GW Instek GRF-3300 Series Student Manual - Manual (Page 2)

GRF-3300 Manual
3 310
Theory
When the signal frequency is high, for example 3GHz, the wavelength no longer
becomes negligible as described in the generic circuit theory: in the example case, it
reaches 100mm in free space. In other words, higher frequency has shorter wavelength.
As the wave (signal) travels (flows) along the conductor, the phase of the voltage and
current changes significantly over the physical length of the conductor. Therefore the
conductor, which works as a short-circuit node in low frequency, now works as a
“distributed component” in high frequency. It is a completely different conception from
the standard circuit theory.
In this chapter we will introduce different methods to implement LPF, HPF and
BPF by using the transmission line technique. The general filter related theory has been
introduced in Chapter 7; here we will just focus on microstrip line filters.
16-1. Transmission Line Basics
Transmission line theory forms the basis of distributed circuits. For detailed study,
we should start from Maxwell’s equations, then move on to electromagnetic wave
analysis, and so on. In this section, we will give the very basic concept of transmission
line since introducing everything in this textbook is impossible.
The key difference between circuit theory and transmission line theory lies in
electrical size. The theory of circuit analysis assumes that the physical dimensions of a
network are much smaller than its electrical wavelength; the size of transmission lines
may be a fraction of single or several wavelengths. Thus we can say that a transmission
line is a distributed-parameter network where voltages and currents can vary in
magnitude and phase over the length of the network.
On the other hand, lumped elements in generic electronic circuit such as inductors
and capacitors are generally available only for a limited range of values and are difficult
to implement at microwave frequencies, where distance between filter components is not
negligible.