Typically a Clock is a combination of an oscillator and an associated counter to count the number of oscillations per second. The resulting waveform is periodic in nature and having a well-defined Frequency (f) and Time period (T).
Any variations in the frequency (f) of the oscillator result in error in the timekeeping accuracy of the clock.
The two most widely used atomic clocks in recent years are Cesium (Cs-133) and Rubidium(Rb-87) clocks. An ideal clock is having a sharp rising and trailing edges and extends from or to infinity as shown below.
We will discuss briefly the important parameters that define the clock.
Frequency is a measure of the occurrence of significant edges of the clock. The frequency associated with this clock is f = (1/T) Hz. Two clocks A & B are said to be frequency synchronized, if the interval between two consecutive leading or trailing edges remains same (within acceptable tolerance) for these clocks for a given observation time. We all know that frequency of any clock should always be measured with reference to an ideal clock. Since the measurement is usually performed over a longer period of time, the resulting data is ideally represented by using a phase plot. Usually slope of the phase plot gives an indication of the Frequency Error of the clock.
Phase is the value of time at significant edges of the clock. Two clocks A & B are said to be phase aligned, if their significant edges align within an acceptable tolerance. As mentioned previously the instantaneous values on a phase plot gives the phase error of the measurement signal with respect to the reference clock at that instant.
Time usually represents the Time of Day (TOD) clock. Two clocks A & B are said to be time aligned, if the time of day occurrences of the significant edges align within an acceptable tolerance.
If two clocks are Time Aligned, then this would imply that they are Phase Aligned and hence they are Frequency Aligned. However the converse is not true.
The following figure from MEF22.1 illustrates these.
ANNAND RODRIGUES : An interesting and simple document to understand the basic of CLOCK.
Sham Naragund : Interesting document which gives basic idea about the clock.