nRF52840 DK- Lower RF output power as expected

[Adricast]

Hello, 

I have some troubles measuring the output power of the nRF52840 DK. I used both radio test and DTM firmware and I have the same problem. I try to measure the maximum output power of the nRF52840 DK, that is 8 dBm as said in the datasheet but I only obtain 4.8 dBm as maximum output power. 

My setup is composed of:

-Spectrum Analyzer Agilent CSA N1996A

- nRF52840 DK

-RF test probe MXHS83QE3000 (referenced by Nordic for RF measurements): 

https://www.mouser.es/ProductDetail/Murata-Electronics/MXHS83QE3000?qs=QnQa2dTVfy7u6%252B1BCmlR%252Bw%3D%3D

-Coaxial SMA to N-Type:

https://www.mouser.es/datasheet/2/418/9/ENG_DS_CSD_SMAM_610_NM_A-3238627.pdf

I expect to have around 1 dB of losses as said in both datasheet connectors, so I expect to measure around 7 dBm of maximum power but I only get 4.8 dBm when I configure 8 dBm as output power and I don't know why. I tried radio test with no modulated and dtm with different types of packets but nothing changes. I hope someone could help me.

Thank you in advance.

 

[Jørgen Holmefjord]

Hi,

Have you tried measuring multiple boards? The RF power accuracy for nRF52840 is +/-4dB, which means that your measurement of 4.8 dBm (I assume this is without insertion loss?) is within specifications.

The MXHS83QE3000 should have a typical insertion loss between 0.5 - 1.0 dB according to this page. The second cable seems to have insertion loss of 0.39*sqrt(f) dB, where f is frequency in GHz, which should correspond to around 0.6 dB for a 2.4 GHz signal. Worst case, the loss can then be around 1.6 dB in your cables.

Best regards,
Jørgen

[Adricast]

Hello,

thanks for your reply. Yes, I tried with two differents nRF52840 DK boards and the measurements are around 4 dBm. I'm not considering the insertion loss of the cables, so the output power without losses would be 6 dBm more or less.  If I understood well the maximum output power is 8 dBm but because of the RF power accuracy is +/- 4 dB, I could measure between 12 and 4 dBm = 8 dBm+/- 4 dB?

Thank you in advance. 

Kind regards.

[Jørgen Holmefjord]

Hi,

Adricast said:

Yes, I tried with two differents nRF52840 DK boards and the measurements are around 4 dBm

Can you post the exact commands you used to configure output power and start the signal output?

How is the spectrum analyzer configured?

Adricast said:

If I understood well the maximum output power is 8 dBm but because of the RF power accuracy is +/- 4 dB, I could measure between 12 and 4 dBm = 8 dBm+/- 4 dB?

Yes, this is correct understaning.

Best regards,
Jørgen