Replacing Obsolete Windows XP DAQ Cards: A Modern CRDS Red Pitaya & LabVIEW Architecture

Bypassing legacy hardware limitations to achieve stable, driver-independent Cavity Ring-Down Spectroscopy data acquisition on Windows 11.

In laser spectroscopy laboratories, equipment longevity is frequently bottlenecked not by optical degradation, but by the obsolescence of control electronics. High-sensitivity analytical setups like continuous-wave Cavity Ring-Down Spectroscopy (CW-CRDS) rely on exceptionally durable components—such as optical cavities, high-finesse mirrors, and acousto-optic modulators (AOMs)—that can operate reliably for decades. However, the data acquisition (DAQ) systems controlling them are often tethered to proprietary, vendor-locked PCI cards that are completely incompatible with modern operating systems.

For many research teams, this creates a critical infrastructure barrier: failing, unstable Windows XP towers are holding multi-thousand-dollar spectrometers hostage. When these legacy data acquisition cards fail, finding discontinued hardware replacements wastes invaluable time and funding.

To resolve this universal laboratory pain point, an elegant migration strategy developed by researcher Roni Vikholm at the University of Helsinki demonstrates a successful modern framework. By integrating the open-source Red Pitaya STEMlab 125-14 with a modern National Instruments LabVIEW environment, the entire CRDS system was successfully migrated to Windows 11. This open-architecture upgrade matched the statistical accuracy of the legacy instrumentation while optimizing the data-handling pipeline to deliver a streamlined, driver-independent scanning environment.

The Operational Bottleneck of Legacy CRDS Hardware

To understand why upgrading a CRDS data acquisition pipeline is historically difficult, one must look at the unique behavior of a ring-down event. CRDS achieves its extreme sensitivity (often exceeding  10^-6 or even 5 x 10^-8  absorption per pass) by measuring the rate of decay of light trapped inside an optical resonator, rather than measuring absolute intensity changes.

Figure 1: Simplified schematic of a standard continuous-wave CRDS signal path.

During a measurement cycle, a continuous-wave laser is coupled into an optical cavity bounded by two highly reflective mirrors. One mirror is scanned back and forth using a piezoelectric actuator to shift the cavity length into resonance. As the mirror moves, the light waves interfere constructively. When the standing wave condition is fulfilled (nλ = 2L), the internal light intensity spikes rapidly.  

At the peak of this resonance, the photodetector senses the intensity surge, and the incoming laser beam must be instantly blocked by an acousto-optic modulator (AOM). This allows the trapped light to leak out exponentially, creating a rapid decay profile.

Figure 2: Visualizing the rapid intensity spike during piezo scanning and the subsequent exponential decay curve used to extract the Ring-Down time (τ).

The resulting decay curve must be captured and digitized with extreme precision to extract the Ring-Down time (τ), defined by the standard decay equation:

Capturing these microsecond-level decay profiles requires hardware capable of high-speed Radio Frequency (RF) sampling, low quantization noise, and deterministic timing synchronization to ensure that fast software jitter does not corrupt the exponential fitting algorithms. The old system relied on an obsolete GaGe CompuScope 12100 PCI card that was physically locked to Windows XP, leaving the entire system prone to frequent computer crashes and component failures.

The Modernized Architecture: Red Pitaya STEMlab 125-14 and LabVIEW

The upgraded architecture completely replaces the legacy Windows XP computer tower and the aging PCI card. Instead, the signal chain is split into an independent hardware digitization layer and a visual, highly maintainable user control layer running natively on Windows 11.

Figure 3: High-throughput block diagram of the modernized data acquisition system.

The data flow is engineered across three interconnected processing stages:

1. High-Speed Edge Digitization

The analog voltage output from the photodetector is routed directly into an RF input channel of the Red Pitaya STEMlab 125-14. Featuring a 14-bit analog-to-digital converter (ADC) sampling at 125 MSps, the Red Pitaya delivers an immediate performance upgrade over the 12-bit resolution of the legacy card. The 14-bit depth provides 16,384 discrete quantization levels, guaranteeing that both the highly intense initial resonance peak and the tail-end of the weak exponential decay are resolved with a minimal noise floor.

2. Embedded Pre-Processing via Custom C Code

To maximize data throughput and bypass standard operating system latency, the Red Pitaya runs an embedded Linux operating system directly from an onboard microSD card. A custom C program utilizes the board's open-source parameters to manage the raw hardware buffers at the chip level. By managing hardware-level threshold triggering directly within the device via localized C code, the Red Pitaya isolates a precise 7,500-point window for each ring-down event. This edge-layer filter reduces network overhead and prevents the host computer from processing massive, un-triggered data segments.

3. Synchronized LabVIEW Consumer Architecture

On the host PC (running Windows 11), a completely redesigned LabVIEW Virtual Instrument (VI) functions as the master control interface. To execute measurements cleanly without host-side memory strain, the software manages operations through a synchronized multi-queue task sequencer designed to balance strict execution ordering with parallel task execution:

• Deterministic Sequence Control: The system utilizes a First-In-First-Out (FIFO) queue order to enforce critical experimental timing. This guarantees that individual decay waveforms are completely isolated, processed, and evaluated for ring-down time () before a command string is dispatched to step the laser to the next wavelength point.
• Parallel Ingestion & Fitting Loops: Multiple independent queues are synchronized across concurrent processing threads. While one loop manages the low-level communication and data packet fetching from the Red Pitaya, a parallel loop processes exponential curve fitting asynchronously.
• Asynchronous Instrument Monitoring: A dedicated parallel While Loop continuously reads background wavelength data from the instrumentation hardware using the device's native broadcast mode. Because this configuration allows the wavemeter to stream data independently of the primary DAQ execution queues, the host PC avoids processing bottlenecks and instrument communication lags during high-density spectral scans.
  

Experimental Validation: Benchmarking Acetylene Absorption

To verify that the modern open-source system could match the analytical accuracy of the legacy hardware, the researchers conducted side-by-side performance benchmarks. Both systems were tasked with profiling a well-documented rovibrational absorption peak of acetylene (C₂H₂) within the 1.5 μm near-infrared spectral band.

By stepping the laser wavelength across the absorption peak, the LabVIEW program calculated the frequency-dependent absorption coefficient α(𝜈) using the relationship:

Where τ(𝜈) is the active ring-down time, τ₀ is the empty cavity ring-down time, and c is the speed of light. Integrating the total area under the resolved absorption curves yielded the integrated absorption coefficient (αint), which served as the metric for evaluating system accuracy.

The statistical analysis confirmed no significant difference in accuracy between the legacy 100 MSps GaGe card and the new Red Pitaya STEMlab architecture. Crucially, while maintaining identical analytical precision, the Red Pitaya configuration lowered total measurement times—achieving a 12.7% speed improvement over the legacy Windows XP setup during standard 10-spectrum benchmarking runs.

Open Hardware: Changing the Paradigm of Spectroscopy Design

The successful implementation of this project highlights why the Red Pitaya STEMlab platform is gaining rapid adoption as an alternative to proprietary laboratory instrumentation.

Figure 4: Structural comparison between legacy and modern DAQ frameworks.

By decoupling data acquisition from the host computer’s motherboard and shifting it to an independent network-accessible SoC, the system becomes isolated from future PC operating system upgrades. If the host computer needs to be upgraded or replaced in the future, the LabVIEW application can be migrated instantly without hunting for proprietary hardware drivers. This upgrade proves that instead of replacing expensive proprietary spectrometers, labs can achieve modern Windows 11 compatibility and superior 14-bit performance simply by integrating a single, cost-effective Red Pitaya board. Furthermore, this open architecture allows future researchers to directly modify the onboard FPGA code or expand the software array via LabVIEW's visual sub-VIs, providing an easily maintainable, future-proof framework for high-precision molecular spectroscopy.

Technical FAQ for Spectroscopy Instrumentation Developers

How does the Red Pitaya handle the high repetition rates of continuous-wave CRDS without losing data frames?

Data handling is optimized by dividing the operational burden. The embedded C program on the Red Pitaya manages raw hardware-level threshold triggers and isolates a tailored data window within the device's RAM. By only packing and streaming data linked to valid ring-down events over a Gigabit Ethernet link, network overhead is minimized, allowing the host PC's LabVIEW environment to sequence the tasks efficiently without overflowing the computer's processing buffer.

What are the main advantages of using a 14-bit system over a legacy 12-bit instrumentation card?

A 12-bit card provides 4,096 vertical quantization steps, which can cause quantization noise distortions at the low-voltage tail of a ring-down curve, degrading the accuracy of the exponential fit. The Red Pitaya’s 14-bit ADC yields 16,384 steps—a fourfold increase in vertical resolution. This allows the acquisition system to capture subtle signal decay dynamics cleanly, ensuring highly repeatable ring-down time calculations even when measuring weak molecular absorptions.

Can this LabVIEW and Red Pitaya pipeline be expanded to handle other laser locking or cavity stabilization tasks?

Yes. Because the STEMlab 125-14 features dual high-speed RF outputs alongside its input channels, the system can be expanded to perform instrumentation control. For instance, the Red Pitaya's onboard signal generator can be programmed via LabVIEW to output specialized error signals or automated voltage ramps to control the laser's internal piezoelectric components, eliminating the need for separate external function generators in the laboratory setup.