Two antenna's and unfitted capacitors in MB1355 schematics

[Anitha]

Hi Sir,

In the MB1355 schematics what is the purpose of two antenna's?How to calculate the unfitted capacitors ? what is the purpose of band pass filter here?

[Remi QUINTIN · Accepted solution]

The MB1355 board is used to prototype application. So, we have implemented 2 RF outputs: one to SMA coaxial connector for external antenna or for conducted measurements, and one PCB antenna to present BLE Over The Air demonstration with a smartphone or another BLE board. the customer chooses one of those 2 RF outputs.

 On the PCB antenna side, the components are indicated. Only one capacitor is unfitted, but it is spare footprint and is never used.

On the SMA side, if you make conducted measurements, no component is necessary. If you put external antenna, the values of the components depend of the antenna impedance connected.

[Anitha]

Hi Sir,

If I want to use only SMA antenna do I need RF switch capacitors? I'm thinking below one is the circuit, if want to use only SMA antenna. let me rectify if I wrong.

may I know the output impedance from RF1 pin in STM32WB55REV?

[Remi QUINTIN]

If you want to use only the SMA antenna, you have to solder the capacitor in the SMA antenna path and remove the capacitor in the PCB antenna path.

It is not necessary to know the output impedance from RF1 pin because the matching network has been calculated to present 50 ohms at the output of the filter towards the antenna path.

[Anitha]

Hi Sir, Like below way only you are saying? Can you tell way to calculate highlighted capacitors(means formula to calculate the these Not fitted capacitors?

[Anitha]

If I use only SMA antenna can I able to use the OTA?

[Remi QUINTIN]

​No issue. The OTA is not linked to the type of antenna.

[Anitha]

Can you please clarify the above question?

[Marc Laury]

In the MB1355 schematics what is the purpose of two antenna's? You have one PCB if you want to use the embedded antenna on the board or you have a SMA connector to connect an external antenna if you have your antenna.

How to calculate the unfitted capacitors ? If you connect an external antenna which presents 50 ohms, you do not need adaptation network for this antenna, but if you connect an external antenna which does not present 50 ohms , you have to define the components of the adaptation network. A lot of documentation is available on the web to do it.

what is the purpose of band pass filter here? This band pass filter is to eliminate the harmonics but also the others unwanted signals from others systems.