109 lines
4.0 KiB
Python
109 lines
4.0 KiB
Python
"""Tests for stability module: gain margin, phase margin from loop gain data."""
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import numpy as np
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import pytest
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from mcp_ltspice.stability import (
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compute_gain_margin,
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compute_phase_margin,
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compute_stability_metrics,
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)
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def _second_order_system(freq, wn=1000.0, zeta=0.3):
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"""Create a 2nd-order underdamped system: H(s) = wn^2 / (s^2 + 2*zeta*wn*s + wn^2).
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Returns complex loop gain at the given frequencies.
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"""
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s = 1j * 2 * np.pi * freq
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return wn**2 / (s**2 + 2 * zeta * wn * s + wn**2)
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class TestGainMargin:
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def test_third_order_system(self):
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"""A 3rd-order system crosses -180 phase and has finite gain margin."""
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freq = np.logspace(0, 6, 10000)
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# Three-pole system: K / ((s/w1 + 1) * (s/w2 + 1) * (s/w3 + 1))
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# Phase goes from 0 to -270, so it definitely crosses -180
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w1 = 2 * np.pi * 100
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w2 = 2 * np.pi * 1000
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w3 = 2 * np.pi * 10000
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K = 100.0 # enough gain to have a gain crossover
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s = 1j * 2 * np.pi * freq
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loop_gain = K / ((s / w1 + 1) * (s / w2 + 1) * (s / w3 + 1))
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result = compute_gain_margin(freq, loop_gain)
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assert result["gain_margin_db"] is not None
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assert result["is_stable"] is True
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assert result["gain_margin_db"] > 0
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assert result["phase_crossover_freq_hz"] is not None
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def test_no_phase_crossover(self):
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"""A simple first-order system never reaches -180 phase, so GM is infinite."""
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freq = np.logspace(0, 6, 1000)
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s = 1j * 2 * np.pi * freq
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# First-order: 1/(1+s/wn) -- phase goes from 0 to -90
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loop_gain = 1.0 / (1 + s / (2 * np.pi * 1000))
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result = compute_gain_margin(freq, loop_gain)
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assert result["gain_margin_db"] == float("inf")
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assert result["is_stable"] is True
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def test_short_input(self):
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result = compute_gain_margin(np.array([1.0]), np.array([1.0 + 0j]))
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assert result["gain_margin_db"] is None
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class TestPhaseMargin:
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def test_third_order_system(self):
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"""A 3rd-order system with sufficient gain should have measurable phase margin."""
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freq = np.logspace(0, 6, 10000)
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w1 = 2 * np.pi * 100
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w2 = 2 * np.pi * 1000
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w3 = 2 * np.pi * 10000
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K = 100.0
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s = 1j * 2 * np.pi * freq
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loop_gain = K / ((s / w1 + 1) * (s / w2 + 1) * (s / w3 + 1))
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result = compute_phase_margin(freq, loop_gain)
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assert result["phase_margin_deg"] is not None
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assert result["is_stable"] is True
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assert result["phase_margin_deg"] > 0
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def test_all_gain_below_0db(self):
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"""If gain is always below 0 dB, phase margin is infinite (system is stable)."""
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freq = np.logspace(0, 6, 1000)
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s = 1j * 2 * np.pi * freq
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# Very low gain system
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loop_gain = 0.001 / (1 + s / (2 * np.pi * 1000))
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result = compute_phase_margin(freq, loop_gain)
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assert result["phase_margin_deg"] == float("inf")
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assert result["is_stable"] is True
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assert result["gain_crossover_freq_hz"] is None
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def test_short_input(self):
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result = compute_phase_margin(np.array([1.0]), np.array([1.0 + 0j]))
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assert result["phase_margin_deg"] is None
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class TestStabilityMetrics:
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def test_comprehensive_output(self):
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"""compute_stability_metrics returns all expected fields."""
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freq = np.logspace(0, 6, 5000)
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w1 = 2 * np.pi * 100
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w2 = 2 * np.pi * 1000
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w3 = 2 * np.pi * 10000
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K = 100.0
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s = 1j * 2 * np.pi * freq
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loop_gain = K / ((s / w1 + 1) * (s / w2 + 1) * (s / w3 + 1))
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result = compute_stability_metrics(freq, loop_gain)
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assert "gain_margin" in result
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assert "phase_margin" in result
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assert "bode" in result
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assert "is_stable" in result
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assert len(result["bode"]["frequency_hz"]) == len(freq)
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def test_short_input_structure(self):
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result = compute_stability_metrics(np.array([]), np.array([]))
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assert result["is_stable"] is None
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