In the past two decades, RF controls have improved by two orders in magnitude achieving meanwhile sub-10 fs phase stabilities and 10e-4 amplitude precision. Analog-to-digital-converters (ADCs) are the main limitation for further increase in detector resolution. Alternative architectures are therefore needed to overcome this limitation. The presented work covers a novel application of the suppressed-carrier detector, which extends conventional heterodyne receivers and improves the residual time jitter of the regulated RF-field in the cavity far below 1 fs. A practical implementation of the proposed principle is presented. The setup was used to drive a superconducting RF cavity at 1.3 GHz frequency at a Cryo Module Test Bench (CMTB) at a gradient of 8MV/m. The measured out-of-loop residual time jitter of the RF field was 189 as (10Hz to 1MHz). The limiting factors of the setup have been identified by feeding the measurements to a system model. In conclusion, a general discussion about future steps is presented.