DON’T BLOW IT
PROTECTING AGAINST RF DAMAGE Adam Purkiss, Field Applications Engineer, Anritsu, EMEA
A few simple guidelines can help protect expensive field test instruments.
T
echnicians such as radio tower mast riggers are responsible for the use of finely engineered RF field test equipment that offers very high levels of accuracy and precision, while also being portable and rugged. The handheld spectrum analyser, for instance, is a mainstay of the technician’s toolkit. With a rugged plastic housing and a toughened display screen, it is easily capable of withstanding the knocks and shocks encountered in everyday use in the field. Rugged it might be, then, but there is one phenomenon that it cannot endure — excessive RF power. For all the recent advances in the design and manufacturing of circuit protection devices, breaching the instrument’s specification for maximum input power will cause damage. Unfortunately, a measurement instrument’s accuracy and precision depend on the use of extremely high-quality components assembled to fine tolerances. This means that the repair or replacement of a damaged instrument can be very expensive. As so often in life, prevention is better than cure. This article describes the main methods and products that field operators should employ to eliminate all risk of exposing their test equipment to harm in ordinary use, whether through excessive RF power, static electricity or connector mismating.
10 Critical Comms - Jul/Aug 2015
Excessive continuous RF power The most obvious way in which incorrect use of a test instrument can damage it is by the application of continuous excessive RF power to the instrument’s inputs. It is the most obvious because the resulting damage can be easily seen – and smelled. Figures 1 and 2 show damage to the front ends of handheld spectrum analysers. Here, continuous RF power above the maximum threshold specified by the manufacturer resulted in the burning of various components inside the case. In Figure 1, it also damaged tracks on the PCB, which rendered it unrepairable, since the PCB was a multilayer assembly and some damaged tracks were therefore not exposed to view. When measuring signals with a handheld spectrum analyser, the correct method to protect the RF input is to use an external attenuator of a known value: this reduces the amplitude of the input signal to an acceptable level. The user should specify carefully both the scale of the attenuation provided by the attenuator (in dB) and its maximum power rating (in W). If a user mistakenly applies RF power in excess of the attenuator’s maximum power rating, it will fail to provide the expected level of attenuation of the input signal, potentially exposing the instrument to the risk of burnout.
www.CriticalComms.com.au