nRF9160 SiP - custom board

[shak2300]

Hi,

In my project I am developing a system to monitor the consumption of electricity taken from the grid. The nRF9160 SiP module is responsible for acquiring analogue signals to calculate the energy consumed over time. Once the measurement is complete, the collected data - such as energy consumption and device location - is transmitted via an LTE connection.

While designing the circuit board and the wireless communication part, I have some technical questions to clarify:

Questions

• Combined GNSS and LTE antenna vs. separate antennas:
  What are the advantages (or disadvantages) of using a single combined antenna for GNSS and LTE, compared to using two separate antennas, one for each function?

• VSWR parameter:
  When selecting antennas, it is stated that they must have a VSWR value < 3:1. What exactly is this parameter and why is it relevant?

• Improving indoor localisation with LTE:
  For indoor localisation, I am using LTE, but the accuracy is generally between 300 and 500 metres. Are there methods to improve it? (e.g. by means of algorithms, higher-performance antennas or other techniques)?

• Interference between stacked cards:
  One of the boards in the system will be responsible for the power supply (230V AC), while another will handle the wireless communication. If these two boards are stacked on top of each other, could antenna interference occur? Is it necessary to maintain a certain distance between them?

[Øyvind]

Hello, 

What are the advantages (or disadvantages) of using a single combined antenna for GNSS and LTE, compared to using two separate antennas, one for each function?

I would recommend reaching out to the antenna designers, e.g. Ignion - they have several application notes. In the case of combined vs separate antennas this boils down to your needs. The Thingy:91 X uses a combined antenna, while the nRF9151DK has separate due to size i.e. the DK has space for two separate while the Thingy:91 X's small form factor needs smaller and combined.

When selecting antennas, it is stated that they must have a VSWR value < 3:1. What exactly is this parameter and why is it relevant?

Voltage standing wave ratio (VSWR) is the ratio of maximum to minimum voltage on a transmission line. https://www.eeworldonline.com/what-is-the-voltage-standing-wave-ratio-vswr-in-rf-systems/

For indoor localisation, I am using LTE, but the accuracy is generally between 300 and 500 metres. Are there methods to improve it? (e.g. by means of algorithms, higher-performance antennas or other techniques)?

I would not recommend using LTE for indoor localization as this will not provide a position indoors. LTE positioning is using either single- or multicell which does not provide a good accuracy. For example, here is a comparison of singlecell vs multicell vs wifi vs GNSS. There are two devices:

These are found on https://world.thingy.rocks/ and are using the nRF Cloud location services

One of the boards in the system will be responsible for the power supply (230V AC), while another will handle the wireless communication. If these two boards are stacked on top of each other, could antenna interference occur? Is it necessary to maintain a certain distance between them?

Are you able to provide more details on these boards? The LTE antenna should be as free as possible, not near any metal or other parts that may affect it. If you are using a PCB mounted antenna, make sure you are following the antenna manufacturers recommended layout as closely as possible. A stacked PCB must not have any parts covering the antenna. Contact the antenna manufacturer for advice if needed

Kind regards,
Øyvind

[shak2300]

LTE is used for indoor localisation, as the device is often located in areas where only the cellular network is available, such as garages or underground car parks. In these environments, Wi-Fi and Bluetooth are not viable options. I also tried A-GPS, but in many indoor situations it is not effective. For this reason, I chose to use multi-cell localisation. The GPS module is included, however, as the device is also used outdoors.

The block diagram below represents an initial test phase. The system is divided into two main sections:

Measurement and power supply: this part takes care of voltage and current measurement and provides the necessary power supply for wireless communication (nRF9160 SiP).

Communication and localisation: this is where the microcontroller, wireless communication module and localisation systems are located. This section receives the data from the ADCs, calculates the power consumed, records the location of the device and sends all information to a server.

Since the device is structured in two separate parts, the measurement and power section will be physically located below the wireless communication section. We are assessing the feasibility of this configuration, both mechanically and in terms of electromagnetic interference.

[Øyvind]

shak2300 said:

LTE is used for indoor localisation, as the device is often located in areas where only the cellular network is available, such as garages or underground car parks

Just as an FYI, this is not indoor localization per definition. Depending on the signal strength and cell tower the device might have location but the accuracy will often not be sufficient to pin point the device. WiFi access points can often provide better localization indoors and uses a database of collected data/static access points to improve the position, based solely on SSID information. Of course, this depends on how many WiFi access points are around the device e.g. underground car parks.

shak2300 said:

I also tried A-GPS, but in many indoor situations it is not effective

That is correct. A-GPS will only provide information about the GNSS satellite position in the sky. But the GNSS antenna must be outside and have an open sky.

From nRF Cloud A-GNSS library documentation

Using A-GNSS reduces the time for a Global Navigation Satellite System (GNSS) module to estimate its position, which is also called Time to First Fix (TTFF). To get a position fix, a GNSS module needs information such as the satellite orbital data broadcasted by the satellites. If nRF Cloud A-GNSS service is used, the broadcasted information can be downloaded at a faster rate from nRF Cloud.

shak2300 said:

The block diagram below represents an initial test phase. The system is divided into two main sections:

Do you have schematics and PCB layout to share with us? This way we can better provide an answer about the layout and board stack.

Kind regards,
Øyvind