Waveguide Components Questions and Answers

Answer: A

The forward-biased model is an inductor in series with a resistor.

Under forward bias the intrinsic region of a PIN diode is flooded with injected carriers, so it behaves as a small resistance R_f that falls as the bias current rises. In series with it is the package and lead inductance L. That simple series L-R circuit is the forward-bias microwave model, which is why PIN diodes are used as current-controlled resistors in switches and attenuators.

The third option, lead inductance in series with a parallel resistance and capacitance, is the reverse-bias model, in which the depleted I-region acts as a capacitor with a large parallel resistance. The fourth, a capacitor in series with a resistor, is a simplified reverse-bias model, and the second, a resistor across the inductor, is not the standard model for either bias.

Answer: C

The transferred electron device is the Gunn diode.

In gallium arsenide, electrons in a strong field transfer from a low-energy, high-mobility valley of the conduction band to a higher, low-mobility valley. Above a threshold field the average velocity therefore falls as the field rises, a negative differential resistance, and the Gunn diode uses it to generate microwave oscillations.

The klystron, the travelling-wave tube and the magnetron are vacuum tubes. They work by velocity modulation and bunching of an electron beam, or by electrons moving in crossed electric and magnetic fields, not by transferred electrons in a semiconductor.

Answer: D

Faraday rotation is used in (1) and (2), circulators and isolators.

In a magnetised ferrite the plane of polarisation of a wave rotates by an amount that does not depend on the direction of travel, so the effect is non-reciprocal. A Faraday rotation isolator uses this to pass a wave in one direction and absorb it in the other, and a Faraday rotation circulator routes power from each port to the next in one rotational direction only.

The directional coupler and the magic tee are reciprocal passive junctions made of ordinary waveguide. They need no ferrite and do not use Faraday rotation, so any option including (3) or (4) is wrong.

Answer: B

The correct matching is a-ii, b-i, c-iv, d-iii.

• Klystron: electrons are velocity-modulated and bunched, and their kinetic energy is converted into RF energy, giving RF amplification (ii).
• Magnetron: a crossed-field oscillator that runs from a DC anode supply and converts that DC power into microwave power (i).
• Gunn diode: a transferred-electron device used as an oscillator in the range of about 10 GHz up towards 1 THz (iv).
• PIN diode: works by high-level injection of carriers into its intrinsic region, which makes it a current-controlled resistor at microwave frequencies (iii).

Only one code pairs the klystron with RF amplification by kinetic energy, its defining principle.

Answer: B

The matching is a-iv, b-iii, c-ii, d-i. Ferrite devices such as circulators and isolators are the non-reciprocal components of microwave systems. A magnetron is a cross field device, since its electric and magnetic fields are at right angles. A klystron works by velocity modulation that leads to bunching of the electrons. A Gunn diode is used as a microwave oscillator. Option (A) gives the magnetron as bunching. Option (C) gives the ferrite device as a cross field device. Option (D) lists the pairs in plain order, which does not fit. Only (B) is right.

Answer: A

Statements 1 and 3 are correct, which is option (A). With power fed at port 1 and the main output at port 2, port 4 is the coupled port and port 3 the isolated port. Coupling is the input power over the coupled-port power, \(C = 10\log_{10}(P_{1}/P_{4})\), which is statement 1. Directivity is the coupled-port power over the isolated-port power, \(D = 10\log_{10}(P_{4}/P_{3})\), which is statement 3. Statement 2 inverts the ratio for coupling, and statement 4 uses \(P_{2}/P_{1}\), the through-path ratio, which is not directivity. Options (B), (C) and (D) each contain one of those wrong statements.

Answer: A

The correct matching is a-ii, b-iii, c-i, d-iv, which is code (A).

A klystron works by bunching of the electrons (ii), a reflex klystron by velocity modulation (iii), a magnetron is a cross-field device (i) and a Gunn diode shows negative resistance (iv).

Codes (B), (C) and (D) attach these properties to the wrong devices.

In a klystron the beam is velocity modulated in the first cavity and the faster electrons catch up with the slower ones, forming bunches that give up energy in the output cavity. The magnetron has the magnetic field crossed with the electric field, hence cross-field.

Answer: D

Frequency pushing and pulling are effects in the magnetron: pushing is the change of oscillation frequency with the anode current and pulling is its change with the load. The reflex klystron is tuned by its repeller voltage, the two cavity klystron is an amplifier, and a pulsed radar system is not a tube. So the magnetron is correct.

Answer: B

The reflex klystron is used as an oscillator only, option (B). It has a single cavity and a repeller, and the electrons that are turned back by the repeller feed energy back into the same cavity, which sustains oscillations at microwave frequency.

It is not used as an amplifier, because it has no separate input and output cavity, so options A and C are wrong. Option D says it is neither an amplifier nor an oscillator, which is false since oscillation is its use. It is a low-power source, suited to use as a local oscillator.

Answer: C

Statements 2, 3 and 4 are correct. In an ideal magic tee all four ports are matched, so the diagonal scattering parameters \(S_{11}, S_{22}, S_{33}\) and \(S_{44}\) are zero; statement 3 is true and statement 1 is false. The junction is passive and reciprocal, so \(S_{jk} = S_{kj}\), which is statement 2. The H-arm and E-arm are isolated from each other, so \(S_{34} = S_{43} = 0\), and the side arms satisfy \(S_{13} = -S_{23}\) and \(S_{14} = S_{24}\), which is statement 4.

Any option that contains statement 1 claims the ports are not matched, which is wrong for the ideal device. Only option (C) leaves it out and includes 2, 3 and 4.