Factory acceptance testing (FAT) is a critical part of completing a new control system. The FAT will have all control system hardware connected, configured and operating as if in the factory before being installed on the site. This testing will allow the integrator to verify a complete system operation prior to the hardware installation to demonstrate to the owner that everything is functioning as ordered.
The wireless hardware needs be part of this testing to verify all timing, recovery and throughput. But this can present challenges, as most factory acceptance testing is conducted in smaller physical areas than where the network will be installed. The difficulty occurs when operating a wireless network designed for miles of operation in such a confined space.
A radio designed for long distance communication seems like it should work as well, if not better, in a shorter distance, but that is not actually the case. The receiver of a radio has a “saturation” level – a point where the receive signal strength is so high that the excess energy creates its own interference when radios are placed close together.
For instance, the receive signal saturation level for the Horizon series radios is -40dBm. Any receive signal strength greater than this can cause the radios to behave unreliably. Operating a radio in these conditions requires specialized hardware or software settings.
To operate a radio with the antennas in close proximity, the receive signal strength needs to be reduced below the radio’s saturation level. This can be done by either lowering the power level in the radios or by adding an RF attenuator.
This attenuator is placed in the transmission path between the radio and antenna and can be of a fixed value (i.e. 10dB or 20dB) or variable. These attenuators will both lower the output power of the radio on the transmit side and lower the receive signal strength on the receive side.
The output power of the radio must be considered for each testing application but generally a fixed 10dB or 20dB attenuator connected to each radio will work well.
As long as the receive signal strengths can be controlled by lower the power level and using RF attenuators, the wireless network can be tested as part of the FAT, giving the customer the peace of mind they need. Once the testing is completed, the attenuators are removed to prepare the system for delivery.