Subharmonic pumping of parametric amplifiers

The purpose of this paper is to present in some detail the operation of a parametric amplifier pumped subharmonically as compared to being directly pumped. As considered here, subharmonic pumping does not involve harmonic pump power generation (external to the varactor) but the utilization of higher-order time-dependent capacitances to yield parametric amplification by employing, basically, only a three-frequency system. The analysis given here is based on an evaluation of the Fourier series representation of the time-dependent capacitance resulting from large-signal ("hard") pumping of varactors. This evaluation indicates that significant values of higher-order time-dependent capacitances suitable for parametric amplification are obtained with relative pump swings in excess of about 90 per cent. Utilizing these higher-order time-dependent capacitances, the amplifier operation for various orders of subharmonic pumping is treated, including such factors as pump power requirements, gain, and noise figure. It is shown that, under certain conditions, less pump power is required to generate the same negative conductance than with direct fundamental pumping. Furthermore, for the same pump power and fundamental pump frequency, it is determined that significant improvements in amplifier noise figure are achieved by employing subharmonic pumping, provided varactor losses are small. From the results obtained by both analysis and experiment, it is concluded that subharmonic pumping, even without harmonic power generation, is not only feasible but can be very useful up to C-band signal frequencies with existing varactors.