If the LO shift ν_LO is not a multiple of the standing wave frequency ν_sw, the quantity f − f^cal can have several roots causing “resonances” in the final spectrum. The left panels have ν_sw = 10   MHz matching ν_LO = 10   MHz, while for the right panels we chose ν_sw = 50   MHz. The upper panels show the resulting spectra (simulations were performed without a noise contribution for a clearer visualisation). In the upper right panel, spikes do appear. The middle panels show the model fitting through . While in the left middle panel f^cal > f for all frequencies, in the right middle panel several crossings occur. The lower panels depict Eq. (56), which represents the condition for resonances to appear.