GTT_2_Introduction nRF52 (Selectivity)

[Eugene]

In the GTT_2_Introduction nRF52 document there is a picture with text -42 dB Selectivity (2'nd CI). As far as i concerned it's about Carrier-to-interference ratio. But what exactly does it mean in that context?

[Øyvind Karlsen]

Hi,

From the RX selectivity section in the product specification we can see a table with selectivity information for a slew of scenarios. In order to get an idea on how the system is affected by interference we look at how strong two equally modulated signals have to be in order to be able to distinguish between them, this is called selectivity. This is tested by setting up a signal on the desired channel with an input power of -67dBm, then the input power of the interfering channel is swept until the Bit Error Rate (BER) reaches 0.1%.

In this context -42dB selectivity means that the radio can detect signals on the desired channel with less than 0.1% BER while there is a signal 2MHz up in frequency that is 42dB stronger that the signal you are detecting. This is the measurement while the radio is configured in the 1Msps mode.

Best regards,

Øyvind

[Øyvind Karlsen]

You have got it right. The reason for the asymmetry is that the mirror frequency (have a look at image frequency in this wikipedia article on the superhetrodyne receiver) for the signal occurs at -2MHz in 1Msps mode and at -4MHz in 2Msps mode.