Merge pull request #51 from fronzbot/add-nsd

Add noise spectral density calculation

Author: fronzbot

adc_eval/spectrum.py

@@ -23,14 +23,15 @@ def enob(sndr, places=1):
     return round((sndr - 1.76) / 6.02, places)
 
 
-def sndr_sfdr(spectrum, freq, nfft, leak, full_scale=0):
+def sndr_sfdr(spectrum, freq, fs, nfft, leak, full_scale=0):
     """Get SNDR and SFDR."""
     # Zero the DC bin
     spectrum[0] = 0
     bin_sig = np.argmax(spectrum)
     psig = sum(spectrum[i] for i in range(bin_sig - leak, bin_sig + leak + 1))
     spectrum_n = spectrum
     spectrum_n[bin_sig] = 0
+    fbin = fs / nfft
 
     for i in range(bin_sig - leak, bin_sig + leak + 1):
         spectrum_n[i] = 0
@@ -62,7 +63,8 @@ def sndr_sfdr(spectrum, freq, nfft, leak, full_scale=0):
         "floor": noise_floor,
         "power": noise_power,
         "rms": np.sqrt(noise_power),
-        "dBHz": round(dBW(noise_floor) - full_scale, 1),
+        "dBHz": round(dBW(noise_floor, 3) - full_scale, 1),
+        "NSD": round(dBW(noise_floor, 3) - full_scale - 2 * dBW(fbin, 3), 1),
     }
     stats["sndr"] = {
         "dBc": dBW(psig / noise_power),
@@ -177,7 +179,7 @@ def plot_spectrum(
 
     pwr_dB = 10 * np.log10(pwr) - scalar
 
-    sndr_stats = sndr_sfdr(pwr, freq, nfft, leak=leak, full_scale=full_scale)
+    sndr_stats = sndr_sfdr(pwr, freq, fs, nfft, leak=leak, full_scale=full_scale)
     harm_stats = find_harmonics(
         pwr,
         freq,
@@ -259,7 +261,8 @@ def get_plot_string(stats, full_scale, fs, nfft, window):
     plt_str += f"SFDR = {stats['sfdr']['dBFS']} dBFS ({stats['sfdr']['dBc']} dBc)\n"
     plt_str += f"Pspur = {stats['spur']['dBFS']} dBFS\n"
     plt_str += f"fspur = {round(stats['spur']['freq']/1e6, 2)} MHz\n"
-    plt_str += f"Noise Floor = {stats['noise']['dBHz']} dBFS/Hz\n"
+    plt_str += f"Noise Floor = {stats['noise']['dBHz']} dBFS\n"
+    plt_str += f"NSD = {stats['noise']['NSD']} dBFS\n"
     plt_str += "\n"
     plt_str += "==== Harmonics ====\n"
 

tests/test_spectrum.py

@@ -67,6 +67,7 @@ def test_sndr_sfdr_outputs(self):
         freq = np.array([100, 200, 300, 400])
         full_scale = -3
         nfft = 2**8
+        fs = 1
         exp_return = {
             "sig": {
                 "freq": 300,
@@ -86,7 +87,8 @@ def test_sndr_sfdr_outputs(self):
                 "floor": 18 / nfft,
                 "power": 9,
                 "rms": 3,
-                "dBHz": round(-11.5297 - full_scale, 1),
+                "dBHz": round(-11.529675 - full_scale, 1),
+                "NSD": round(36.6351 - full_scale, 1),
             },
             "sndr": {
                 "dBc": 10.0,
@@ -101,6 +103,6 @@ def test_sndr_sfdr_outputs(self):
             },
         }
 
-        result = spectrum.sndr_sfdr(data, freq, nfft, 0, full_scale=full_scale)
+        result = spectrum.sndr_sfdr(data, freq, fs, nfft, 0, full_scale=full_scale)
         for key, val in exp_return.items():
             self.assertDictEqual(result[key], val, msg=key)

tests/test_spectrum_plotting.py

@@ -98,7 +98,7 @@ def test_plot_string(self):
         """Test proper return of plotting string."""
         self.bin = 13
         (freq, pwr) = self.gen_spectrum(3)
-        stats = spectrum.sndr_sfdr(pwr, freq, self.nfft, leak=0, full_scale=0)
+        stats = spectrum.sndr_sfdr(pwr, freq, 1, self.nfft, leak=0, full_scale=0)
         harms = spectrum.find_harmonics(
             pwr, freq, self.nfft, self.bin, self.arms, harms=3, leak=0
         )
@@ -121,7 +121,7 @@ def test_plot_string(self):
             f"SFDR = {all_stats['sfdr']['dBFS']} dBFS" in plt_str, msg=msg_txt
         )
         self.assertTrue(
-            f"Noise Floor = {all_stats['noise']['dBHz']} dBFS/Hz" in plt_str,
+            f"Noise Floor = {all_stats['noise']['dBHz']} dBFS" in plt_str,
             msg=msg_txt,
         )
         self.assertTrue(