  We present a continuous-filtering Vernier spectrometer operating in the
3.15-3.4 ${\mu}$m range, based on a femtosecond doubly resonant optical
parametric oscillator, a cavity with a finesse of 340, a grating mounted on a
galvo scanner and two photodiodes. The spectrometer allows acquisition of one
spectrum spanning 250 nm of bandwidth in 25 ms with 8 GHz resolution,
sufficient for resolving molecular lines at atmospheric pressure. An active
lock ensures good frequency and intensity stability of the consecutive spectra
and enables continuous signal acquisition and efficient averaging. The relative
frequency scale is calibrated using a Fabry-Perot etalon or, alternatively, the
galvo scanner position signal. We measure spectra of pure CH${_4}$ as well as
dry and laboratory air and extract CH${_4}$ and H${_2}$O concentrations by
multiline fitting of model spectra. The figure of merit of the spectrometer is
1.7x10${^-}$${^9}$ cm${^-}$${^1}$ Hz${^-}$${^1}$${^/}$${^2}$ per spectral
element and the minimum detectable concentration of CH${_4}$ is 360 ppt
Hz${^-}$${^1}$${^/}$${^2}$, averaging down to 90 ppt after 16 s.
