Add functions to find specific frequency or gain value in transfer function.

applications/lofar2/model/rtdsp/fourier.py

@@ -133,6 +133,45 @@ def dtft(coefs, Ndtft=None, zeroCenter=True, fftShift=True):
     return h, f, HF
 
 
+def estimate_gain_at_frequency(f, HF, freq):
+    """Find gain = abs(HF) at frequency in f that is closest to freq.
+
+    Input:
+    . f, HF: frequency axis and spectrum as from dtft() with fftShift=True
+    . freq : frequency to look for in f
+    Return:
+    . fIndex: index of frequency in f that is closest to freq
+    . fValue: frequency at fIndex
+    . fGain : gain = abs(HF) at fIndex
+    """
+    fDiff = np.abs(f - freq)
+    fIndex = fDiff.argmin()
+    fValue = f[fIndex]
+    fGain = np.abs(HF[fIndex])
+    return fIndex, fValue, fGain
+
+
+def estimate_frequency_at_gain(f, HF, gain):
+    """Find positive frequency in f that has gain = abs(HF) closest to gain.
+
+    Input:
+    . f, HF: frequency axis and spectrum as from dtft() with fftShift=True
+    . gain : gain to look for in HF
+    Return:
+    . fIndex: index of frequency in f that has abs(HF) closest to gain
+    . fValue: frequency at fIndex
+    . fGain : gain = abs(HF) at fIndex
+    """
+    pos = len(f[f <= 0])
+    HFpos = HF[pos:]
+    HFgain = np.abs(HFpos)
+    HFdiff = np.abs(HFgain - gain)
+    fIndex = pos + HFdiff.argmin()
+    fValue = f[fIndex]
+    fGain = np.abs(HF[fIndex])
+    return fIndex, fValue, fGain
+
+
 ###############################################################################
 # Radix FFT
 ###############################################################################