External RF fields or similar disturbances may cause a malfunction when they couple into an electronic circuit.
Malfunctions can range from a slight variation in analogue operating conditions to complete corruption of digital operation. The actual definition of a malfunction must be provided by the equipment manufacturer, although guidelines for definition of performance criteria are offered in most immunity standards. Susceptibility may vary for different operating configurations or for different operating modes.
RF disturbances are almost invariably due to local radio transmitters, which may produce field strengths high enough to induce levels of several volts into a victim. In the commercial environment, field strengths up to 10V/m can be expected, but in more extreme situations such as aircraft flying close to radars then levels of hundreds of V/m must be considered.
As with emissions out of a circuit, an external field can couple either directly, with the internal circuitry and wiring in differential mode, or with the cables to induce a common mode current. Coupling with internal wiring and PCB tracks is most efficient at frequencies above a few hundred MHz, since wiring lengths of a few inches approach resonance at these frequencies and the internal dimensions of partially-screened cases may also form a resonant cavity. In either case, the structures responsible for the coupling are acting as accidental antennas.
At frequencies below 200-400MHz coupling with the connected cables is more efficient, and this will induce a common mode current in each exposed cable which can be modelled as a common mode voltage appearing at the interface with the enclosure. This in turn will cause common mode currents to flow internally, and differences in internal circuit impedances will convert this to a differential mode interference voltage to which the circuit will respond. Typically a radiated field of 1V/m will induce a current of 1–3mA in the cables, and the resultant interface voltage will depend on the common mode impedance ZCM at the interface. A well designed interface will have either a low ZCM – which diverts the current via good filtering or shielding to the enclosure, or a high ZCM – which prevents the current flowing into the interface, and is achieved with effective common mode chokes.
As with emissions, the presence of a ground reference plane is necessary for a repeatable RF test. The commercial conducted RF immunity test is specified in IEC 61000-4-6 and the radiated test in IEC 61000-4-3. Military requirements appear in DEF STAN 59-411 tests DCS02 and DRS02, and MIL STD 461 tests CS114 and RS103. The frequency range for each of these tests is shown in Table 1 (at the bottom of this page). Automotive RF immunity tests are referenced in the ISO 11451 and 11452 series and now IEC standards due to the electrification of modern vehicles.
The following video illustrates a typical instance of a product experiencing radiated interference. The external noise source is generated by a signal generator and an RF amplifier (with AM at 1 kHz). The noise is then directed towards the PCB under test through a small PCB antenna. You can observe that the tone produced by the beacon changes in response to the external noise.
In this particular scenario, the vehicle's screens exhibited flickering behaviour during the radiated immunity test. It's worth noting that due to the potentially hazardous nature of radiated immunity testing, the assessment was conducted within an anechoic chamber. Cameras were positioned to focus on the infotainment area inside the vehicle. The image presented here captures the view from the control room.
LF magnetic field susceptibility
The power-frequency (50-60 Hz) magnetic field is a direct result of currents flowing in power networks. When low-frequency currents flow in the entire power network, depending on the size of the current-circulating loop, the impact on equipment/products in the environment can be significant. A typical case is an equipment with a cathode ray tube (CRT) screen. The display on a CRT screen would appear to wobble due to the presence of a nearby low-frequency field. Professional audio equipment such as electric guitars, tape recorders, and loudspeakers are also sensitive to external magnetic fields. IEC 61000-4-8 defines the test method for basic power-frequency magnetic fields. In recent years, many low-frequency magnetic field issues have been identified in new product applications, such as products using electron-beam technology and electric vehicles (EVs). Products such as additive manufacturing equipment using electron-beam technology are also sensitive to power-frequency magnetic fields and poor immunity could lead to inaccuracy in the manufacturing process. In the case of EVs, traction motors generate fluctuating currents up to 2 – 3 kHz, and wireless power transfer (WPT) systems for battery charging are operated at about 85 kHz.
The issue with low-frequency magnetic fields in this case is often related to health and safety. The International Commission on Non-Ionizing Radiation Protection (ICNIRP) Guidelines 20204 describes the potential health and safety impacts of human exposure to electromagnetic fields. According to the Guidelines, the main physiological effects of electromagnetic field exposure include the electro-stimulation of the nervous system, resulting from electric fields being induced in biological tissues under exposure to time-varying magnetic fields with frequencies up to 10 MHz.
Not only can low-frequency magnetic fields pose health hazards to human beings, but they can also affect some electric control units (ECUs) in a vehicle. An ECU that consists of Hall-effect sensors located near the battery pack or powertrain modules could be affected by the low-frequency magnetic field if no sufficient shielding is provided.
A CRT screen wobble can be seen in the video below
AF susceptibility
There are no requirements for immunity of commercial products to audio frequency disturbances, but the military sector does have such requirements. MIL STD 461 CS101 applies low frequency disturbances to power lines, while DEF STAN 59-411 applies to power lines (DCS01) and to control and signal lines (DCS03). Both standards have a radiated magnetic field susceptibility requirement (RS101 and DRS01). In the latter case, susceptibilities are usually related either to unnecessarily large loop areas which allow a voltage to be induced in a sensitive circuit, such as an AF or sonar amplifier, according to Faraday's law:
V =A dB/dt
where A is the area of the loop, and dB/dt is the rate of change of flux coupling the loop
or to particular components which are sensitive to magnetic fields, such as microphones or transducers.
Conducted AF susceptibility on power lines is tested by applying a ripple voltage through a transformer in series with the power supply to the equipment; the DCS03 test applies a current through a test wire which is wrapped around the cable to be tested.
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