Reference design files

Electrical and mechanical design files are available at the W-Modbus Module support page.

PCB mounting

Antenna

The antenna connector of the module is of U.FL type. A coax cable with a characteristic impedance of 50 ohm shall be used to connect an antenna. The antenna shall have a characteristic impedance of 50 ohm at 2.45 GHz.

Layout considerations for the main board

The W-Modbus module has been specifically designed to achieve good RF performance. To maintain this, we stress the following guidelines:

• The use of ground planes on the main board cannot be emphasized enough. Good decoupling of any high-speed digital circuitry is a must. Many embedded microprocessors today have clock frequencies with clocks or overtones that reach well into the GHz range. It is perfectly possible for an embedded design to pass any EMC certification and still cause disturbances that will block the RF reception of the W-Modbus module. The sensitivity of the W-Modbus module receiver is -96 dBm; it is therefore recommended to keep disturbances below -100 dBm in the frequency range of operation.

• A near field probe connected to a spectrum analyzer will show if there are any disturbances present on the 2.45 GHz band generated by the microprocessor or any other device that is placed on the main board. Pay special attention to pre-built LAN-products; they pass EMC certifications, but some of them radiate poorly on 2.45 GHz. If disturbances can be seen on a spectrum analyzer - then the W-Modbus module will have impaired reception.

• The W-Modbus module has supply voltage decoupling on the circuit board. The supply voltage must still be properly filtered. If interference or intermittent communication failures occur, check the supply voltage for dropouts, switch supply ripple, etc.

• The TOP layer within the footprint must be free of copper. There is a ground plane on the W-Modbus module, but there are also supply lines. It is an unnecessary risk to rely on solder mask lacquer for isolation.

Guidelines for optimal performance of internal antenna

The older W-Modbus MWA-N2 module only has the option of using an external antenna connector option of U.FL. type. The new W-Modbus MWA-N3 module has the option of an internal antenna. If the internal antenna is to be used, the following guidelines must be followed; failure to do so may result in poor wireless performance.

• The W-Modbus module has been tested on 1.6mm carrier boards of the brands ITEQ IT180 and Isola 370HR. For optimal performance it is recommended to use those for the carrier board design or a PCB with similar specification.

  • Iteq IT-180
  • Isola 370HR

• Minimum dimensions for ground plane clearance for optimum antenna performance are shown below:

• Red diagonal striped area is a keep out area “air gap” from any solid objects or carrier board PCB material.

• A minimum of 5 mm distance to solid objects under and above the W-Modbus module antenna area. Note that carrier board thickness can be included in the total distance to solid objects under the antenna. That is, for a 1.6 mm carrier board, standoffs needs to be min 3.4 mm.