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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def start_fsweep(self, start=None, stop=None, step=None):
 """Starts a frequency sweep. :param start: Sets the start frequency. :param stop: Sets the target frequency. :param step: Sets the frequency step. """

        if start:
            self.frequency_start = start
        if stop:
            self.frequency_stop = stop
        if step:
            self.frequency_step = step
        self._write(('SWEEP', Integer), 1)