NRF54L15 reference design onboarding questions

[astomi]

Hi,

We are migrating from NRF52840 to NRF54L15 (QFN48).

I am using nrf54l15-qfaa-reference-layout-0_8 as reference but really struggle to understand some design choices.

Aplologies if these questions have been answered before my search couldn't find what I was looking for.

It would be very helpful if somebody from Nordic hw team could look at questions below:

1. C13 (reset pin bypass) is 3.9pF. This value is less than parasitic pin capacitance - should it be 3.9nF?

2. C11 (last cap in antenna matching network) is 0.3pF. A u.FL connector pin geometric tolerances or a speck of dust would introduce same order of magnitude error.

3. The reference design is 4 layers. I have seen mentions of previous 8 or 6 layer designs. Is 4 layer a latest recommendation?

4. It seems that C1-FB1-C2 are a just a filter for DECD output, and DECA and DECRF are locally bypassed by C12 and C5. Meaning that the track at the bottom layer connecting DECA and DECRF pins should probably be replaced with a normal power island on Layer 2?

5. Given that C9 is referenced to the bottom ground and copper windows under RF circuitry, bottom layer is a primary RF ground. So why RF ground has tracks but not Layer 2 for example?

6. What is the rationale of having 3 ground layers in a 4 layer pcb?

7. With a window under RF circuitry it looks like RF clearances were calculated for the bottom ground referenced waveguide. Is that correct? I have measured 0.15mm clearace along the RF path and 0.16mm around ground island.

8. The stackup is 1.56mm with 1.2 core and not impedance controlled. Was it intentionally selected or just being a simplest option?

9. What is the rationale of having a ground island C9 connects to? Is it just becuase of 1.6mm stackup? Yet C11 connects to RF ground using nearby vias.

10. What was the rationale to migrate to 0201 parts from 0402 before? It is a same working frequency/wavelength as NRF52.

11. What is the drive strength for X1 and X2 crystals?

Thanks. Your commects will be greatly appreciated.

[Kazi Afroza Sultana]

Hello,

''1. C13 (reset pin bypass) is 3.9pF. This value is less than parasitic pin capacitance - should it be 3.9nF?''

This is tested and verified. The RC filter (1 kΩ + 3.9 pF) on the NRESET pin is specifically chosen due to coupling of the 2nd harmonic from the neighboring ANT pin to the RESET pin ((+) nRF54Lxx - reset circuitry - Nordic Q&A - Nordic DevZone - Nordic DevZone).

2. C11 (last cap in antenna matching network) is 0.3pF. A u.FL connector pin geometric tolerances or a speck of dust would introduce same order of magnitude error''

I believe you meant. This is for the best RF performance; getting the utmost output power while keeping the harmonics below regulatory limits. 

''3. The reference design is 4 layers. I have seen mentions of previous 8 or 6 layer designs. Is 4 layer a latest recommendation''

The latest recommendation is 4 layer PCB design. 

5.Given that C9 is referenced to the bottom ground and copper windows under RF circuitry, bottom layer is a primary RF ground. So why RF ground has tracks but not Layer 2 for example?

If we look at the schematic, you can see C6 is grounded through VSS_PA; not directly to the ground plane for creating additional filtering of harmonic components.

To give you accurate answers for rest of the questions, I will talk to team. In the meantime you can send your hardware design to us if you have any.

Thanks.

BR
Kazi

P:S: This is easter vacation time in Norway. So, it may take a bit longer time than usual. Sorry for this incovenience.

[astomi]

"The RC filter (1 kΩ + 3.9 pF) on the NRESET pin is specifically chosen due to coupling of the 2nd harmonic from the neighboring ANT" 
What is the exact frequency of the second harmonic you are referring to? How it correlates with RC(1 kΩ + 3.9 pF) cutoff frequency of 40MHz?

What would be better: A/ leave 'Reset' pin unconnected or B/ Bypassed by small cap?

[Kazi Afroza Sultana]

(1 kΩ + 3.9 pF) is recommended and you should not leave  the pin unconnected (floating).