Below are a few examples of the types of questions that we will answer for you at the NIST Time and Frequency Seminar. More detailed answers are provided at the seminar.
QUESTION: What is frequency stability?
ANSWER: An oscillating signal with good frequency stability produces a sine wave at a desired frequency. If the signal frequency deviates over any time interval, this is a result of something which is undesirable. These undesirable noise mechanisms cause random or systematic processes to exist on top of the sine wave signal of the oscillator. To account for the noise components at the output of a sine wave signal generator, we can express the output voltage as
V(t)=V0[1+a(t)]sin(2pn0t+ Æ(t)),
where V0 = nominal peak voltage amplitude, a(t) = deviation of amplitude from nominal, i.e., DV/V0, n0 = nominal fundamental frequency, Æ(t) = deviation of phase from a desired phase.
Ideally "a(t)" and "Æ(t)" should equal zero for all time, but in actuality, they are the measurable consequences of various kinds of noise.
QUESTION: Of the five noisy signals on the right (amplitude vs. running time is shown), which of these have you encountered?
ANSWER: You have probably encountered all five. They are (in descending order) white, flicker, random-walk, flicker-walk, and random-run noise. Each has a specific cause.
QUESTION: Of the two plots of phase-noise below, do you know which would be characteristic of an amplifier and which would be characteristic of an oscillator?
ANSWER: The blue plot, showing flicker PM and white PM noises, is characteristic of the residual noise introduced by an amplifier. The red plot, showing flicker FM and white PM noises, is typical of a high-quality oscillator.
QUESTION: Why are the Total and Allan deviations (shown on the top plot to the right)
recommended for characterization of fractional frequency fluctuations of an oscillator (shown on the bottom plot)?
ANSWER: The standard deviation applied to the measurement of the frequency of an oscillator implies a false
assumption that there exists a true mean frequency. The Total and Allan deviations can estimate frequency stability even
with non-white noise where the mean is changing, such as the frequency step that is shown in the figure.