According to some embodiments, a synthetic nuclear magnetic resonance (NMR) reference signal is injected into the receive path of an NMR spectrometer, after the NMR probe and before the receive amplifier. The synthetic signal is generated using a transmit path for the same channel or for a different channel than the reference signal detection channel. The reference signal is coupled from a transmit-path reference signal coupler situated before a transmit amplifier to a receive-path reference signal coupler situated between the probe and the receive amplifier. The reference signal couplers may include passive directional couplers and/or active switches. The synthetic signal samples the receive path of the system but is not substantially affected by intra-probe interactions. Using a well-defined existing NMR channel to generate the reference signal allows superior control of reference signal characteristics, and does not require a dedicated spare channel for the reference signal.