Microwave Engineering Questions and Answers – Frequency Multipliers

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

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

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

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

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

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.
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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

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 fo 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

Explanation: In a diode frequency multiplier, an input signal of frequency fo applied to the diode is terminated with reactive impedance at all frequencies other than required harmonic nfo. 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

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

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 %

Explanation: For a resistive frequency multiplier of multiplication factor 2, the maximum theoretical conversion efficiency is given by 1/m2 where m is the multiplication factor. For a factor 2 multiplier, maximum theoretical conversion efficiency is 25 %.

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