# Microwave Engineering Questions and Answers – Oscillator Phase Noise

This set of Microwave Engineering online quiz focuses on “Oscillator Phase Noise”.

1. A practical oscillator has a frequency spectrum consisting of a single delta function at its operating frequency.
a) True
b) False

Explanation: An ideal oscillator has a frequency spectrum consisting of a single delta function at its operating frequency, but a practical oscillator has a spectrum in the form of a Gaussian curve consisting of some noise component.

2. ____________ due to random fluctuations caused by thermal and other noise sources appear as broad continuous distribution localized about the output signal.
a) Phase noise
b) White noise
c) Thermal noise
d) Shot noise

Explanation: Phase noise is defined as the ratio of power in one phase modulation sideband to the total signal power per unit bandwidth at a particular offset fm. phase noise due to random fluctuations caused by thermal and other noise sources appear as broad continuous distribution localized about the output signal.

3. The phase variation for an oscillator or synthesizer is given by:
a) ∆f*sin ωmt/ fm
b) ∆f / fm
c) Sin ωmt/ fm
d) None of the mentioned

Explanation: The phase variation for an oscillator or synthesizer is given by ∆f*sin ωmt/ fm. here, fm is the modulating signals frequency, ∆f is the change in the frequency.

4. The expression for phase noise in an oscillator is given by:
a) θrms2
b) θrms2/√2
c) θrms2/2
d) θrms2/ 3

Explanation: The expression for phase noise in an oscillator is given by θrms2/2. θrms is the rms value of the phase deviation. Phase noise is directly proportional to the square of the RMS value of the phase deviation. Greater the deviation, higher is the phase noise.

5. Phase noise at the output of an oscillator is given by:
a) kBFGT0
b) kT0F/Pc
c) kT0F/Pc
d) None of the mentioned

Explanation: Phase noise at the output of an oscillator is given by kT0F/Pc. here k is the Boltzmann’s constant. B is the operating bandwidth of the system, here the equation is considered for a bandwidth of 1 Hz as per the definition of phase noise. F is the figure of merit of system.
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6. Noise power versus frequency for an amplifier has spikes at the operating frequency without the application of an input voltage.
a) True
b) False

Explanation: Noise power versus frequency for an amplifier has spikes at the operating frequency with the application of an input voltage. When an input voltage is applied to the amplifier, noise component also is added. Along with the signal, noise is also amplified and peaks at the operating frequency.

7. An idealized power spectral density of amplifier has a straight line parallel to X axis and the noise is constant at all frequencies.
a) True
b) False

Explanation: The curve has a negative slope up to a frequency called fα due to the thermal noise also called as 1/f noise. Above this frequency, the graph is a straight line parallel to X axis.

8. At higher frequencies of operation of an oscillator, induced noise is mostly:
a) Thermal noise
b) White noise
c) Shot noise
d) Flicker noise

Explanation: At higher frequencies of operation of an oscillator, induced noise is mostly thermal, and constant with frequency. This noise is also proportional to the noise figure of the amplifier.

9. A GSM cellular telephone standard requires a minimum of 9 dB rejection of interfering signal levels of -23 dBm at 3 MHz from the carrier, -33 dBm at 1.6 MHz from the carrier, and -43 dBm at 0.6 MHz from the carrier, for a carrier level of -99 dBm. Determine the required local oscillator phase noise at 3 MHz carrier frequency offset.
a) -138 dBc/Hz
b) -128 dBc/Hz
c) -118 dBc/Hz
d) None of the mentioned

Explanation: Phase noise is given by the expression C (dBm)-S (dB) –I (dBm)-10log (B). Substituting the given values in the above expression, the oscillator phase noise is -138.

10. The most affected parameter of a receiver by the phase noise is signal to noise ratio.
a) True
b) False

Explanation: The effect of phase noise in a receiver is to degrade both the signal to noise ratio and the selectivity. Of these, the most severely affected is the selectivity. Phase noise degrades the receiver selectivity by causing down conversion of signals located near by the desired signal frequency.

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