This set of Microwave Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Frequency Multipliers”.

1. Oscillators operating at millimeter wavelength are difficult to realize and are also less efficient.

a) true

b) false

View Answer

Explanation: As frequency increases to the millimeter wave range, it becomes increasingly difficult to build fundamental frequency oscillators with good power, stability and noise characteristics. An alternative approach is to produce a harmonic of a low frequency oscillator through the use of frequency multiplier.

2. __________ is an example for a frequency multiplier.

a) resistor

b) inductor

c) capacitor

d) transistor

View Answer

Explanation: A non linear device has the ability to generate the harmonics of the input sinusoidal signal. Transistor and diodes are non linear devices and hence can be used as a frequency multiplier.

3. The major drawback of frequency multipliers is that they have:

a) higher attenuation

b) complex construction methods

c) complex design

d) none of the mentioned

View Answer

Explanation: Designing a good quality frequency multiplier is more difficult since it non-linear analysis, matching at multiple frequencies, stability analysis and thermal considerations. Considering all these issues for designing a multiplier makes it very complex.

4. A reactive diode multiplier uses _______ as the key electronic component for frequency multiplication.

a) zener diode

b) light emitting diode

c) varactor diode

d) Gunn diode

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Explanation: Reactive diode multipliers use either a varactor diode or step recovery diode biased to present a non linear junction capacitance. Since losses in these diodes are low, the fraction of RF power converted to the desired harmonic is relatively high.

5. A major disadvantage of frequency multipliers is that they multiply the noise factor along with frequency.

a) true

b) false

View Answer

Explanation: A disadvantage of frequency multipliers is that noise levels are also increased by the multiplication factor. Frequency multiplication process is a phase multiplication process as well, so phase noise variations get multiplied by the same factor as the frequency gets multiplied.

6. If a frequency multiplier has a multiplication factor of 10, then the increase in noise level due to frequency multiplication is:

a) 10 dB

b) 20 dB

c) 25 db

d) 15 dB

View Answer

Explanation: For a frequency multiplier, the increase in noise power is given by 20 log n, where n is the multiplication factor of the multiplier. Substituting in the below equation, increase in noise level is 20 dB.

7. In a diode frequency multiplier, an input signal of frequency f_{o} applied to the diode is terminated with_________ at all frequencies other than required harmonic.

a) real impedances

b) reactive impedance

c) complex impedance

d) none of the mentioned

View Answer

Explanation: In a diode frequency multiplier, an input signal of frequency f

_{o}applied to the diode is terminated with reactive impedance at all frequencies other than required harmonic nf

_{o}. if the diode junction capacitance has a square –law I-V characteristic , it is necessary to terminate unwanted harmonics with short circuit.

8. Resistive multipliers are more efficient as compared to reactive multipliers.

a) true

b) false

View Answer

Explanation: Resistive multipliers generally use forward biased Schottky-barrier diodes to provide non linear characteristic. Resistive multipliers have low efficiency but have better bandwidth.

9. Reactive multipliers have a disadvantage that they cannot be used at very high frequencies and they become less efficient.

a) true

b) false

View Answer

Explanation: At millimeter frequencies, varactor diode exhibits resistive property. Hence, at high frequency the multiplier becomes lossy and also does not offer high bandwidth, which is a major disadvantage.

10. For a resistive frequency multiplier of multiplication factor 2, the maximum theoretical conversion efficiency is:

a) 50 %

b) 25 %

c) 75 %

d) 12.5 %

View Answer

Explanation: For a resistive frequency multiplier of multiplication factor 2, the maximum theoretical conversion efficiency is given by 1/m

^{2}where m is the multiplication factor. For a factor 2 multiplier, maximum theoretical conversion efficiency is 25 %.

**Sanfoundry Global Education & Learning Series – Microwave Engineering.**

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