# Microwave Engineering Questions and Answers – Transistor Multipliers

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

1. Transistor multipliers are more efficient compared to diode multipliers from all operational aspects.
a) True
b) False

Explanation: Transistor multipliers offer better bandwidth and the possibility of conversion efficiency greater than 100% (conversion gain). FET multipliers also require less input and DC power than diode multipliers.

2. A major characteristic property required by frequency multipliers for frequency multiplication to happen is:
a) High gain
b) High conversion efficiency
c) Non linearity
d) None of the mentioned

Explanation: Non linearity property of devices like transistors and diodes is exploited in frequency multipliers. In transistor amplifiers, FET are used since several nonlinearities exist in FET can be used for harmonic generation.

3. If the input power for a frequency doubler is 10.7 mW and the output measured after the frequency doubling process is 21 mW, then the conversion gain for the frequency doubler is:
a) 4.5 dB
b) 8.4 dB
c) 9.8 dB
d) 2.9 dB

Explanation: The conversion gain for a frequency doubler is given by the expression, P2/Pavail. Here P2 is the power measured at the output of the frequency doubler and Pavail is the power input. Substituting the given values in the equation, the conversion gain is 2.9 dB.

4. An ideal _______ produces an output consisting of the sum and difference frequencies of the two input signals.
a) Mixer
b) Amplifier
c) Product modulator
d) Phase modulator

Explanation: A mixer is a three port device that uses a mixer or a time varying element to achieve frequency conversion. An ideal mixer produces an output consisting of the sum and difference frequencies of the two input signals.

5. A mixer consists of a non-linear device that produces various harmonics of the input frequency.
a) True
b) False

Explanation: A non linear device can generate a wide variety of harmonics and other products of the input frequencies. A filter is used to extract only the desired frequency components.
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6. A mixer can be used for both up conversion and down conversion at the transmitter and receiver respectively.
a) True
b) False

Explanation: At the transmitter, mixer is used to convert the baseband signal to a broadband signal with use of a high frequency local oscillator. At the receiver, a mixer is used to convert the received broadband signal to baseband signal using a local oscillator. This is called down conversion.

7. A mixer having high conversion loss is said to have very high:
a) Gain
b) Loss
c) Bandwidth
d) None of the mentioned

Explanation: Conversion loss for a mixer is defined as the ratio of available RF input power to the available IF output power. A higher value of conversion loss implies that large amount of power is lost in down converting the frequency from RF to IF range. This makes them less efficient.

8. The IS-54 digital cellular telephone system uses a receive frequency band of 869-894 MHz, with a first IF frequency range of 87 MHz, one possible range of local oscillator frequency is:
a) 956 to 981 MHz
b) 750 to 784 MHz
c) 869 to 894 MHz
d) None of the mentioned

Explanation: The two possible local oscillator frequency range is given by fLO = fRF ± fIF.. fLO is the local oscillator frequency, fRF is the received frequency and fIF is the intermediate frequency range. Substituting the given values in the above equation, one possible frequency range is 956 to 981 MHz.

9. The curve of FET transconductance v/s gate-to-source voltage is a straight line through origin.
a) True
b) False

Explanation: The gate to source voltage of an FET is slowly increased from negative voltage towards zero. As the voltage applied becomes more positive, the transconductance increases up to a certain level and then remains a constant. This property of a FET is used in single-ended FET mixer.

10. RF input matching and RF-LO isolation can be improved through the use of:
a) Balanced mixer
b) Single-ender diode mixer
c) Single ended FET mixer
d) Image reject mixer