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By applying time-domain methods the measurement time for emissions measurements is reduced by several orders of magnitude. In this paper a novel real-time capable ambient noise cancelling system is presented to perform measurements of electromagnetic interference (EMI) in time-domain during the presence of ambient noise. The hardware implementation of the least-mean-square (LMS) algorithm in combination with a digital down-conversion is described and it is shown that in comparison to systems presented in the past a continuous processing of the measurement data in real-time is achieved. Based on such a system a full gap-less evaluation of the emission spectrum can be realized. This is a basic requirement of the CISPR 16-1-1 standard for applying the quasi-peak detector mode. Measurements were carried out at an inner urban site showing the cancellation of ambient noise and detecting of covered device-under-test signals.