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:
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:
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
You mentioned that the pins 53 and 54 should be connected with the ground loop connected along the circumference of the copper clearance area. Does this loop trace have a specific thickness? Also if you look at the latest datasheet there is some copper clearance area above the pins 53 and 54. Do we need to implement that as well and follow the ground loop along it to make sure it resonates at the right frequency?
What is the radiation efficiency of the module when the board width is the minimum (11m)?
The thickness of the GND loop trace is the standard 35 um in our reference layout designs. The thickness should not have significant effects, it is more important to follow the keep-out area dimensions and trace width. Extra keep-out above those pins can only be seen if the module cannot be placed right directly at the board edge, but there is some distance, e.g. due to Reset line routing for instance.
The minimum recommended board design width is 10 - 11 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.