What does XRFTEST RX do on NRF9160?

[bsams]

I'm using AT%XRFTEST=0,1,2,19300,-40,1 to test reception at 1930MHz on the NRF9160 DK with a CW generator, and I don't understand the effect of the -40 parameter. With my generator set to -10dBm RMS, I get a reading of -23226 (it varies, but its around this number). If I instead use AT%XRFTEST=0,1,2,19300,-50,1 I get a result of -8951 (it also varies, but most of the time its around this number, but I do sometimes get numbers like -25240). What's going on? I've verified the connection since I can see the signal generated by TX testing from the NRF9160 to my equipment.

[Håkon Alseth]

Hi,

 

That measures the energy going into the SiP at that specific frequency.

< -20000 means < -78 dBm signal.

I get ~ -23k to -25k when there's no signal present.

Provided that you get ~-9000, it means ~-35 dBm (-9k / 256) at the input.

 

With my generator set to -10dBm RMS, I get a reading of -23226

Are you sure the generator is setup with the correct frequency? Are you measuring conducted or radiated?

 

Kind regards,

Håkon

[bsams]

So I think my main question here is more "what does the 5th parameter do" since it seems to affect the results, and the explanation in the datasheet is not really clear.

[Håkon Alseth]

It allows for AGC settings in the RX part.

Are your measurements radiated or conducted?

Kind regards,

Håkon

[bsams]

Thanks for this explanation, that makes sense. I think I've fixed the issue that was causing unusual readings, but there is one thing I still dont get: in the datasheet, it suggests a 300KHz offset for CatM1 for measuring the signal with XRFTEST. This does work, but If you run XRFTEST for frequencies around the test signal, there is a low point at the frequency, and high points everywhere else. For example:

If you put in a signal at 1900.0MHz, XRFTEST will show a signal at 1899.8MHz, 1899.9MHz, 1900.1MHz etc, but at 1900.0MHz it will show low or no signal. Why is this?

[Håkon Alseth]

Hi,

 

My apologies for the late reply. It has been Easter holiday period here in Norway.

bsams said:

If you put in a signal at 1900.0MHz, XRFTEST will show a signal at 1899.8MHz, 1899.9MHz, 1900.1MHz etc, but at 1900.0MHz it will show low or no signal. Why is this?

 This is due to the nature of the RF modulation scheme that LTE-M (and LTE in general) uses:

https://www.tutorialspoint.com/lte/lte_ofdm_technology.htm 

https://www.youtube.com/watch?v=rKy5dOl3Et4 

 

Based on your results, you have setup the subcarriers to be 100 kHz apart.

  

Kind regards,

Håkon