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Note: Many answers contain passages quoted in whole or in part from the text.
Return to RF Cafe Quiz #19
1.
Which circuit law states that the sum of all currents into and out of a node equals zero?
c) Kirchhoff's
Current Law.
This diagram illustrates the principle.
2. How much power is dissipated by
an ideal 100 pF capacitor fed by a 1 V
_{pk-pk} sinewave at 10 MHz? Hint: Z = 1/(2
πfC)
d) 0 W.
An ideal capacitor or inductor dissipates no power, since power is a function of the real part
of a complex impedance (R ± jX). Ideal capacitors and inductors store and release energy without consuming it (by
releasing heat).
3. What causes a ground current loop?
c) More than one ground potential in
a circuit.
A ground current loop occurs when "ground," or the supposed 0 V point, exists at more than one
potential in a common circuit. The difference of potential causes a current to flow, often creating a noise
component.
4. What
type of filter does this circuit represent?
c) Bandpass.
An ideal series tank circuit passes energy
at its resonant frequency with zero impedance (short circuit), and a parallel resonant circuit presents infinite
impedance (open circuit). So, if both are tuned to the same frequency (although usually there is an offset), then
at resonance the circuit appears as a short between the upper terminals and an open circuit wrt the lower
terminals.
5. How many poles does the filter in Q4 have?
c) 2 poles.
Each resonant LC tank generates a
pole in the transfer function.
6.
What is the configuration of the opamp shown here?
a) Integrator.
The transfer function is
7. What is the secondary voltage
of this transformer?
d) 2 V.
The secondary:primary turns ratio is 20:100, or 1:5. Voltage ratio is
proportional to the turns ratio, so the secondary voltage is 10/5 = 2V
8. What is the impedance of
the transformer secondary in Q7?
d) 4 Ω.
The impedance ratio is proportional to the square of the
turns ratio, so the secondary impedance 100/5
^{2} = 4 Ω.
9. In an ideal system, what is the
minimum sampling rate to prevent aliasing of a signal with a maximum frequency of 100 kHz?
b) 200 kHz.
The Nyquist Sampling Theorem demonstrates that a band-limited signal must be sampled at a rate at least twice the
highest frequency in the original signal in order to prevent aliased content in the reconstructed signal. Here is
a nice little applet that illustrates the
sampling principle. Set the original signal for 150 Hz, and the sample rate to 300 Hz (2x the original) to see
that the reconstructed signal is an exact duplicate of the original. Ditto for 350 Hz. Now, set the sample rate to
200 Hz or 100 Hz and note the added frequency components in the reconstructed signal.
10. Which
type of diode typically has the lowest forward bias voltage for a given current?
d) Schottky.
A
Schottky diode is very similar to a standard P-N junction diode, except instead of using an implanted p-layer, the
rectification occurs at the interface between a barrier metal and the silicon layer. Here are typical I-V charts
for the four types of diodes (compare at 25°C)
.
GaAs Diode
Germanium Diode
Silicon Diode
Schottky Diode