Question About Band Edge Performance of nRF54LM20 at 2480 MHz

[Louis Chang]

Hi Team,

We are planning to use the nRF54LM20 + nRF21540 combination in our new design.

Currently, we are evaluating the band-edge performance on the highest BLE channel (2480 MHz) using the nRF54LM20A evaluation board together with the nRF21540.

Under DTM mode, during our testing, we found that, for BLE 1M PHY at 2480 MHz, the RF output power needs to be reduced to approximately 11 to 12 dBm in order to comply with the band-edge regulatory limit (FCC restricted band above 2483.5 MHz).

To compare the behavior, we replaced the nRF54LM20 with an nRF52840 while keeping the same nRF21540 FEM and repeated the measurement. In this configuration, the transmit power could reach approximately 20 dBm while still meeting the band-edge requirement.

We captured both measurements using a spectrum analyzer. Please refer to the screenshots below:

• Blue trace: nRF52840 + nRF21540
• Black trace: nRF54LM20 + nRF21540

The marker is placed at 2483.5 MHz. As shown in the plots, the band-edge level of the nRF54LM20 + nRF21540 combination is noticeably higher than that of the nRF52840 + nRF21540.

In addition, we also tested the nRF52840 and nRF54LM20 without the nRF21540 FEM. The same trend was observed: the nRF54LM20 showed a higher band-edge level than the nRF52840.

Based on these results, we believe the elevated noise floor or out-of-band emission may originate from the transceiver itself rather than from the nRF21540 FEM.

Do you have any suggestions regarding this behavior? Is there any recommended configuration, calibration, or design guideline that could help improve the band-edge performance of the nRF54LM20?

Thank you for your support.

Best regards,

[ketiljo]

Hi Louis

Can you do the same measurements but using Radio Test (https://docs.nordicsemi.com/bundle/ncs-latest/page/nrf/samples/peripheral/radio_test/README.html) instead? Use the command "start_tx_modulated_carrier". This will setup a modulated carrier with 100 % duty cycle and tell if there's a timing problem with the PA. Band edge and restricted bands measurements should be performed with a constant carrier, 100 % DC, if possible. 

The may be a timing issue with DTM wthere the PA is turned on after the carrier so the step in power causes the bandwidth to increase. 

Also test the nRF54LM20 with and without the FEM with 100% duty cycle.

Best regards,

Ketil

[Louis Chang]

Hi Ketil,

We verified this in Radio Test mode using start_tx_modulated_carrier. With a 100% duty cycle (I can see it by zero span), the band-edge emissions are reduced significantly.

However, when using start_duty_cycle_modulated_tx, the result looks very similar to the DTM mode result shown above.

Under radio mode, we also compared the results with and without the FEM, and observed no noticeable difference between them.

So if you think 100% duty is the correct way for restricted bands measurement, ok, we may use this method to do it.

Thank you 

[ketiljo]

Hi Loius

The FCC DTS measurements guidelines specifically says that 100% duty cycle shall be used if it's supported by the DUT.

DTM Used 60% duty cycle in 1 Mbps mode. Radio test is more suitable for general radio testing than DTM since you have a lot more options. 

Cheers,

Ketil