). It get's reset to relative 0 when switched on. And some bypassing caps and filters are added as necessary. As already mentioned I used the same IPEX connectors for the antennas as FrSky does. The project is not 100% done yet, so I'll post some updates in the future and maybe make the Hard and Software OpenSource.
If you have any questions about this project don't hesitate to ask!
