It is present everywhere along the cable, because it results from the cable’s inherent capacitance and inductance. Characteristic impedance is not a function of cable length.Characteristic impedance is determined by the physical properties of the transmission line in the case of a coaxial cable, it is a function of the inner diameter (D1 in the diagram below), the outer diameter (D2), and the relative permittivity of the insulation between the inner and outer conductors.Here are some salient points that help to clarify the nature of characteristic impedance: Thus, it’s important to have a clear idea of what we mean by “characteristic impedance.” It is not the resistance of the signal conductor inside the cable-a common characteristic impedance is 50 Ω, and a DC resistance of 50 Ω for a short cable would be absurdly high. However, we sometimes use “impedance” where “resistance” would theoretically be more appropriate for example, we might refer to the “output impedance” of purely resistive circuit. “Impedance” is used in the context of AC circuits and often refers to a frequency-dependent resistance. Overall this is a fairly straightforward concept, but initially it can cause confusion.įirst, a note on terminology: “Resistance” refers to opposition to any flow of current it is not dependent on frequency. The most important property of a transmission line is the characteristic impedance (denoted by Z 0). However, at 1 GHz many PCB traces must be treated as transmission lines, and as frequencies climb into the tens of gigahertz, transmission lines become ubiquitous. For medium frequencies, only very long cables require special consideration. So for very low frequencies, transmission-line effects are negligible. The corresponding transmission-line thresholds are the following: If we assume a propagation velocity of 0.7 times the speed of light, we have the following wavelengths: Recall that wavelength is equal to propagation velocity divided by frequency: If the interconnect length is greater than one-fourth of the signal wavelength, transmission-line effects become significant, and the influence of the interconnect itself must be taken into account.The interconnect itself does not significantly affect the electrical behavior of the circuit. If the interconnect length is less than one-fourth of the signal wavelength, transmission-line analysis is not necessary.A more specific guideline is one-fourth of the wavelength: The general idea is that transmission-line effects become significant when the length of the line is comparable to or greater than the wavelength of the signal. So when do we need to incorporate transmission-line effects into our analysis? Not every high-frequency interconnect is a transmission line this term refers primarily to the electrical interaction between signal and cable, not to the frequency of the signal or the physical characteristics of the cable. When laying out an RF PCB, we can easily customize the dimensions-and thus the electrical characteristics-of the transmission line according to the needs of the application. When we buy a cable, its physical properties are fixed we simply gather the necessary information from the datasheet. PCB transmission lines are particularly important because their characteristics are controlled directly by the designer. The “stripline” transmission line consists of a PCB trace and two ground planes: The “microstrip” transmission line consists of a trace and a nearby ground plane, as follows: The coaxial cable is certainly a classic example of a transmission line, but PCB traces also function as transmission lines. “Cable” is a convenient but imprecise word in this context. The behavior of RF interconnects is very different from that of ordinary wires carrying low-frequency signals-so different, in fact, that additional terminology is used: a transmission line is a cable (or simply a pair of conductors) that must be analyzed according to the characteristics of high-frequency signal propagation. RF signals do not travel along wires or PCB traces in the straightforward fashion that we expect based on our experience with low-frequency circuits. This aspect of circuit design and analysis changes dramatically as frequency increases. The resistance of these conductive elements is low enough to be negligible in most situations. In low-frequency systems, components are connected by wires or PCB traces. High-frequency interconnects require special consideration because they often behave not as ordinary wires but rather as transmission lines.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |