RF module can communicate when the frequency difference is 4MHz

[harryAI]

We are using Nordic nRF24L01+ RF module. The RF library software is "nRF24L01P-EK" (nRF24L01+ Evaluation kit files) obtained from the official Nordic website. RF module is initialized by the API `radio_pl_init`, where the key settings are with auto-ack, auto re-transmit 15 times, dynamic payload length, 2Mbps air rate, etc.

If we set two RF modules at the same frequency, for example, both 2438 MHz, these two RF modules can communicate well for sure.

However, we meet a problem:

RF Module A: set frequency as 2434 MHz

RF Module B: set frequency as 2438 MHz

We found when module A send packet, module B will receive and decode the packet data correctly. However, when module B send packets, module A does not receive anything. This situation also happens at other frequency if the frequency difference is 4 MHz, for example, 2440 MHz and 2444 MHz, etc.

My question is in theory 2434 MHz and 2438 MHz should not communicate, why they can communicate? In addition, why the communication is in unidirectional but not bidirectional?

[run_ar]

This is expected behavior for the L01+. And happens because of the way the radio is designed. Please note that the received signal strength is much lower in this case, than when both tx and rx is transmitting on the same channel.

[harryAI]

One more question. If this is expected behavior, why it is not bi-directional? In addition, in our test, if we test several different RF modules, some RF modules can communicate when the frequency difference is 4 MHz, but some cannot not. Why does it have such difference.

[run_ar]

It is not bidirectional because of the way frequencies are mixed in the radio (or something like that). I assume the difference is there because the signal will be very low on the receiver side. So small changes to distance and some production variation would make some devices better at blocking this frequency.