vibration_analyze.py: Don't use the peak power for D_guess

Use the power at the Nyquist frequency.  This is what I'd meant to use
back in 82cd9c1 (Massive rewrite (v 0.6) to base everything on Cython
and revamped pypiezo, 2011-04-21), but I goofed.  Luckily, the fitting
algorithm was generally able to recover from my poor heuristic, but
using a borked heuristic is not ideal.

The fixed heuristic should reduce the number of poorly fit cases, and
improve convergence efficiency for all cases.

diff --git a/calibcant/vibration_analyze.py b/calibcant/vibration_analyze.py
index 487d027ce88509185b2432c5fdc7001231b807cd..f5b83dec8cfa7f338262fb90f41de697d8fe5d2b 100644 (file)
--- a/calibcant/vibration_analyze.py
+++ b/calibcant/vibration_analyze.py
@@ -390,7 +390,7 @@ def fit_psd(freq_axis, psd_data, min_frequency=500, max_frequency=25000,
     A_guess = Q_guess*B_guess
     C_guess = max_psd * (-res_freq**4 + A_guess**4)
     if offset:
-        D_guess = psd_data[max_psd_index]
+        D_guess = psd_data[-1]
         C_guess -= D_guess
     else:
         D_guess = 0