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RF Cafe Quizzes.
Note: Some material based on books have quoted passages.
Return to RF Cafe Quiz #66
quiz is based on the information presented in
Spectrum and Network Measurements, 2nd Edition, by Robert A. Witte.
"This comprehensive treatment of frequency domain measurements successfully consolidates all the pertinent theory into one
text. It covers the theory and practice of spectrum and network measurements in electronic systems. It also provides thorough
coverage of Fourier analysis, transmission lines, intermodulation distortion, signal-to-noise ratio and S-parameters."
The book was graciously provided by Keysight Technologies.
Note: Some of these books are available as prizes in the monthly RF Cafe Giveaway.
1. Which type of FFT (Fast Fourier Transform) window should be used when measuring a fast transient
A decaying exponential response leaves the beginning portion of the measured waveform undisturbed while forcing the end
of the time record to zero. (p 60)
2. Which statement regarding the DFT (Discrete Fourier Transform) is true?
a) A DFT is only an approximation of a Fourier transform
The DFT differs from the Fourier transform in many ways, including an assumption that the waveform begin measured is periodic, which
can cause significant 'leakage' artifacts on the display, and the limited dataset can limit the dynamic range and noise
performance. (p 38)
3. When might a non-zero coherence (between input and output signals) measurement be false?
b) When a third signal affects both the input and output signals
A significant third signal that gets into both the input and output signal can cause the instrument to include its contribution
to the result. (p 70)
4. Where is the video filter of a swept spectrum analyzer located in the signal chain?
d) Between the detector and the display
The video filter smooths out the data displayed on the monitor and therefore sits between the detector and the display.
5. Where, if at all, on the spectrum analyzer display does LO (local oscillator) feedthrough show
a) At 0 Hz (DC)
When the LO frequency is the same as or very near the IF (intermediate frequency), the difference frequency of the mixing
process generates signal content at or near 0 Hz. (p 95)
6. What is the purpose of the 'Zero Span' function on a spectrum analyzer?
c) It shows the amplitude characteristic of a specific frequency over time
In the Zero Span mode, the output of the detector is swept over time at the center frequency of the spectrum analyzer
(with a bandwidth that includes its sidebands), showing how the amplitude of signal changes over time. (page 114)
7. What is the term given for a long-term frequency shift?
Long-term changes in frequency are considered drift due to temperature and/or mechanical changes to the circuitry.
8. What is the term given for a short-term frequency shift?
b) Phase noise
Short-term changes in frequency are considered phase noise due to perturbations such as jitter, white noise, unintentional
modulation, or any other short-lived phenomenon. (p162)
9. Spectrum analyzer predetection filtering reduces which aspect of noise?
d) Noise present in the input signal
Predetection and filtering works on the raw input signal before it is affected by noise components introduced by the
analyzer circuitry. (p 177)
10. What is the rule-of-thumb swept spectrum analyzer correction factor used for noise power measurements?
c) 2.5 dB
Because of the effective noise bandwidth of the filter (usually Gaussian) and the log video amplifier in the display
circuitry, many - if not most - spectrum analyzers display a noise level around 2.5 dB lower than the actual noise
present. (p 158)