|
In all cases of far-field radiated emissions or immunity, the coupling mechanism involves a structure which is acting as an antenna. Most of the time, this is not intentional. We need to understand how structures act as antennas and then minimise their efficiency, or control the currents that drive them to radiate, or divert the currents that they pick up harmlessly away from sensitive circuits.
In the majority of cases, we can identify the antenna structure as being in one of three categories:
.gif) Directly to/from a PCB
To/from a cable
To/from a metal enclosure
Radiation from/to the PCB structure
In most electronic equipment, the primary emission sources are currents flowing in high frequency circuits (clocks, video and data drivers, and other oscillators) that are mounted on printed circuit boards (PCBs).
 Radiated emission directly from a PCB can be modelled as a small loop antenna carrying the interference current – this is equivalent to a differential mode of emission. A small loop is one whose dimensions are smaller than a quarter wavelength (λ/4) at the frequency of interest (1 metre at 75MHz). The maximum electric field strength from such a loop over a ground plane at 3 metres distance is proportional to the square of the frequency (f in MHz):
E = 87.7 · 10-8 (f2 · A · IS) volts per metre (1)
where A is the loop area in cm2, and IS is the source current in amps
If A = 10cm2, IS must be less than 4.5mA at 50MHz for an equivalent field strength of 40dBμV/m – which is equivalent to the European Class B emissions limit (adjusted from 10m to 3m).
Immunity to incoming RF fields can be modelled in the same way - by regarding the PCB tracks as loop antennas, in which a voltage is induced by Faraday's law. The interference voltage then appears in series with the wanted signal voltage in each susceptible node.
This mode of coupling can be controlled either by reducing the radiating loop area (typically, this is a simple consequence of using a ground plane) or by screening the offending circuit. But it isn't the only mode.
Radiation from/to the cable structure
 A PCB is typically connected to cables and housed within an enclosure. Cable emission is mainly due to common-mode interference current flowing in the cable. The interference current is generated from ground noise developed across the PCB or elsewhere in the equipment and referred to the ground or the enclosure, and may flow along the conductors, or along the shield of a shielded cable. (Common mode emission may also be created by currents on any structure, including current flow on an unshielded PCB itself – it's just that cables are the easiest to visualise and model.)
The simple model for cable radiation at lower frequencies is a short (L < λ/4) monopole antenna over a ground plane. (When the cable length approaches resonance, e.g. above 75MHz for a 1m length, the model becomes invalid.) The maximum field strength at 3m over a ground plane due to this radiation is directly proportional to frequency:
E = 0.42 · (f · L · ICM) volts per metre (2)
where L is the cable length in metres and ICM is the common-mode current in amps at f MHz flowing in the cable
For a 1m cable, ICM must be less than 4.8μA at 50MHz for an equivalent field strength of 40dBμV/m – very much lower than the equivalent PCB-based differential mode current, calculated above. And, of course, a screen around the circuit by itself is of no use; what must be achieved is a reduction in the current fed into the radiating structure.
Henry Ott and Clayton Paul outlined a method to convert RF current probe measurements into electric field strength radiated from cables carrying RF currents. It is a derivation/simplification of the full treatment outlined in “Antenna Theory – Analysis and Design” (C. Balanis). Engineers may find this application note from Tekbox useful if they want to explore the details. Min has also demonstrated this method in his popular Youtube video.
Again, immunity to incoming RF fields can be modelled in the same way, by regarding the cable as a simple monopole antenna so that the RF field induces a common mode current into the point of connection to the circuit; this current then creates differential mode voltages within the circuit as it passes through differential parasitic or circuit impedances.
For both emissions and immunity control, the design of the cable interface is paramount.
Radiation from/to an enclosure
 Once an electronic circuit is housed in a conducting enclosure, the radiation model is more complex. Surface currents on the enclosure will radiate, but so too will currents on any cables connected to the enclosure. If the cables are not RF-bonded to the enclosure (as with a good screened cable connection or with adequate interface filtering) then the cable radiation model dominates, as above. If they are bonded, then the total mechanical assembly, enclosure plus cable(s), is the radiating structure. This will be driven by any sources that create outer surface currents. Typically these will be apertures or seams in the enclosure, or inadequate cable screen bonds, which are illuminated by internal fields or currents from the operating circuit. Vice versa, incoming fields will create such surface currents which will then couple to the internal circuit and potentially cause susceptibilities.
Being composed of a complex assembly of three-dimensional conducting structures, this description is not amenable to a simplified model for determining emissions or susceptibility levels. If you need to know the level of coupling before performing a test, then only a three-dimensional electromagnetic simulator will give valid answers: although this is now a possibility with available commercial software packages.
Try your own calculation for field strength from a loop, based on equation (1):
And for field strength from a monopole, based on equation (2):
|
.png)
Send comments to an expert