  We propose a method to subtract a photon from a double sideband mode of
continuous-wave light. The central idea is to use phase modulation as a
frequency sideband beamsplitter in the heralding photon subtraction scheme,
where a small portion of the sideband mode is downconverted to the carrier
frequency to provide a trigger photon. An optical Schr\"odinger's cat state is
created by applying the propesed method to a squeezed state at 500MHz sideband,
which is generated by an optical parametric oscillator. The Wigner function of
the cat state reconstructed from a direct homodyne measurement of the 500MHz
sideband modes shows the negativity of $W(0,0) = -0.088\pm0.001$ without any
loss corrections.
