code: def set_fm_ext(self, freq, amplitude, peak_freq_dev=None,
                     output_state=True):
          
  
          if peak_freq_dev is None:
              peak_freq_dev = freq
          commands = ['FUNC SIN',  # set to output sine functions
                      'FM:STAT ON',
                      'FREQ {0}'.format(freq),
                      'FM:SOUR EXT',
                      # 'FM:FREQ {0}'.format(freq),
                      'FM:DEV {0}'.format(peak_freq_dev),
                      'VOLT {0}'.format(amplitude),
                      'VOLT:OFFS 0']   # set to frequency modulation
          if output_state is True:
              commands.append('OUTP ON')
          else:
              commands.append('OUTP OFF')
          command_string = '\n'.join(commands)
          print_string = '\n\t' + command_string.replace('\n', '\n\t')
          logging.info(print_string)
          self.instr.write(command_string)
docs: Sets the func generator to frequency modulation with external modulation.
          freq is the carrier frequency in Hz.