TABLE OF CONTENTS The oscillator loaded Q is a critical parameter. The loaded Q is a direct indication of many oscillator performance parameters. A high loaded Q
Reduces phase noise
Reduces frequency drift
Isolates performance from active-device variation
Phase noise is inversely proportional to the square of the loaded Q [4]. Drift is reduced because the resonator solely determines the oscillation frequency in high- Q designs. Isolating the resonator from active device reactances reduces the effect of temperature. Many oscillator designs have low loaded Qs. The phase noise and long-term stability of these designs are far from optimum. An oscillator with a low loaded Q is often the root problem even though designers offer imaginative and esoteric descriptions of the problem. Noise is discussed further in Chapter 4.
The open-loop loaded Q of a cascade is
| (2.8) |  |
For the 100 MHz example the loaded Q is approximately 5.2. The loaded Q in terms of the phase slope is
| (2.9) |  |
where ? is in radians or
| (2.10) |  |
where ? is in degrees.
When the phase zero crossing does not occur at maximum phase slope, the loaded Q should be calculated using one of the latter two equations instead of using the amplitude response bandwidth. The loaded Q is less than optimum in that situation. If the phase zero crossing occurs at maximum phase slope, the first equation for loaded Q may be used.
