ELECTRICAL PRINCIPLES
ELECTRICAL PRINCIPLES
RF effects in components and circuits: skin effect; real and reactive power; electrical length of conductors
What is the result of conductor skin effect?
The AC current density is strongest at the surface of a conductor, and the magnitude decreases exponentially as you get farther away from the surface. Several variables affect this distribution, with frequency being one of them. You just have to remember that the current density at the surface increases with increasing frequency, leading to a 'thinner' RF current.
The skin effect governs how far RF signals penetrate a given material.
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Why is it important to keep lead lengths short for components used in circuits for VHF and above?
Any wire has self inductance, which increases with the length of the wire (among other things). Since the impedance of an inductor is proportional to frequency, it is usually safe to ignore the self inductance of short wires at low frequencies. But for VHF and above a wire's self inductance may have significant inductive reactance. This reactance is often unwanted and can be minimized by keeping connections short.
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What is the phase relationship between current and voltage for reactive power?
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Why are short connections used at microwave frequencies?
The answer is somewhat bogus as with microstrip and other high frequency designs, you use controlled lengths of connections (transmission lines) to purposely introduce phase shift which is part of tuning and matching.
In other words, short connections are not necessary other than to cut down on loss and parasitic radiation.
But if you wanted to minimize phase shift (which is rarely a design goal), then you would want short connections.
Just remember it is the only answer with "phase shift" in it.
It's also the only answer with the word "connection" in it, which is also in the question.
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What parasitic characteristic causes electrolytic capacitors to be unsuitable for use at RF?
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What parasitic characteristic creates an inductor’s self-resonance?
Self-resonance happens when you have both inductance and capacitance in series.
Real world components have parasitics-- every component contains resistance, inductance, and capacitance in addition to the values it is designed to have.
In an inductor, adjacent wire acts similarly to the plates of a capacitor to create a small amount of parasitic capacitance between each turn. This combination of inductance and capacitance causes.
HINT: Both "self-resonance" in the question "inter-turn" in the answer are hyphenated words.
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What combines to create the self-resonance of a component?
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What is the primary cause of loss in film capacitors at RF?
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What happens to reactive power in ideal inductors and capacitors?
The question states both ideal inductors and capacitors, so think perfect. The current just passes from the inductors (magnetic field) to the capacitors (electric field), back and forth so none of the power is lost or dissipated.
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As a conductor’s diameter increases, what is the effect on its electrical length?
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How much real power is consumed in a circuit consisting of a 100-ohm resistor in series with a 100-ohm inductive reactance drawing 1 ampere?
Only resistance (real component of impedance) consumes power. The values for the resistor, 100 ohms, and current, 1 A, are given.
\begin{align} P_{\text{real}} &= I^2 R\\ &= (1 \text{ A})^2(100 \:\Omega)\\ &= 100 {\text{ W}} \end{align}
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What is reactive power?
Capacitors resist change in voltage and inductors resist change in current each by storing energy and releasing it as voltage and current fluctuate. This is called reactance. Unlike resistance, no actual power is dissipated by reactance. In purely reactive circuits there will still be measurable voltage and current. The product of this voltage and current is called "wattless" power, measured in volt-ampere reactive (VAR).
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