RF Design

Signal Integrity Design, RF Design and EMC Design

What does Signal Integrity design, RF design and EMC design have in common?

Signal Integrity Design is a rapidly expanding field in the digital design world due to the increasing operational speeds at which electronics are processing data. Signal Integrity Design involves board layout issues such as trace impedance, printed circuit board (pcb) stackup, power supply bypassing, trace routing, reflections, ringing, risetimes, falltimes, parasitic inductance/capacitance, signal/noise ratio, filtering, etc. All these parameter are considered with respect to a wide band of frequencies that are required to uniformly propagate down a transmission path in order to retain the signal’s original waveshape. Fourier analysis tells us that any waveshape may be described as a sum of sinusoidal signals with appropriate amplitudes, frequencies and phase relationships. In order for the digital signal to be received correctly at a given load a certain number of these sinusoidal signals must maintain their relationships relative to each other throughout the transmission path. Signal integrity design typically defines a waveform from the time domain perspective.

RF Design is considered by a number of students today to be an older discipline. Digital is the new age or is it? RF design for many years has required concentration and care to avoid reflections, unwanted resonances, maintain phase relationships, reduce parasitic inductance/capacitance, power supply bypassing, filtering, bandwidth limiting, signal/noise ratio, etc. RF design sometimes defines a transmission path from an S-parameter perspective (S₁₁, S₁₂, S₂₁, S₂₂). These parameters define the voltage relationships of a two port network. Actually, RF design is concerned with the same issues as signal integrity design just from a frequency domain perspective typically. One can look at signal integrity design from a frequency domain perspective and RF design can be looked at from a time domain perspective. The two are interchangeable if proper relationships are maintained between the perspectives. Impedance matching between the source, transmission media and load(s) are critical to both types of design perspectives.

Electromagnetic Compatibility (EMC) Design requires a unit or system to operate in its intended environment without interfering with or being interfered with, itself and/or other electronics. In order to achieve this harmony with the surrounding electromagnetic environment, the signals inside a unit whether RF or Digital require a stable controlled transmission path between the respective source(s) and load(s), or transmitter(s) and receiver(s) (depending on your terminology of choice), clean/stable waveshapes, controlled current return paths and protection from unwanted stray or intentional energy. Whether you call it signal integrity design techniques or RF design techniques, the combination of properly applied perspective are required to meet the EMC Design goal with minimum cost and schedule impacts. Another advantage to applying these techniques or concepts early in a design is improved design margins, and increased reliability.

These three disciplines are not mutually exclusive even though many see them as such. In order for a company to get the biggest bang for their buck, a unification of these three concepts is required. When designing a pcb, before placing the first component in a layout, the number of layers, board stackup, characteristic impedance and general signal flow needs to be considered. Use only the frequency ranges necessary for the signal you need (i.e. bandwidth limiting via filtering or component choice). Provide controlled return currents back to the sources. Pay attention to the unseen circuits created by parasitic inductances and capacitances that rob the signal of energy. Be diligent to minimize reflections, because once created they are eliminated only by radiating through antenna structures or as heat. Provide a stable power distribution system because this is a common link or path between functions in a unit or system that stray energy can propagate through. Isolate dissimilar function from on another either by physical location spacing or shielding. All the above basic recommendation of what to consider in a design are common to all three disciplines. Practical Engineering has experience working from all three perspectives providing a unified design approach for new and existing designs.