Revise docstring for BroadbandPulse

Author: weiliangjin2021

tidy3d/components/source/time.py

@@ -623,11 +623,11 @@ class BroadbandPulse(SourceTime):
         lt=0.5,
     )
     offset: float = pydantic.Field(
-        5.0,
+        0.0,
         title="Offset",
-        description="Time delay of the maximum value of the "
-        "pulse in units of 1 / (``2pi * fwidth``).",
-        ge=2.5,
+        description="An automatic time delay of the peak value of the pulse has been applied under the hood "
+        "to ensure smooth ramping up of the pulse at time = 0. This offfset is added on top of the automatic time delay "
+        "in units of 1 / [``2pi * (freq_range[1] - freq_range[0])``].",
     )
 
     @pydantic.validator("freq_range", always=True)
@@ -676,13 +676,10 @@ def frequency_range_sigma(self, sigma: float = DEFAULT_SIGMA) -> FreqBound:
         return self._source.frequency_range(sigma)
 
     def frequency_range(self, num_fwidth: float = DEFAULT_SIGMA) -> FreqBound:
-        """Frequency range where the source amplitude is within ``exp(-sigma**2/2)`` of the peak amplitude."""
+        """Delegated to `frequency_range_sigma(sigma=num_fwidth)` for computing the frequency range where the source amplitude
+        is within ``exp(-num_fwidth**2/2)`` of the peak amplitude.
+        """
         return self.frequency_range_sigma(num_fwidth)
 
-    @cached_property
-    def _freq0(self) -> float:
-        """Central frequency from frequency range."""
-        return np.mean(self.freq_range)
-
 
 SourceTimeType = Union[GaussianPulse, ContinuousWave, CustomSourceTime, BroadbandPulse]