Reducing oscillator PM noise from AM–PM noise correlation

Introduction: In most oscillators, the close-to-the-carrier amplitude modulation (AM) noise is usually lower than the phase modulation (PM) noise and does not pose serious problems. However, in highperformance systems where every decibel of PM noise is important, the AM noise cannot be neglected. Numerous applications consider AM noise effects on the overall performance of an oscillator or a system. However, very little is known about the relationship between AM and PM noise in an oscillator. Past studies have focused on the relationship between AM and PM noise in high-performance quartz oscillators, reported in [1, 2], but did not involve spectral-correlation measurements as in [3]. The close-to-the-carrier AM and PM noise of an oscillator can exhibit varying degrees of correlation [3, 4]. By measuring the PM noise of an oscillator in real time, the oscillator noise can be corrected for and reduced. However, it can be difficult to measure the PM noise without a higher-quality reference or a highquality factor element to measure against. The AM noise is relatively easy to measure, requiring only a diode and a few capacitors. If the PM noise is correlated to the AM noise, the measurement of the AM noise can be utilised to improve the PM noise. In this Letter, we present a feed-forward scheme for reducing PM noise from AM–PM noise correlation.