docs: 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