TABLE OF CONTENTS In an earlier section we listed desirable attributes of high loaded Q. However, there are fundamental limitations to the maximum loaded Q. As the cascade loaded Q approaches the unloaded Q of components in the resonator the resonator insertion loss approaches infinity. The insertion loss for the resonator is
| (2.25) |  |
where IL is a positive decibel number. IL is therefore equal to - S 21 dB. For example, if Q u = 100 and Q l = 21.5, IL = 2.1 dB. If the cascade amplifier has adequate gain then significant loss can be tolerated in the resonator. Nevertheless, the loaded Q can not exceed the component unloaded Q.
A second factor which may limit the maximum loaded Q is resonator voltage. This is particularly a problem with high-power oscillators and oscillators with varactor tuning elements. The voltage at resonance across the shunt inductor and capacitor in LC8, the top-C coupled parallel resonator, is given by
| (2.26) |  |
where V s is source voltage into R o ohms and B L8 is the admittance of the shunt inductor.
The insertion loss, Q l and V r versus X s are given in Figure 2-6 for Q u = 200, R o =50 ohms, V s = 0.707 V rms ( +10 dBm,) and B L8 = .01 mhos. Notice with only 0.707 volts drive...
