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RF TECHNIQUES


At higher frequencies, basic circuit theory runs into problems. For
example, if wires are electrically long, transmission line effects can occur.
The basic theory no longer applies because electromagnetic wave reflections
bouncing back and forth along the wires cause problems. These
electromagnetic wave reflections can cause constructive or destructive
interference resulting in the breakdown of basic circuit theory. In fact,
when a transmission line has a length equal to one quarter wavelength
of the signal, a short placed at the end will appear as an open circuit at
the other end! Certainly, effects like this cannot be ignored. Furthermore,
at higher frequencies, circuits can radiate energy much more
readily; that is circuits can turn into antennas. Parasitic capacitances and
inductances can cause problems too. No component can ever be truly
ideal. The small inductance of component leads and wires can cause
significant voltage drops at high frequencies, and stray capacitances
between the leads of the component packages can affect the operation
of a high-frequency circuit. These parasitic elements are sometimes
called “the hidden schematic” because they typically are not included
on the schematic symbol.

How do you define the high-frequency regime? There is no exact
border, but when the wavelengths of the signals are similar in size or
smaller than the wire lengths, high-frequency effects become important;
in other words, when a wire or circuit element becomes electrically long,
you are dealing with the high-frequency regime. An equivalent way to
state this is that when the signal period is comparable in magnitude
or smaller than the delay through the interconnecting wires, highfrequency
effects become apparent. It is important to note that for digital
signals, the designer must compare the rise and fall times of the digital signal
to the wire delay. For example, a 10 MHz digital clock signal may only have
a signal period of 100nsec, but its rise time may be as low as 5nsec.
Hence, the RF regime doesn’t signify a specific frequency range, but
signifies frequencies where the rules of basic circuit theory breakdown.
A good rule of thumb is that when the electrical length of a circuit element
reaches 1/20, RF (or high-speed digital) techniques may need to be used.
When working with RF and high-frequency electronics it is important
to have an understanding of electromagnetics. At these higher frequencies,
you must understand that the analogy of electrons acting like
water through a pipe is really more of a myth than a reality. In truth,
circuits are characterized by metal conductors (wires) that serve to guide
electromagnetic energy. The circuit energy (and therefore the signal) is
carried between the wires, and not inside the wires. For an example, consider
the power transmission lines that deliver the electricity to our
homes at 60Hz. The electrons in the wires do not directly transport the
energy from the power plant to our homes. On the contrary, the energy
is carried in the electromagnetic field between the wires. This fact is
often confusing and hard to accept for circuit designers. The wire electrons
are not experiencing any net movement. They just slosh back and
forth, and through this movement they propagate the field energy down
the wires.





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