Estimating the Output Impedance of the nRF52840

When implementing an RF design, it is often sufficient to follow the layout provided with the manufacturer’s evaluation board. In some cases, however, we may want to create a more compact layout or use different component sizes or values. An electromagnetic (EM) simulation tool can then be used to model the parasitic effects associated with the PCB layout and substrate.

In the analysis below, I demonstrate how to estimate the Bluetooth output impedance of Nordic Semiconductor’s nRF52840 using Sonnet Software, a 2.5D EM analysis tool.

Nordic Semiconductor provides Altium design files for evaluation boards that use the nRF52840-CKAA device. This example uses the following Altium project:

nrf52840_ckaa_var1.PrjPCB

https://www.nordicsemi.com/Products/nRF52840/Compatible-downloads#infotabs

The RF output of the nRF52840 is connected to a T-matching network consisting of three components: L1, C3, and L5.

Without accounting for the electromagnetic effects of the PCB layout, the circuit simulation produces the following impedance.

The impedance presented to the nRF52840 is 82.6 − j17.1 Ω, which is equivalent to 82.6 Ω in series with 3.8 pF. The conjugate impedance is 82.6 + j17.1 Ω, which is equivalent to 82.6 Ω in series with 1.1 nH.

The PCB layout was exported from Altium Designer as a DXF file. After the DXF file was cleaned up, it was imported into Sonnet.

The cleaned DXF file used for the Sonnet model is shown below. Removing unnecessary geometry reduces the simulation’s memory requirements and execution time. The RF output connections of the nRF52840 are identified in the image. Ball E1 is the RF output, and ball C1 is the RF ground reference.

The Sonnet model, shown below, contains nine ports. Ports 1 and 2 connect to the RF output of the nRF52840. Port 9 is located at the output of the evaluation board, where the RF connector attaches to the PCB. The remaining ports provide connection points for the components in the LC T-matching network.

Sonnet can display the model in three dimensions, which is useful when constructing and reviewing the model. The top-layer metal is shown in purple, and the vias are shown in dark green. The z-axis is exaggerated, making the substrate appear thicker relative to the dimensions in the x-y plane.

The PCB stackup information was obtained from the Altium design files.

After the Sonnet model was created and simulated, an S-parameter file was exported from Sonnet and imported into a linear circuit simulator. QUCS was used to simulate the S-parameter data together with ideal inductors and capacitors.

This simulation produces a significantly different result. The impedance presented to the nRF52840 is 54.9 − j13.3 Ω, which is equivalent to 54.9 Ω in series with 4.9 pF. The conjugate impedance is 54.9 + j13.3 Ω, which is equivalent to 54.9 Ω in series with 0.86 nH.

The model can be improved further by representing the inductors with manufacturer-supplied S-parameter data. S-parameter files for the 3.3 nH and 4.8 nH Coilcraft inductors were imported into QUCS.

This simulation provides a more complete representation of how the matching network transforms the impedance at the RF output of the nRF52840. The impedance presented to the device is 48.6 − j12.3 Ω, which is equivalent to 48.6 Ω in series with 5.28 pF. The conjugate impedance is 48.6 + j12.3 Ω, which is equivalent to 48.6 Ω in series with 0.8 nH.

Summary

The estimated Bluetooth output impedance of the nRF52840 is close to 50 Ω and slightly inductive:

48.6 Ω in series with 0.8 nH