NRF905 external PA

[Edriano Carlos Araujo]

Hello all,

I have been using nrf905 for about 5 years without problems. now I would like to ask if is there any way to use an external PA? I am not a RF engineer and i am not sure if it is a good idea.

thanks in advance.

[ketiljo]

Hi

It depends a bit where in the world you are located. There are different restrictions on output power in different regions. In Europe for instance, there's no point in using higher output power as there are no frequency in the 868MHz band that allows this. This is dependent on the country, so some countries may allow this at certain frequencies.

In the US, you may want to do frequency hopping also to use higher output power to satisfy the regulations.

Using a PA will increase the complexity and cost of the product. Also battery lifetime is greatly impacted as a PA may easily draw 100mA. It's also more difficult to attenuate the harmonics sufficiently, and external low pass filters may be needed.

Often, optimizing the antenna is a better way to increase range, but it depends a bit on the requirements you have,

[Edriano Carlos Araujo]

Thanks for your replay.

I am located in Brazil and there are some circumstances where an increase in power can be accepted. in the case where transmission is not continue at 433MHz.

Lets say i want to turn on a pump far way i will only transmit once in a while.

May principal question is that looking at the databook as the PA is located internally and i don't have access to tx rx part. i don't know how could implement the RX part.

If you know any paper or application note that could help would be nice.

thanks in advance

[ketiljo]

Connecting the PA is quite straight forward. You need two RF switches that are used to switch in the PA in TX mode and bypass it in RX mode. All this is placed after the matching network so you will have a 50 ohm system the switches and PA's input and output.

VDD_PA can be tapped and buffered to control the switches and the PA it selves. VDD_PA goes high (1.8v) only when the TX is active.

Most RF switches have a DC voltage on the input and output pins, so make sure you are using DC block capacitors where appropriate. Refer to the manufacturers datasheet.