Antenna questions on nRF7002 - nRF5340

[dario.sortino]

Hello everyone!
I'm developing a custom board in which the two above mentioned chip will live.

As the nRF7002 Prod. Spec file states, I can use two different antennas, one dedicated to the 5340, one to the 7002.

I already have a working design for a PCB antenna for the nRF5340, so that's not a problem. I'm more curious about the nRF7002, since my guess is that, if I want to use a patch antenna, I could only work with 2.4 OR 5 Ghz. This problem would be solved by the diplexer and the chip antenna as it is used in the nRF7002DK. Is the whole reasoning right? 

Also, If i decide to go for a PCB antenna for the 7002, what would be the RF trace that I need to adapt to 50 oHm impedance?

Do I need matching circuitry? If not, it's correct to assume that my antenna start directly from the PIN?

[Bendik Heiskel]

Hi,

There are some designs for dual band PCB antennas, but they would require simulation to optimize the shape for both bands simultaneously. Using just one antenna is OK, but if you just want to use the 2.4GHz band it would be simplest just to share the antenna between the nRF5340 and the nRF7002. Both radios can't be transmitting simultaneously anyways so sharing the antenna is not a problem. If you want to try this you can use parts of the nRF7002 DK design files as reference, as the DK have a option to use the same 2.4GHz antenna for both radios:

just remove the diplexter and connect the output of the RF switch to the antenna matching network directly.

Do I need matching circuitry? If not, it's correct to assume that my antenna start directly from the PIN?

You don't need a matching network for the nRF7002 side as the TXRF0 pin already have an impedance of 50 Ohm, but the antenna will require a matching network. The type of matching network is dependent on the type of antenna.

The TXRF1 pin can just be left floating when not using 5GHz.

 

Best regards,

Bendik

[dario.sortino]

Thank you for your answer.

What do you mean that both can't be transmitting simultaneously even if I have different antennas? I thought it was not a problem since the antennas are different. I was planning to collect data from a set of devices via ESB (hence running in the network core) and forward them via WiFi (in the application core). Is there any problem with this?

[Bendik Heiskel]

Since both ESB and WiFi uses the 2.4GHz band the two antennas will be optimized for the same frequency band, this means a good amount of power will be coupled from one antenna to the other. WiFi can transmit at a higher power than the nRF5340, leading to the nRF5340 being deafened by the nRF7002. If the nRF5340 is in RX mode it may not be able to receive the anything as the receiver may be saturated by the TX power of the nRF7002.

This is solved by using coexistence, which ensures only one device is using the radio, most protocols allows for this as there isn't continuously radio activity. We have a sample that demonstrates this with WiFi and BLE coexistence:

https://developer.nordicsemi.com/nRF_Connect_SDK/doc/latest/nrf/samples/wifi/sr_coex/README.html

This also allows the nRF7002 and nRF5340 to share a 2.4GHz antenna as both won't be using it at the same time. The nRF7002 have dedicated pins for coexistence which must be connected to the nRF5340, as well as a dedicated pin for controlling the RF switch when sharing antenna with the nRF5340.

[dario.sortino]

Super clear. Just one last question: even if the example works with BLE, is it possible to make WiFi coexist with ESB?

[Bendik Heiskel]

Yes, it should be possible to make WiFi coexist with ESB,

[dario.sortino]

I have one more question about this topic.

The component U16 is the switch between the 5340 own antenna and the shared one with the 7002. This is controlled by GPIO 1 10 that in the devicetree I see its the btrf-switch-gpios field under the coex interface.

Does this mean that in "regular" applications like normal BT, without having WiFi, let's say by default I should have a 2.4Ghz antenna that can interface directly with the 5340?

Also, If I want to make my matching+patch antenna instead of using A2 as a chip antenna, should I control the impedance of the traces (inside the blue pen drawing) to be 50 ohms?