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

[Eugene]

So -42 dB (2nd CI) means that next channel signal is 15849 times stronger than our channel but we still can get data with 0.1% BER. Have i got it right? But what causes an asymmetry? -42dB +2 MHz and -19dB -2 MHz?

[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.

[Eugene]

What's the LO frequency? It always higher than receiver freq. in order to keep mirror channel on the right side from the LO. Higher freq is easy to suppress. Am I right? And why freq. range (-2 or -4) depends on the bit rate?

[Øyvind Karlsen]

LO is the local oscillator, this is used in mixers in order to get a signal to the desired frequency. Higher frequencies generally have higher attenuation and will be easier to suppress because of this.

The properties of the reciever changes when the bitrate changes, this is partly due to lowered sensitivity for higher bitrates.

[Eugene]

I meant what the LO frequency was (i understand that it changes because of hopping). What's the IF (intermediate frequency) in this case?