Software defined radio with ZIF architectures commonly use a lower frequency (10 MHz) precision reference from which a tunable local oscillator (LO) can be derived for the direct down conversion process. Accuracy and stability of that reference can have big impact on the receiver’s ability to recover the transmitted signal. Accuracy or stability errors in the reference can be magnified in the derived LO.
Frequency accuracy is the degree of conformity of a measured or calculated frequency to an ideal or specified reference. The difference between the measured frequency and the specified (desired) frequency is known as the frequency offset. The concept of stability illustrated in Figure 2 shows how the frequency changes over time.
Figure 2 Frequency vs Time (Courtesy of NIST)
Frequency stability breaks down into three categories: long term stability, which is usually measured over periods of hours, days or more; short term stability, which is usually measured over periods of fractional seconds to one day; and phase noise, which deals with very short time scales and produces effects that look more like unwanted modulation changing the shape of the waveform rather than a wandering frequency.
Long term stability is dominated by a progressive change in frequency, called drift, that all oscillators undergo. Aging is often used synonymously with drift, but technically drift doesn't have to arise from an aging process. Drift often proceeds in one direction and may be predictable based on past performance, at least for a few days. In some oscillators drift may be more random and can change direction. Long-term drift will affect the accuracy of the oscillator's frequency unless it is corrected for.
Short term stability resembles noise and is not normally predictable. It reflects the uncertainty of the oscillator's frequency at a given instant in time. Phase noise is usually thought of in the frequency, rather than the time, domain. Like short term stability, it results from the inherent noise in any component that contains moving atoms (i.e., everything that's not either a total vacuum, or at absolute zero temperature).
Some oscillators are also specified for stability over variables other than time, such as temperature, power supply voltage, shock, and even gravitational effects.