Abnormal spurious at ~2432MHz for nRF51422

[Hungli3333]

Dear Sir/Madam,

We have a customer to develop a product based on nRF51422 and then they passed their product for pre-registration test.

They used DTM interface to connect to nRF go Studio for transmission test and it is using coaxial cable to connect DUT to equipment.

When they set the payload model to "11110000" mode and select either channel for transmission, they will give the following spectrum throughout a whole 2.4GHz band.

The critical problem is an high spurious at ~2432MHz (-36dBm) and I found this spurious is not related to the channel selection so you can another picture of selection channel 19 for transmission.  

This spurious problem can be duplicated for using Nordic kit with PCA10028 with nRF51422 used and no such problem for using PCA10040 kit (nRF52832 inside)

Please advise how to suppress on this abnormal spurious even if -36dBm is still not over on the regulation requirement but margin is very low.

  

[ketiljo]

The spurious you see is caused by harmonics of the 16 MHz system clock. This is normal and does not cause any problems with radio regulatory. The nRF52 have a completely different radio architecture, so you won't see this here. 

[Hungli3333]

Our test lab claimed that the regulation required on the spurious not greater than -33dBm and it is Ok on -36dBm today but do you know any circuit change to suppress this 16MHz clock harmonics to allow more margin on it?

[ketiljo]

The spurious are inband, so it's not easy to suppress.

The radio in the nRF51 series is the same as in the nRF24L-series as well. This has been on the marked for 1++ years and we haven't gotten any reports that this has been a problem.

What regulation paragraph are they referring to that requires inband spurious to be at -33 dBm?

[Hungli3333]

It is for China SRRC regulation check.