Nrf24L01 based module has been very popular, because it’s easy to implement in wireless communication projects. The module could be found under 1$ with a PCB printed version, or monopole Antenna.The problem with these cheap modules is that they have many issues and become easily defective. Mainly because the IC is not originally made by Nordicsemi, but also because of the poor printing quality of the PCBs.
Throughout this article I will show you how to build your own nrf24L01 based module, and how to add PA (Power amplifier ), LNA (Low noise amplifier) to extend range and output power.
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Step 1: Typical Application Circuit
Here is the typical circuit for an nrf24L01 based module; this one is commonly used in commercial modules based on this chip. The circuit contains some decoupling capacitors connected between VDD and ground. 16 MHZ crystal oscillator is used and must fulfil the specifications found in the datasheet. ANT1 and ANT2 provide RF output to the antenna, according to datasheet a 15ohm+j88ohm load is recommended for a maximum output power of 0dbm, a 50ohm load impedance can be obtained by fitting a matching network, ANT1 and ANT2 have a DC path to VDD_PA (more about this later). Finally an SMA connector connects the circuit to a dipole antenna.
Step 2: Adding a Front End Module to Increase Power and Range
The circuit discussed above have 4 levels of output power: 0dBm, -6dBm, -12dBm, -18dBm. Power level controls directly range, of course there is other characteristics related to the antenna (impedance, Power rate, type …) and to the propagation environment, but let’s focus on the module itself.
To extend output power a front end module could be used. I found this RFX2401C from Skyworks Solutions just perfect; it’s a 2.4GHZ ZigBee/ISM front-end module, with 50ohm input and output ports, 25db of small signal gain and 22dBm of saturated output power (All of these characteristics are related to Transmit mode). Skyworks offers also an evaluation board which help to prototype easily with their IC.
This module has a relatively simple control logic (See logic table). To activate receiving (RX mode), TXEN should be pulled LOW and RXEN pulled HIGH and to activate transmission (TX mode) TXEN pulled HIGH the state of RXEN isn’t important. According to nrf24L01 datasheet CE pin must be pulled HIGH whenever the transceiver has to enter RX mode. Using an oscilloscope I’ve measured the state of VDD_PA pin, it turns out that it’s HIGH whenever the Transceiver is in TX mode and LOW in RX mode. This way TXEN should be connected to VDD_PA and RXEN to CE
Step 3: Bill of Material
This table contains the list of componenets you need to build this circuit, I've ordered them at: https://lcsc.com/
Step 4: Schematics
This is the typical circuit of our transceiver with its RF output connected to the front end module; this one receives commands from VDD_PA and CE pins, some decoupling capacitors where added. The output is connected to a discrete LC filter with an SMA connector at the end.
Step 5: Conclusion and Improvements
after extracting gerber files I ordered 10 pcb and did soldering using a stencil and reflow station.
It turns out that making such an RF circuit require taking any possible electromagnetic interference into account, especially when performing pcb routing. It is strongly recommended a non-vented shield and connect this one to ground, which helps reducing capacitive and magnetic coupling between the module and its environment.