Channel Frequency mismatch

[Suresh C]

Hi,

I am using NRF52833 DK  as a transmitter. I have analyzed it with spectrum Analyzer. The frequency I got, does not match with the frequencies of channel given as per the BLE specification. What may be the issue?

Eg: 1. For Channel 0 - Frequency as per BLE Specification is 2404MHz but I got it as 2399MHz  

       2. For Channel 39 - frequency as per specification is 2480MHz but I got it as 2438.7MHz.

[ketiljo]

How do you test this? What FW and what spectrum analyzer are you using? 

[Suresh C]

I am using FPC 1000 Spectrum analyzer and Radio check SDK from Nordic to test this.

[ketiljo]

Can you show some screenshots?

[Suresh C]

This was the image I referred. the above image is based on BLE specification. I am attaching a image below that contains the data we got from the Spectrum analyzer.

[ketiljo]

That doesn't make much sense. The carrier frequency should be less than 30 ppm off, so a few 100 kHz at most. 

Can you share a screenshot of the spectrum analyzer please?

Use a unmodulated carrier when you do these measurements. Also, when you use the radio test, the frequency is set at 2400 MHz + channel. This is not related to the BLE channel map. So channel 40 = 2440 MHz etc. 

[Suresh C]

You mean unmodulated carrier will result in these data's. So how can I check the frequency of modulated signal using spectrum analyzer. Is there any way to check that?

Spectrum Analyzer Image:( Image when measuring 39th channel)

[ketiljo]

If you want to check the frequency of the carrier, it needs to be unmodulated. Else it's too wide. Set the span to 100 kHz and RBW to 200 Hz and try again. It should look like this:

[Suresh C]

Hi,

Thank you for you response. I have tried changing the TX power of the transmitter(NRF52833 DK). That mean I vary the power to +8dBm but the maximum power I can see in spectrum analyzer is -24 dBm.  It doesn't go beyond that.

[ketiljo]

https://infocenter.nordicsemi.com/topic/ug_nrf52833_dk/UG/dk/hw_rf_measure.html