Lock-in Modeler

Overview

The Lock-in Modeler performs real-time frequency response and distortion sweeps using Synchronized Swept Sine (SSS) signals and digital lock-in detection algorithms to measure the magnitude and phase response of a device under test (DUT) with extremely high precision and speed.

In addition to standard frequency sweep measurements (tracking the fundamental and up to 5th-order harmonics simultaneously), this tool features a nonlinear Hammerstein model identification capability and allows exporting identified models to JSON files.

Operation

1. Latency Calibration (Mandatory)

Before running any sweep, the round-trip latency of the audio input and output channels must be measured and compensated. The sweep start button remains disabled until calibration is complete.

Click the Calibrate Latency button.
A test signal is sent, and the system latency (in samples and milliseconds) is automatically calculated and displayed.
Once calibration is successful, the Start Sweep button will be enabled.

2. Running a Sweep Measurement

Configure parameters such as Sweep Mode and frequency range in the Settings tab.
Click the Start Sweep button to begin the measurement.
During the sweep, progress, current signal frequency, analysis frequency, and Equivalent Noise Bandwidth (ENBW) are displayed in the Overview panel.
Click Stop Sweep to halt the measurement early.

Settings

Settings Tab

Sweep Mode: Choose between three measurement modes:
- Sweep Measurement (Default): Performs a standard frequency sweep, plotting the magnitude and phase responses for the fundamental and configured harmonics.
- Nonlinear Model (Forward): Identifies a Hammerstein nonlinear model by automatically performing multiple sweeps at different test amplitude levels to resolve higher-order kernels using a standard forward model.
- Nonlinear Model (Parallel Complex): Identifies a Hammerstein nonlinear model using a Parallel Complex Hammerstein structure, capable of capturing more intricate nonlinear behaviors.
- Predistortion Sweep (Adaptive DPD): Performs an adaptive digital predistortion (DPD) sweep, adjusting the inverse compensation iteratively to minimize harmonic distortion.
Start Freq / End Freq (Hz): Sets the frequency boundaries for the sweep (2.0 Hz up to the Nyquist frequency).
Duration (s): The duration of a single sweep cycle (2.0 s to 600.0 s).
Amplitude (dBFS): Sets the output signal level in dBFS (-100.0 dBFS to 0.0 dBFS).
Max Harmonic: Sets the highest harmonic order to analyze alongside the fundamental (1st to 5th).
Averages: Sets the number of sweep cycles to average for each amplitude level, which improves the SNR.
Amplitude Steps: Active only in the Nonlinear Model mode. Specifies the number of amplitude steps (5 to 10) to use for system identification.
Adaptive Algorithm: Active only in the Predistortion Sweep mode. Selects the optimization algorithm (Secant (Empirical Jacobian) or Newton (LM Regularized)) to use for iteratively updating the predistortion model.
DPD Iterations: Active only in the Predistortion Sweep mode. Specifies the number of iteration sweeps (1 to 20) for the adaptive algorithm to converge.
Learning Rate (mu): Active only in the Predistortion Sweep mode. Sets the step size for each DPD update (0.01 to 1.0).
Mu Decay: Active only in the Predistortion Sweep mode. Sets the decay factor for the learning rate (0.50 to 1.00) to ensure stable convergence over multiple iterations.

Advanced Settings

Advanced Settings: Opens a dialog to configure advanced algorithm parameters.
- Time-Synchronized Averaging (TSA):
  - Cycles: Number of cycles to average (1-100).
  - Rejection Threshold: Standard deviation threshold for outlier rejection.

Display Tab

Show Relative to Fundamental: When enabled, the magnitude and phase of the harmonics are plotted relative to the fundamental frequency response (in dB / degrees) rather than as absolute values.
Unwrap Phase: When enabled, phase transitions over $\pm 180$ degrees are smoothed out into a continuous curve instead of wrapping.
Show Raw Lock-in (Unprocessed): When enabled, displays the raw, unprocessed data points from the lock-in amplifier before any smoothing or interpolation is applied (disabled in Hammerstein mode).
Display Data: In Hammerstein mode, selects whether to display the overall 'Model Kernels' or the individual response at specific test amplitudes.
Graph Smoothing: Selects the smoothing level (None, Low, Medium, or Heavy) for the plotted curves, applying a Savitzky-Golay filter.

Routing Tab

Output Ch: Selects the physical channel to output the sweep signal (Left, Right, or Stereo).
Input Mode: Configures how the input signal is acquired:
- Single Ch (Left Input): Absolute measurement using the left input channel only.
- Single Ch (Right Input): Absolute measurement using the right input channel only.
- 2-Ch Relative (Ref=Left, Meas=Right): Dual-channel Transfer Function (XFER) mode. Measures the ratio between the left channel (reference) and right channel (measurement). This cancels out systemic response variations and nonlinearity in the test gear.
- 2-Ch Relative (Ref=Right, Meas=Left): Dual-channel Transfer Function mode with the right channel as reference and the left as measurement.
REF Phase Lock Only (Absolute): Available only in 2-Ch Relative modes. When enabled, the reference channel is used strictly for phase synchronization (correcting time delays/phase drift), while the magnitude response is calculated as an absolute measurement of the measurement channel rather than a relative ratio.

Advanced Tab

Analysis Cycles: Sets the analysis window size in cycles per frequency bin for the digital lock-in calculation (2.0 to 2048.0 cycles). A larger number improves frequency resolution and noise rejection, but increases vulnerability to transient settling time.
Meas Points: Total number of frequency sample points measured in the sweep (100 to 5000 points).
Min Window (ms): The minimum analysis window duration in milliseconds. This prevents the window from becoming excessively short at high frequencies, which would inflate the ENBW.
Preset: Allows selecting from predefined configurations (e.g., "Fast & Dynamic (16 cyc / 30 ms)", "Normal (64 cyc / 100 ms)", "High Resolution (128 cyc / 500 ms)", "High Stability (256 cyc / 1.0 s)", "Maximum Stability (512 cyc / 2.0 s)") that automatically set the optimal balance of Analysis Cycles and Min Window for specific measurement scenarios. Selecting "Custom" allows manual tuning.
Real-time Display: When enabled, plots and updates curves in real-time during the sweep. When disabled, analysis calculations are buffered during the sweep and plotted all at once after completion, which reduces CPU load and prevents audio buffer underruns (dropouts).

Analysis Results & Model Export

Plot Views

Frequency Response Tab: Displays the magnitude and phase responses for the fundamental and harmonics (up to 5th-order, based on Max Harmonic). Harmonic characteristics can be viewed as absolute values or relative to the fundamental. The magnitude, phase, and quality plots share a sweep-aware 1-2-5 logarithmic frequency axis.
Impulse Responses (Kernels) Tab: Enabled after a Hammerstein (Nonlinear Model) sweep. Displays the identified 1st-order to maximum 5th-order time-domain impulse response kernels ( $h_1(t)$ to $h_5(t)$ ).
Lock-in SNR Tab: Displays the signal-to-noise ratio (SNR) in dB for the lock-in estimation, providing a dynamic metric to evaluate the reliability of the measurement data. Higher values (e.g., +40dB / +60dB) represent higher lock-in quality.

Upon completing a Nonlinear Model (Hammerstein) sweep, the Export Model... button is enabled.

Click Export Model....
Specify the path and filename (JSON) in the file save dialog.
The exported JSON model contains metadata (sample rate, sweep duration, parameters, etc.) alongside both time-domain and frequency-domain kernel datasets.
Saving the model also automatically registers it to MeasureLab's active model cache, making it instantly available for other processing components (e.g. distortion compensation).