A new design philosophy for high phase stability odd order frequency multipliers

Abstract

Phase instability (in particular versus temperature) of the first frequency multiplication stages in the frequency synthesis chain can be a limitation for the medium term stability of atomic frequency standards of the new generation. These initial stages are based on a (passive or active) nonlinear device, and a resonator tuned to the desired harmonic of the input. A frequency shift of this filter induces in the output a phase variation, which is proportional to the selectivity of the filter. Striving for the highest possible Q values in this filter is therefore detrimental for phase stability. Nevertheless this is necessary with traditional distortion methods, for spectrum cleanup reasons, because they have little capability of selecting a desired harmonic. In fact, the best that can be done with them in this sense is to produce a nice and symmetric square wave, which has in principle no even harmonic content. In this paper we propose a new design philosophy, in which the distortion of the input sinewave is produced in such a way that most of the energy of the distorted signal is already in the desired harmonic before filtering. In this way the selectivity of the output filter can be greatly relaxed, and the phase stability of the multiplier stage correspondingly improved. The proposed nonlinear network is made of transistors used as current switch and current mirrors used as current amplifiers, and piecewise synthesizes a signal as similar as possible to the desired output sinewave by simply reversing and amplifying the signal at suitable points of the input period. By working in current we also hope to minimize the phase noise of the device.