Set the code to calculate results at just one frequency.

        **Call signature**

        *f_ghz*
          The frequency to sample, in GHz.
        Returns
          *self* for convenience in chaining.

def set_one_freq(self, f_ghz):
        """Set the code to calculate results at just one frequency.

        **Call signature**

        *f_ghz*
          The frequency to sample, in GHz.
        Returns
          *self* for convenience in chaining.

        """
        if not (f_ghz >= 0):
            raise ValueError('must have f_lo_ghz >= 0; got %r' % (f_lo_ghz,))

        self.in_vals[IN_VAL_NFREQ] = 1
        self.in_vals[IN_VAL_FREQ0] = f_ghz * 1e9 # GHz -> Hz
        self.in_vals[IN_VAL_LOGDFREQ] = 1.0
        return self