Electromagnetic Interference Suppression
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Analog Electromagnetic Interference
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Suppression of EMI is necessary for all electronic designs, both wireless and nonwireless, due to rigid European and strict Federal Communications Commission (FCC) regulations, as well as to protect your design against interferers There are many ways to accomplish this, many of which must be used in concert: Metallic shielding of synthesizers and oscillators, which is used to prevent coupling energy into other circuits or to prevent other circuits from coupling energy into our frequency sources, is crucial This is because any such EMI that is propagating out of a frequency source may decrease the isolation between other stages and/or cause harmonic or mixing products to appear in the rest of the radio, while EMI that successfully enters a phase-locked loop (PLL) or voltage controlled oscillator (VCO) may create undesirable spurs Therefore, traces leaving a frequency source should be protected by placing them on a lower PCB layer or within an RF shield box, which is especially essential when dealing with high-power frequency sources such as those used to feed passive diode high-level mixer conversion stages
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EMI Control and Printed Circuit Board Layout
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Although placing RF shielding over any EMI-generating circuit (or shielding a circuit that is susceptible to EMI) will aid in decreasing this radiation substantially, the EMI emissions may still find another path: they may actually travel through the dielectric of the PCB itself For either the fundamental or its harmonics, the method used to attenuate this particular EMI propagation mode is to utilize a barrier of throughhole vias all the way from the top of the PCB to its very bottom groundplane This is referred to as a via fence, and is located around some or all of the affected circuit At high microwave frequencies, or if high amplitude harmonics of the fundamental are present, these vias will have to be placed in a relatively dense pattern Preventing EMI from corrupting the system s DC power supply, and thus the entire wireless device, is critical A generous amount of decoupling capacitors that are capable of shunting the entire spectrum of possible EMI frequencies, along with a high frequency choke, should be placed as close as possible to all active devices This will help prevent unwanted oscillations and/or contamination of our desired signal, and is especially important for VCOs, PLLs, low-noise amplifiers (LNAs), and PAs, which must all be heavily decoupled from any noise-producing conducted emissions from the DC supply, or prevented from injecting any of their own circuit-generated EMI from reaching the common DC supply and other circuits And since saturated power amplifiers and other nonlinear stages can produce high-amplitude harmonics and distortion products, the EMI created within these particular circuits may be the fundamental itself or the fifth harmonic of the fundamental
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Digital Electromagnetic Interference
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The electromagnetic interference created within digital circuits located in or attached to the radio system is quite similar to the above analog circuits However, due to certain design variances, and the huge amount of harmonics generated by the high-speed square-like waveforms of a digital circuit, there are some added considerations If traces of any significant length are left unterminated, or terminated into a high impedance, they can be capable of radiating substantial EMI, especially at high frequencies, almost as if they were small monopole or loop antennas The trace may also be located close enough to another trace or component to couple capacitively or inductively between the two objects, causing significant or subtle crosstalk between the circuits The capacitive coupling can be visualized as a virtual capacitor that has been placed in parallel between the two circuits, creating a low impedance path for the EMI current to flow Crosstalk created by inductive coupling can best be visualized as each of these traces being comparable to a series inductor, or a winding of a transformer, and thus causing induced coupling of EMI currents to flow within the victim trace EMI radiation from traces may not even be at the expected fundamental frequency, but can be at one of its many harmonics In fact, these harmonics are more easily radiated than the fundamental due to their shorter wavelength: as the length of the trace gets closer to onetenth of a wavelength of the signal of interest or longer, the transmission line effect must be considered in order to preserve signal integrity The transmission line effects infers that after a trace becomes longer than this one-tenth of a wavelength, the signal s response to the trace itself must be considered and, as the frequency of operation increases further, reflections will become more of a problem on a simple noncharacteristic impedance trace, sometimes causing extreme signal degradation of the original digital waveform We can prevent much of these transmission line issues by confirming that the trace itself has the proper impedance, by using shorter line lengths versus wavelength,
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