Define tilt target over the full amplifier bandwidth

Currently the tilt_target defined by a user is applied over the band of
propagating channels. This means for example that if only two channels
are propagated, the difference in gain between the two channels will be
equal to the tilt_target, independently of how close the two channels
are in frequency. I think it makes more sense to always define the
tilt_target over the full operational bandwidth of the amplifier.

Signed-off-by: Jonas Mårtensson <jonas.martensson@ri.se>
Change-Id: I4f29de2edc4d0de239b34e0d8d678d964b6a0af3

gnpy/core/elements.py

@@ -805,24 +805,21 @@ def _gain_profile(self, pin, err_tolerance=1.0e-11, simple_opt=True):
         if len(self.interpol_dgt) == 1:
             return array([self.effective_gain])
 
-        nb_channel = arange(len(self.interpol_dgt))
-
         # TODO|jla: find a way to use these or lose them. Primarily we should have
         # a way to determine if exceeding the gain or output power of the amp
         tot_in_power_db = self.pin_db  # Pin in W
 
         # linear fit to get the
-        p = polyfit(nb_channel, self.interpol_dgt, 1)
+        p = polyfit(self.channel_freq, self.interpol_dgt, 1)
         dgt_slope = p[0]
 
-        # Calculate the target slope - currently assumes equal spaced channels
-        # TODO|jla: support arbitrary channel spacing
-        targ_slope = self.tilt_target / (len(nb_channel) - 1)
+        # Calculate the target slope
+        targ_slope = self.tilt_target / (self.params.f_max - self.params.f_min)
 
         # first estimate of DGT scaling
-        if abs(dgt_slope) > 0.001:  # check for zero value due to flat dgt
+        try:
             dgts1 = targ_slope / dgt_slope
-        else:
+        except ZeroDivisionError:
             dgts1 = 0
 
         # when simple_opt is true, make 2 attempts to compute gain and