Use f_max for bandwidth

src/python_spectrometer/daq/zurich_instruments.py

@@ -171,28 +171,14 @@ class ZurichInstrumentsMFLIDAQ(_ZurichInstrumentsDevice):
 
         return MFLIDAQSettings
 
-    def setup(self, bandwidth: Union[str, float] = 'auto', filter_order: Optional[int] = None,
-              freq: float = 50, **settings: Mapping) -> Dict[str, Any]:
+    def setup(self, filter_order: Optional[int] = None, freq: float = 50,
+              **settings: Mapping) -> Dict[str, Any]:
         r"""Sets up the daq module to acquire time series data.
 
         See [1]_ for information on lock-in measurements.
 
         Parameters
         ----------
-        bandwidth : Union[str, float], optional
-            The demodulator noise-equivalent power (NEP) bandwidth.
-            The default is 'auto', in which case it is set to f_max/2.
-
-            The bandwidth is related to the time constant of the RC
-            filter by
-
-            .. math::
-
-                \tau = \frac{\Gamma\left(n - \frac{1}{2}\right)}
-                            {4\sqrt{\pi}f_\mathrm{NEP}\Gamma(n)},
-
-            where :math:`n` is the filter order
-
         filter_order : int, optional
             The filter order. Not changed if not given.
         freq : float, optional
@@ -211,9 +197,23 @@ class ZurichInstrumentsMFLIDAQ(_ZurichInstrumentsDevice):
         **settings : Mapping
             Additional settings for data acqusition.
 
+        Notes
+        -----
+        The demodulator 3 dB bandwidth is chosen as $f_\text{max}$. The
+        noise-equivalent power (NEP) bandwidth is related to the time
+        constant of the RC filter by
+
+        .. math::
+
+            \tau = \frac{\Gamma\left(n - \frac{1}{2}\right)}
+                        {4\sqrt{\pi}f_\mathrm{NEP}\Gamma(n)},
+
+        where :math:`n` is the filter order. By default, it is set to
+        ``fs/4``.
+
         Raises
         ------
-        RuntimeError
+        RuntimeError :
             If settings are incompatible with the hardware.
 
         Returns
@@ -226,18 +226,22 @@ class ZurichInstrumentsMFLIDAQ(_ZurichInstrumentsDevice):
         .. [1] https://www.zhinst.com/europe/en/resources/principles-of-lock-in-detection
 
         """
-        settings = self.DAQSettings(freq=freq, **settings).to_consistent_dict()
+        settings = self.DAQSettings(freq=freq, **settings)
+
+        if 'bandwidth' in settings:
+            warnings.warn('The bandwidth parameter has been replaced by f_max',
+                          DeprecationWarning)
 
-        if bandwidth == 'auto':
-            bandwidth = settings['f_max'] / 2
+        if 'f_max' not in settings:
+            settings.f_max = settings.fs / 4
 
-        if filter_order:
+        if filter_order is not None:
             self.device.demods[self.demod].order(int(filter_order))
 
         # BW 3dB = √(2^(1/n) - 1) / 2πτ
         # BW NEP = Γ(n - 1/2) / 4τ √(π)Γ(n)
         n = self.device.demods[self.demod].order()
-        tc = gamma(n - 0.5) / (4 * bandwidth * np.sqrt(np.pi) * gamma(n))
+        tc = gamma(n - 0.5) / (4 * settings['f_max'] * np.sqrt(np.pi) * gamma(n))
 
         # Do not use context manager here because somehow settings can get lost
         # with device.set_transaction():