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youtube-at-vach edited this page Jul 23, 2026 · 1 revision

Lock-in Suite

Relevant source files

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The Lock-in Suite provides a family of high-precision measurement tools based on synchronous demodulation and matrix projection. These modules allow for the extraction of extremely low-level signals buried in noise by leveraging a known reference frequency (either internally generated or externally tracked via hardware loopback).

Lock-in Amplifier

The LockInAmplifier performs dual-phase demodulation to extract the magnitude and phase of a signal relative to a reference. Due to the lack of hardware-level clock synchronization in standard PC audio interfaces, this module typically requires a hardware loopback (reference channel) to maintain a stable phase relationship.

Implementation Details

  • Dual-Phase Demodulation: The input signal is multiplied by in-phase ($I$) and quadrature ($Q$) reference waves.
  • Post-mix LPF: To achieve high dynamic reserve, the baseband result is passed through a cascade of up to 8 one-pole IIR sections src/gui/widgets/lock_in_amplifier.py:79-82.
  • Harmonic Ratios: Supports arbitrary integer harmonic ratios ($N/D$) for the demodulation frequency src/gui/widgets/lock_in_amplifier.py:94-118.

Data Flow

  1. Input Capture: Signal and reference channels are captured into a ring buffer src/gui/widgets/lock_in_amplifier.py:190-213.
  2. Phase Tracking: If in external mode, the reference channel is analyzed to track continuous phase src/gui/widgets/lock_in_amplifier.py:175-175.
  3. Complex Baseband: Multiplication by $e^{-j\omega t}$ followed by IIR filtering src/gui/widgets/lock_in_amplifier.py:80-86.

Sources: src/gui/widgets/lock_in_amplifier.py:40-180, tech_docs/implementation/lock_in_amplifier.md (referenced).

Lock-in Harmonic Analyzer

The LockInHarmonicAnalyzer provides ultra-precision Total Harmonic Distortion (THD) measurement. Unlike FFT-based analyzers, it uses parallel reference-locked matrix projection to extract up to 200 harmonics simultaneously.

Key Features

  • Matrix Projection: Projects the input signal onto a basis of sine/cosine pairs for each harmonic order src/gui/widgets/lock_in_harmonic_analyzer.py:120-123.
  • Distortion Compensation: Features an iterative calibration system that generates anti-phase harmonics to cancel generator-side non-linearity src/gui/widgets/lock_in_harmonic_analyzer.py:83-94.
  • High Order: Supports analysis up to the 200th harmonic src/gui/widgets/lock_in_harmonic_analyzer.py:35-35.

Sources: src/gui/widgets/lock_in_harmonic_analyzer.py:40-95

Lock-in Frequency Counter

The LockInFrequencyCounter measures precision frequency and phase drift. It uses a Phase-Locked Loop (PLL) style architecture with a PID controller to track an incoming signal.

Technical Components

  • PID Controller: Adjusts the Numerically Controlled Oscillator (NCO) to minimize the phase error src/gui/widgets/lock_in_frequency_counter.py:33-62.
  • Kalman Filter: A 1D Kalman filter smooths the frequency estimates, providing an uncertainty metric ($P$ covariance) src/gui/widgets/lock_in_frequency_counter.py:65-102.
  • Stability Metrics: Calculates Allan Deviation and distribution statistics for long-term stability analysis src/gui/widgets/lock_in_frequency_counter.py:175-181.

Sources: src/gui/widgets/lock_in_frequency_counter.py:108-209

Lock-in Spectrum Finder

The LockInSpectrumFinder is designed for identifying periodic interference (e.g., mains hum, SMPS noise, USB frame noise).

Implementation

  • Matrix Projection: Instead of a full FFT, it projects data onto specific target frequencies defined in a target dictionary src/gui/widgets/lockin_spectrum_finder.py:68-104.
  • Scan Targets: Includes presets for mains harmonics, musical scales (12TET, 24TET, Just Intonation), and common switching power supply frequencies src/gui/widgets/lockin_spectrum_finder.py:106-218.
  • Sonification: Uses the Sonifier class to map detected spectral components into the audible range for "listening" to electromagnetic interference src/gui/widgets/lockin_spectrum_finder.py:37-38.

Sources: src/gui/widgets/lockin_spectrum_finder.py:57-220

Lock-in Modeler

The LockInModeler utilizes the Synchronized Swept Sine (SSS) method to identify system transfer functions and non-linear kernels.

Architecture: Modeler to Core Engine

The Modeler widget acts as a frontend for the RealtimeSSSEngine, which handles the heavy DSP lifting in background threads.

Logic Entity Code Entity Role
Module Frontend LockInModeler Manages UI state and audio callback registration src/gui/widgets/lock_in_modeler.py:122
DSP Engine RealtimeSSSEngine Implements SSS excitation and least-squares fitting src/gui/widgets/lock_in_modeler.py:30
Worker Thread SSSCalculationThread Prevents UI blocking during kernel extraction src/gui/widgets/lock_in_modeler.py:64
Kernel Storage HammersteinModel Saves extracted $h_1 \dots h_n$ kernels src/gui/widgets/lock_in_modeler.py:31

Workflow Diagram: SSS Kernel Extraction

This diagram illustrates how the system moves from "Natural Language" measurement goals to "Code Entity" execution.

Title: SSS Measurement Pipeline

graph TD
    User["User: Start Sweep"] --> LIM["LockInModeler.start_analysis()"]
    LIM --> Engine["RealtimeSSSEngine.prepare_sweep()"]
    Engine --> Gen["_get_reference_signals()"]
    
    subgraph AudioThread ["Audio Engine Callback"]
        Gen --> Out["outdata (Excitation)"]
        In["indata (Response)"] --> RingBuf["RingBuffer"]
    end
    
    RingBuf --> CalcThread["SSSCalculationThread.run()"]
    CalcThread --> LS["RealtimeSSSEngine.process_input_block()"]
    LS --> Result["Least-Squares Fit (I/Q)"]
    Result --> Kernel["Hammerstein Kernel Extraction (h1-h5)"]
    Kernel --> Save["save_hammerstein_model()"]
Loading

Sources: src/gui/widgets/lock_in_modeler.py:186-210, src/core/analysis.py:96-107, src/gui/widgets/lock_in_modeler.py:64-116

Technical Summary Table

Module Core Algorithm Primary Use Case
Amplifier Dual-phase I/Q Demodulation Small signal recovery, phase measurement
Harmonic Analyzer Parallel Matrix Projection Ultra-low THD (< -120dB) verification
Frequency Counter PID-tracked NCO + Kalman Clock drift, Allan Deviation, jitter
Spectrum Finder Target-based Projection EMI/RFI identification, sonification
Modeler Synchronized Swept Sine (SSS) Non-linear system identification (Hammerstein)

Sources: src/gui/widgets/lock_in_amplifier.py, src/gui/widgets/lockin_harmonic_analyzer.py, src/gui/widgets/lock_in_frequency_counter.py, src/gui/widgets/lockin_spectrum_finder.py, src/gui/widgets/lock_in_modeler.py

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