High-frequency interconnects require unique consideration since they typically behave not as simple wires yet as transmission lines.
In systems with low frequency, parts are attached by PCB traces or wires. The resistance of these conductive components is reduced sufficiently to be negligible in a power line hardware.
This circuit’s aspect design, as well as evaluation adjustments significantly as regularity increases. RF signals don’t simply follow cables or PCB traces that we anticipate based on the experience of professionals having low-frequency circuits.
The Transmission Line
The behavior of the RF interconnects is different from that consisting of ordinary wires having low-frequency signals, very different, actually, that extra terminology is utilized: a transmission cable is a wire or a couple of conductors, that should be analyzed according to the features of signals having high-frequency signal proliferation.
First, let’s make clear two things:
Cord vs. Trace
“Cord” is hassle-free; however, an inaccurate word in this context. The coax is definitely a traditional example of a transmission line, yet PCB traces additionally function as the transmission lines. A “microstrip” transmission line contains a trace, as well as a nearby ground plane, as complies with:
The “strip line” transmission line includes a PCB trace as well as two ground planes:
The PCB transmission lines particularly are essential due to the fact that their features are regulated directly by the designer. When professionals purchase a cable, its properties are fixed; they simply collect the needed information taken from the datasheet. If laying down an RF PCB, they can easily customize the measurements, and also thus the electric qualities of the transmission line according to the needs of the application.
The Transmission Line Criterion
Not every high-frequency adjoin is a transmission line; this term refers mostly to the electrical interaction between a signal, as well as a cord, not to the regularity of the signal or the physical features of the cable. So, when do professionals require to incorporate transmission-line results right into the analysis?
The general concept is that transmission-line impacts become substantial when the length of the line approaches or is higher than the wavelength of the signal. A more particular standard is a quarter of the wavelength:
- If the interconnect size is less than one-fourth of the signal wavelength, transmission-line evaluation is not necessary. The interconnect itself does not dramatically influence the electrical behavior of the circuit.
- If the interconnect length is more than a quarter of the signal wavelength, transmission-line impacts come to be substantial, as well as the impact of the interconnect itself has to be thought about.
