EMC Simple source models; Signal waveforms + spectra

Simple source models

For many EMC situations such as coupling by radiation, effective prediction and analysis are achieved by reference to simple mathematical expressions. For nearly all products experiencing EMC problems, the equipment will work perfectly in the development laboratory and only when it’s subjected to external electromagnetic disturbance do other facets of its characteristics emerge. Under these circumstances, circuit conductors are considered as antennas capable of both transmitting and receiving radio energy. The circuit can usually be assessed as either a short monopole antenna, for instance where one end of the conductor is terminated in a high impedance, or as a loop where both ends are terminated in low impedance.

For the monopole equivalent at low frequencies and at distances greater than a wavelength, the field strength E, at distance d (in meters) is given by: …where Z is the current in amps, h is the length of the conductor and A is the wavelength (= 300 + frequency in MHz), both in meters. It can be seen that the field strength is greater for shorter distances and higher frequencies (shorter wavelengths). For loop radiators at low frequencies, the field is given by: where n is the number of turns and A the area of the loop. The field strength is greater at shorter distances and higher frequencies.

At high frequencies where the conductor lengths are comparable with a wavelength, a good approximation of the field at a distance can be taken if the source is considered as a half-wave dipole. The field is then given by: …where P is the power in watts available in the circuit.

Receptor efficiency:

To estimate the degree of coupling in the radiated path, empirical data give values of induced currents of about 3 mA for an incident electromagnetic wave of 1 volt/meter.

This relationship can be used to good effect in converting the immunity test levels in the standards into an engineering specification for induced currents impressed at an input port due to coupling via an attached cable.

Signal waveforms and spectra

For many digital electronic systems, the main concerns in emission control at low frequencies are associated with power line disturbance generated by switch mode power supplies. These devices switch at a relatively high rate, in the order of 30-100 kHz, and produce a line spectrum of harmonics spreading over a wide frequency band. Frequency (MHz)

___ Typical conducted emission spectrum of a switch mode power supply With no mains filtering, the emission levels from the individual harmonics are considerably in excess of the common emission limits. Care is needed in the sourcing of these subassemblies to ensure that they are compliant with the relevant standards.

At higher frequencies, noise from digital circuits switching at very high rates (clock frequencies in excess of 30 MHz are not uncommon) couples via external cables and radiates in the VHF band (30-300 MHz) or it may radiate directly from circuit boards in the UHF band (300-1000 MHz). A clock oscillator has a waveform and a spectrum of the type.

' Time T Waveform 1 lnt, 1 /nt,

Frequency spectrum

___ Waveform and frequency spectrum of a digital signal

It can be seen that the turning points in the spectrum are (1/pi) x the pulse width t_ above which frequency the spectrum decreases inversely proportionally to frequency, and (1/pi) x the rise time t_r. At frequencies greater than 1/pi the spectrum decays rapidly, in inverse proportion to the square of the frequency. Thus for longer pulse durations the high-frequency content is reduced; if the rise time is slow then the content is further reduced. For good emission control, slower clock speeds and slower edges are better for EMC. This is contrary to the current trends where there are strong performance demands for faster edges and higher-frequency clocks.

Many equipment malfunction problems in the field are caused either by transient disturbances, usually coupled onto an interface cable, or by radar transmitters if close to an airfield. The disturbance generated by a pair of relay contacts opening comprises a series of short duration impulses at high repetition rate. As the contacts separate the energy stored in the circuit inductance is released, causing a high voltage to occur across the contacts. The voltage is often sufficient to cause a discharge and a spark jumps across the gap. This is repeated until the gap is too wide. When coupled into a digital electronic circuit, the disturbance can change the state of a device and interference in the form of an unwanted operation or circuit 'lock-up' occurs. Control is exercised by ensuring that the receptor circuit has adequate immunity to this type of disturbance.

___ Voltage across opening relay contacts. Time.

The radar transmission is one example of a modulated radio frequency signal, and this can often cause interference even in low-frequency electronic circuits. The radio frequency energy is rectified at the first semiconductor junction encountered in its propagation path through the equipment, effectively acting as a diode rectifier or demodulator. The rectified signal, an impulse wave in the case of radar, is thus processed by the circuit electronics and interference may result if the induced signal is of sufficient amplitude.

RF Radar signal pulse modulated RF

Time Radar signal after circuit rectification

___ Radar signal

Most equipment is designed and constructed to be immune to radio frequency fields of 3 volts per meter (or 10 volts per meter for industrial environments) at frequencies up to 1 GHz. Many radars operate at frequencies above I GHz ( For example, 1.2,3 and 9.5 GHz) and equipment close to an airfield may suffer interference because it’s not designed to be immune. Under these circumstances, architectural shielding is required, often comprising the use of glass with a thin metallic coating which provides adequate light transmittance and, more importantly, effective shielding at these high radio frequencies. 66 dB (PV) ; 50 46 -- - EN55022 Class A I EN55022 Class B

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