Poor RSSI level of custom board based on BMD340

Question

Hi, 
 We have made a custom board based on BMD340 for a vital signs monitor. The board will be attached onto various parts of human body. The body shadowing effect plays an important role in the performance of wireless systems. This effect can lead to loss of packets or can even break the wireless link. In some of the designs we have made till date, we face this issue. Before going ahead with the design, we would love to make sure that BMD340 is the best option we have and we have not done any major mistake with respect to RFdesign. 
 To check for body shadowing effect, we performed a simple RSSI comparison test using NRF connect android app. We placed the BMD340 based design (PCB) on body as well as a Movesense device (with enclosure) on body and compared the both RSSIs reported by the app at various orientations. The values were monitored at the following scenarios at different distances

Subject facing the phone (receiver) 
 Subject facing away from the phone (receiver) 
 
 In all cases, the custom design one showed RSSI value in the range from -80 to -101 dBm whereas Movesense device showed RSSI in the range -57 to -85 dBm. We have few concerns with respect to the custom design. The design once kept inside enclosure will have further reduction in signal strength and this can severely affect the performance and our intended use case. The current BMD340AR10 uses integrated PCB antenna, so we were wondering if using a BLE module with external antenna will help in overcoming this signal strength degradation issue.

Currently we have 3 options

Use BMD341 series with external antenna 
 Use BMD345 series with external antenna and PA, LNA 
 Use existing BMD340 
 
 We have shared the schematics and layout details of our current design for review. It would be great if someone can help us in understanding whether this RSSI level variations is due to something in our layout or these levels are expected and is absolutely normal?

BMD-340.pdf layout.pdf

Marjeris Romero · Accepted Answer

Hi, 
 What output power is the radio set to when you are performing your measurements? If you are running a BLE example from one of our SDK you can modify the code and set the output power to up to +8dBm for the nRF52840, this will extend the range considerabely compared to the default 0dBm. 
 Currently we have 3 options

Use BMD341 series with external antenna 
 Use BMD345 series with external antenna and PA, LNA 
 Use existing BMD340 
 To discuss this briefly. I don't think the type of antenna (if it's internal or external) will have so much to say for the performance in your case. 
 The datasheet of the BMD840 specifies an antenna gain of max -1dBi, if you are curious you can compare this figure with other antenna manufacturers of any external antenna you are considering using instead. A bigger antenna (monopole) will usually give you better performance, but it will take more space, which for wearables is also something to consider. Regardless of the antenna choice, the size of the ground plane will influence the gain of the antenna, and for a smaller PCB this will usually be the limiting factor. 
 A PA/LNA will also give you extended range, as you can set the output power to up to +18dBm, but will also mean higher current consumption: 
 So for a wearable device the module with the integrated PCB antenna will probably be the best option of the three, even at the cost of lower range. 
 But usually the problem when designing wearables is their small PCB size. If you take a look at the BMD340 datasheet the recommended dimentions for the ground plane is 90mm x 50mm, yours look like it's on the smaller side. A smaller PCB will always have less antenna gain than a larger one due to the smaller ground plane, this will also increase the sensitivity to external factors as fro example the effect of the human body in close proximity. 
 You are also missing ground vias right underneath the module, this will probably result in poor connection between the ground plane of the module and the rest of the board, I recommend adding some ground vias as specified in the module datasheet: 
 
 For the enclosure I recommend avoiding metal, specially around the antenna area as a general rule. If possible keep the antenna area free of any enclosure, as the module is designed to be used in free space. 
 If you are still unsure of your module choice I recommend asking these questions also directly to the module manufacturer: https://portal.u-blox.com/s/ 
 Best regards, 
 Marjeris

Poor RSSI level of custom board based on BMD340

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