Low-noise power-efficient oscillators: theory and design

The paper proposes a new linear theory with experimental verification which shows how the spectrum of a high-efficiency oscillator varies with the amplifier noise figure (F), the voltage gain of the amplifier (G), the unloaded quality factor of the resonator (Qo), the loaded quality factor (QL) and the total RF power in the oscillating system (PFED). By defining the parameters, F, G, PFED, QL and hence QL/Qo fundamentally and precisely, the theory shows that F, G and QL/Qo are interdependent. The noise equation is therefore expressed in only a few variables, F, Qo, QL/Qo and PFED. Optimum operating conditions occur when the differential of sideband noise with respect to QL/Qo = O, so that if F is assumed to be constant, minimum sideband noise occurs when QL/Qo = 2/3. Many LC oscillators therefore operate far from the point at which minimum sideband noise occurs. By defining the oscillator power as the total power in the system, the variation of sideband noise power with DC input power can be described, permitting the optimum design of oscillators with minimum sideband noise for any given DC input power. Based on this theory a new highly efficient oscillator configuration has been designed satisfying this criterion. This configuration also reduces the pulling effect of the load sufficiently to permit its use directly as a transmitter. Experimental verification of the theory has been obtained at 1 MHz between the limits 0.09 < QL/Qo < 0.93. A 150 MHz low-noise oscillator, designed using the same approach, has been successfully demonstrated.