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HFXO Capacitor Bank (CTune) calibration on EFR32

CTUNE Access

Saving CTUNE value as manufacturing token

Question

How do I use the CTUNE token for EFR32?

Answer

The EFR32 38.4MHz crystal does not require external loading caps, as there is a tunable capacitor bank in the EFR32 that can be used instead.  Different tune values can be written to registers to observe the corresponding frequency offset, and the CTUNE token is used to store the value for use by the application.  Here in this example, we refer to nodetest commands.

Once your EFR32 device is programmed with nodetest, if you enter “tokdump” you should see the ctune token amongst the tokens reported:

[C354] MFG_CTUNE FF FF

Note that there are also nodetest commands "setCtune" and "getCtune" where you can set and read the CTUNE register values. This is helpful if you want to try multiple specific ctune values and measure their affect prior to writing the token. From the nodetest "help" output:

setCtune u4 - set CTUNE registers (CMU->HFXOSTEADYSTATECTRL_CTUNE and CMU->HFXOSTARTUPCTRL_CTUNE)

getCtune - get CTUNE register value

A default / erased value for MFG_CTUNE corresponds to using the nodetest default ctune register value of 0x142. The EFR32 reference manual provides a formula for ctune:

Sets oscillator tuning capacitance.

Capacitance on HFXTAL_N and HFXTAL_P (pF) = Ctune = Cpar + CTUNE<8:0> x 40fF.

Max Ctune 25pF (CLmax ~12.5pF)

CL(DNLmax)=50fF ~ 0.6ppm (12.5ppm/pF)

With this calculation, the default of 0x142 (322 decimal) and no loading caps installed (as is the case with the BRD4151A modules in the WSTK) corresponds to a capacitance of 12.8pF.

You can issue the nodetest command "tokWrite c354" to write this token:

tokWrite u2 - Write all data of a token (u2=creator code)

Similar set/get of CTUNE register values can be achieved with RAIL test as well.  See the RAIL test user documentation for additional details.

Setting CTUNE token can also be achieved via Simplicity Commander.  See the Simplicity Commander User Guide UG162 for additional details on setting tokens.

Introduction

This article shows how tune the antenna integrated into the BGM12x with the specific use case of placing a coin-cell battery behind the SiP, therefore enabling extremely compact designs.

To better follow this article it is recommended to watch our BGM12x related videos, BGM121/123 Hardware Design Guidelines and BGM121/123 Antenna Robustness.

Effects of placing coin-cell battery in tuned design

To understand the effects of the coin-cell battery on the antenna matching we took a BGM121 test board and placed a coin-cell in the back with a piece of blu-tak to keep it in place.

Figure 1 - BGM121 test board with coin-cell battery attached

The antenna matching can be seen in the following figures, first for the test board without battery and then after attaching the coin-cell battery.

Figure 2 - Original antenna matching for the BGM121 test board

Figure 3 - Antenna matching for BGM121 test board after attaching the coin-cell battery

Markers 1, 2 and 3 correspond to the beginning, middle, and end of the Bluetooth band. As it is visible in Figure 3 the presence of a metallic body (coin-cell) shifts the antenna tuning outside of the Bluetooth band. This is mostly due to the fact that the coin-cell is covering part of the copper clearance area and therefore reducing the antenna current loop.

Re-tuning the antenna 

To counteract this frequency shift we can extend the clearance area simply by scrapping away the ground layer as shown in Figure 4. Due to the the underlying uncertainty on how much it needs to be extended we’ll just extend it about 50% more as it is then easier to re-add a metal plane in order to fine-tune the clearance area.

Figure 4 - Extending the copper clearance area

The antenna matching of the modified board is show in the next figure.

Figure 5 - Antenna matching after extending the copper clearance area

The matching is still off-band but this time it is towards the lower frequencies which can be compensated by reducing the copper clearance. To reduce the copper clearance we simply cover it with a piece of metal (a shield from one of our modules in this specific case) until we get the correct matching as shown in the next figure.

Figure 6 - Reducing the copper clearance area to fine tune antenna matching

The antenna matching is now back into the Bluetooth band and very close to the original design.

Figure 7 - Antenna matching after reducing size of the copper clearance area

Conclusion

The presence of metal very near to the BGM12x can have a negative impact on the antenna matching. However, it is easy to compensate the effect by adjusting the size of the copper clearance area to bring the antenna matching back into the Bluetooth band and ensure the best RF performance for your design.

The BGM121/BGM123 Blue Gecko Bluetooth® SiP Module family is targeted for applications where ultra-small size, reliable high RF performance, low-power consumption and easy application development are key requirements.

At 6.5 x 6.5 x 1.4 mm the BGM121/BGM123 module fits applications where size is a constraint. BGM121/BGM123 also integrates a high performance, ultra robust antenna, which requires minimal PCB, plastic and metal clearance. The total PCB area required by BGM121/BGM123 is only 51 mm2.

The BGM12x SiP module has an internal chip antenna which is properly tuned with a GND loop, i.e. copper clearance area, on the carrier board. For the best possible antenna performance Silicon Labs recommends to precisely follow the layout suggestions around the SiP module including the copper clearance area, as shown in the following figures.

In order to have the internal antenna tuned the GND pins 53 and 54 of BGM12x shouldn’t directly be tied together. They need to be connected to each other through the GND loop which is routed along the circumference of the copper clearance area. This GND loop makes the antenna resonate and its size determines the resonant frequency, so its layout parameters should exactly be followed as shown in the figure above. The SiP module should be placed at the board edge of the carrier PCB.

If these layout recommendations above are precisely followed on the carrier board of the SiP module then the BGM12x has the following unique features:

1. It remains always tuned on any application board as well. Its antenna tuning is not sensitive to the size of the carrier board. However, the minimum recommended board design width is 10mm (to ensure enough place for the module together with the copper clearance area and GND loop).
2. It can always be tuned on any application board if needed. For instance, the BGM12x SiP module antenna gets de-tuned if any metal object, e.g. coin cell battery, is mounted or attached onto the carrier board beneath the antenna area of the SiP module on the opposite layer – this de-tuning effect due to close metal proximity can generally be seen with any antenna. However, this de-tuning effect can be compensated out and the BGM12x SiP module can get tuned again by adjusting the antenna matching, i.e. modifying the antenna clearnace area on the carrier board.
3. The module antenna is not sensitive to any dielectric material in its close proximity. It is nearly immune against any dielectric (e.g. plastics, conformal coating) in touch with the module as well.
4. In an optimal board design it has as good performance (in terms of antenna efficiency) as what could be achieved with any external antenna (-1 to -2 dB antenna efficiency).

The impedance of the BGM12x SiP module’s built-in antenna can be fine-tuned by adjusting the copper clearance area (i.e. GND loop) on the carrier board. The figure below shows the return loss (S11) – where the resonant frequency can also be observed from the notches – of the built-in antenna with different copper clearance width. This tuning method is a unique feature for BGM12x, so the module antenna can get tuned on any custom design layout and thus exceptional RF performance can always be achieved with this SiP module.

The generally recommended copper clearance area dimensions are 5.0 x 2.2 mm on the top layer - when there isn't any metal object in the close proximity of the antenna and copper clearance area. If any metal object, e.g. coin cell battery, is mounted or attached onto the carrier board beneath the antenna area of the SiP module on the opposite layer, then the suggested GND loop dimensions are 6.2 x 2.2 mm - this assumes that the front edge of the coin cell battery is in line with the front edge of the PCB.  

A more detailed guidance on the antenna tuning of the BGM12x SiP module can also be seen under the below KB article here:

http://community.silabs.com/t5/tkb/articleprintpage/tkb-id/BluetoothandWiFi@tkb/article-id/563

https://www.silabs.com/community/wireless/bluetooth/knowledge-base.entry.html/2017/01/10/antenna_tuning_forb-R5av

The optimal board design mentioned above in point 4 refers to the generally recommended GND plane size of monopole-type antennas. Since the GND plane itself is also the part of the monopole-type antennas it is important to ensure big enough GND reference for the antenna. The smaller GND reference is ensured for any monopole-type antenna the weaker antenna performance can be expected. If the GND plane size goes below quater-wavelength then the antenna performance drops quickly.

To have the best antenna performance with BGM12x Silicon Labs recommends to use 40...50 mm wide carrier board. If the module antenna is kept tuned by following the layout recommendations, especially around the copper clearance area and GND loop, then the following curve describes the effects of the GND plane on the antenna efficiency.

RF performance when using BGM12x SiP module on a very small carrier board:

The minimum recommended board design width is 10 mm. The antenna efficiency in that case is around -12 dB. Under these conditions, the tested practical RF range with the BGM12x SiP module in office environment is about 15 meters.

The following figure shows the antenna efficiency versus board size curves for different antennas. As the curves show below the considerable antenna efficiency degradation is a general feature of any monopole-type antenna.

Training & demonstration videos:

BGM121/123 Hardware Design Guidelines: https://www.youtube.com/watch?v=5DCn7AzIyig

BGM121/123 Antenna Robustness: https://www.youtube.com/watch?v=LA2j4AXqd7Q