Targeting amplitude noise from microwave generators

Low noise microwave generators are necessary for experiments in various fields, including cavity optomechanics, force sensing, quantum mechanics, and dark matter detection. However, all sources of microwaves produce phase noise — fluctuations in the frequency of the output tone — and amplitude noise — fluctuations in the power of the output tone. Both types of noise can heat and limit the sensitivity of experiments.

While most studies have focused on reducing phase noise from microwave generators, Depellette et al. developed a technique to reduce amplitude noise using feedback cancellation. They used a field-programmable gate array (FPGA) that reproduces the amplitude noise to cancel it out. Using this technique on a 4 GHz microwave tone, the authors reduced the total noise by a factor of 20. The remaining noise was mostly phase noise.

“A very large number of experiments need microwave sources with low noise,” said author Joe Depellette. “The circuit is also made entirely out of commercially available equipment, so any lab that is already familiar with microwave electronics should be able to replicate this.”

In cavity optomechanics experiments, sideband cooling is used to lower the temperature of a mechanical resonator, such as a membrane, interacting with a cavity. By cancelling amplitude noise in such an experiment, the authors reduced heating in the cavity, allowing the membrane to be cooled further.

Future work could increase the bandwidth of cancellation, which currently ranges from a few hundred kilohertz to less than one kilohertz, or it could combine this technique with phase noise cancellation to reduce both types simultaneously for a very low noise microwave source.

Source: “Amplitude noise cancellation of microwave tones,” by Joe Depellette, Ewa Rej, Matthew Herbst, Richa Cutting, Yulong Liu, and Mika A. Sillanpää, Review of Scientific Instruments (2025). The article can be accessed at https://doi.org/10.1063/5.0283567 .