There are different types of noise that can be affect the performance of an RFID system. The most common indication of a noise problem is reduced read range. In some situations noise can cause the reader electronics to operate at a high temperature which can shorten the operational life.
Some noise sources can affect both HDX and FDX while other sources may only disturb one type of tag but not the other.
Noise can enter the reader through the power source (conducted) or received through the antenna (radiated).
Conducted noise is caused by small voltage variations (ripple) on the power cables. It is important that the DC power source be electrically quiet.
Power sources for our stationary HDX readers should produce no more than 30 millivolts of ripple. Because FDX uses amplitude signaling it can be more sensitive to interference from supply noise and the amount of ripple should be lower. FDX readers often have additional noise filters on the power inputs.
A battery is an extremely quiet source from the chemical reaction that produces a smooth flow of electrons. Because of this, they can be used to perform a simple test for conducted noise. Measure the read range of a tag when using battery power and again when a connected to a power supply being considered. If the read range is better with the battery the difference is from electrical noise from the power source.
It is important to do conducted noise testing over a range of situations. A power supply may be quiet at room temperature but the read range might drop as the electronics heats up on a sunny day. A solar power controller may be quiet while charging a battery but generate noise when it is fully charged.
Radiated noise comes in through the antenna and feedline cable and can be generated by nearby electric equipment. This is a problem particularly with large antennas which are more sensitive and can limit the size of the loop. The use of FM signaling is one reason HDX antennas can be larger than FDX which uses AM.
Common radiated noise sources are florescent light ballast, switching power supplies, laptop computer chargers, electric motors, vehicle ignitions and overhead electrical power lines.
Make sure power wires are kept separate from antenna wires and cables carrying antenna signals. Crossed cables can cause supply ripple to be induced into the antenna signals (conducted noise becomes a radiated source).
Test for radiated noise by turning off all electrical devices in the vicinity of the antenna to see if the read range changes. Turn devices back on one at a time to find the one(s) causing the problem.
Another way to test for noise is to measure the read range with a standard antenna with known performance. Use a battery for power while testing to eliminate any conducted noise effect. This is a good test to run when looking for antenna sites before installing an antenna.
FDX uses amplitude changes (AM signals). Vibrations on the antenna wire can also cause small changes in the inductance which will change the signal amplitude and become a noise source. Magnetic guitar pickups use this property of inductive coils to generate electrical signals from vibrating metal strings.