Reference design files¶
Electrical and mechanical design files are available at the W-Modbus Module support page.
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 2.45GHz.
Layout considerations for the main board¶
The W-Modbus module has been specifically designed to achieve good RF performance. To maintain this, there are some guidelines that we would like to stress:
The use of ground planes also on the main board cannot be emphasized enough. Good decoupling of any high-speed digital circuitry is a must. Many embedded type 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 receiver is -96dBm therefore it is recommended to keep disturbances below -100dBm 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 readymade LAN-products "Server in a RJ connector". They pass EMC certifications, but some of them radiate badly on 2.45 GHz. If disturbances can be seen on a spectrum analyzer - then the W-Modbus module will have impaired reception.
W-Modbus has a supply voltage decoupling on the circuit board. The supply voltage still needs to be adequately filtered. If any disturbance or intermittent communication failures occur, as one of the troubleshooting steps; check the supply voltage for drop-outs, switch supply ripple etc.
The TOP layer inside the footprint must be free from 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 future optional internal antenna.¶
The W-Modbus module has an external antenna connector option of U.FL. type and will have a future option for an internal antenna. If the internal antenna is planned to be used the following guidelines must be followed. Failure to do so may result in inferior 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.
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.
Min 5mm 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. I.e. for a 1.6mm carrier board standoffs needs to be min 3.4mm