Frequency Range of Interest for EMI Analysis
Most EMI algorithms are typically based on quasi-static assumptions. Hence, the examined structures such as TL segments must be small compared to the wavelength. An upper frequency limit of 1GHz is therefore applied. The EMI analysis starts at 30 MHz. Signals that have a fundamental frequency lower than 1 MHz are ignored, as they are considered to not contribute significantly to the overall radiation. This is because their spectral energy content is too low in typical PCB applications.
The Frequency Spectrum
The spectral content within the EMI Algorithms is based on all clock frequencies for Medium-Utilization and High-Utilization E-Nets. This is derived as a Fourier Series expansion of a trapezoidal, periodic clock signal. The frequencies are derived from the signal's current computation. For each E-Net, the clock frequency can be defined in the Classification dialog, Net table, in the PI/EMI Analysis Module. The default duty cycle of the digital signals is defined as 45%. Settings made in the Constraint Browser for clock frequency and duty cycle are applied in the PI/EMI Analysis Module. The Fourier Series expansion considers the rise and fall times of the signals, as well as their magnitude. You can also define these in the PI/EMI Analysis Module, Net table.
Those harmonics which have a small magnitude only, and which may not result in any significant emission (<20 dB µV/m in 3 m antenna distance), are suppressed in the spectrum and are not considered any further. Frequencies of different E-Nets which may appear closely separated are treated as a single contribution when their distance in the spectrum is smaller than 0.1 MHz. This is similar behavior to measurement equipment with a respective bandwidth. As a result of the above, not all integer multiples of the E-Net's clock frequencies may appear in the spectrum. The actual frequencies can also be slightly different from these multiples.
PI-Analysis Frequency Range
You can define a wider frequency range for PI Impedance Analysis.
- Minimal frequency: 100 kHz.
- Maximal frequency: 10 GHz.
Far Field Analysis
The EMC Expert System accounts only for far field effects. The minimum antenna distance is therefore required to be over 3 m. Even this distance is not really in the far field at low frequencies. All field values are computed for an antenna in this distance. All antennas are assumed to be an-isotropic. This means that only the maximum emitted field strength is reported. As no phase information about the fields is known, fields of multiple sources at the same frequency are added as an RMS value.
All antennas are assumed to be in resonance. However, this is not true in reality for all frequencies. This worst case assumption is made in order not to suppress any potential EMI problems, by assuming a fixed antenna length.
EMI Effects not accounted for in the PI/EMI analysis module
Some EMI mechanisms are not accounted for in the PI/EMI Analysis Module, such as the direct far field emission from ICs. Compared to other sources on the PCBs, this is a minor EMI effect because the antenna size is small and therefore inefficient up to 1 GHz.
