---
title: PWM Audio Analysis with FPGA + Red Pitaya Gen 2
description: Analyze PWM audio signals with Red Pitaya Gen2. See how resolution impacts spectrum, filtering, and Class-D amplifier design.
image: https://content.redpitaya.com/hubfs/blog%20featured%20image%20-%202026-05-05T152338.466.jpg
---
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# PWM Audio Analysis with FPGA + Red Pitaya Gen 2
- Posted by  [Red Pitaya Technical Editorial Team](https://redpitaya.com/Red-Pitaya-Blog-team/) , May 12, 2026
***A practical look at spectrum behavior in Class-D–style signals***
Engineers working with audio DSP or power electronics already know the trade-offs in PWM-based systems. What this project does well is make those trade-offs measurable—using real hardware, not just simulation.
In a recent experiment published by [**Pablo Trujillo (ControlPaths)**](https://controlpaths.com/2026/03/15/red-pitaya-gen2-audio-pwm-signal/), Red Pitaya [**STEMlab 125-14 PRO Gen 2**](https://redpitaya.com/stemlab-125-14-gen2) is used as a spectrum analyzer to evaluate how PWM modulation affects an audio signal across different resolutions. The setup combines FPGA signal generation with high-speed analog measurement, offering a compact but capable test bench for real-world validation.
## The Setup: FPGA-Generated PWM, Measured in Hardware
The experiment pairs two platforms:
- Red Pitaya STEMlab 125-14 PRO Gen 2 → used for acquisition and spectral analysis
- Digilent Arty A7 → generates the PWM audio signal
A simple Verilog-based PWM modulator runs at 100 MHz on the FPGA, encoding a 3.9 kHz sine wave. The resulting signal is then captured and analyzed using Red Pitaya’s built-in spectrum analyzer.
This approach is notable because it replaces a traditional lab stack (signal generator + spectrum analyzer) with a single, software-defined instrument that is remotely accessible and FPGA-reconfigurable.
## What’s Actually Being Tested?
The core question is familiar to anyone designing Class-D amplifiers:
**How does PWM resolution affect spectral distribution—and what does that mean for filtering and audio quality?**
PWM encoding introduces a carrier frequency and harmonic structure that must be filtered out to recover clean audio. The position of that carrier depends directly on resolution:
- 8-bit → ~390 kHz carrier
- 9-bit → ~195 kHz
- 10-bit → ~97 kHz
Higher resolution improves amplitude granularity, but compresses the spectral spacing—bringing switching artifacts closer to the audio band.
## What the Measurements Show
Using Red Pitaya as a spectrum analyzer makes the trade-offs immediately visible:
- **8-bit PWM**
- Carrier far from audio band
- Easier filtering
- Lower amplitude resolution
- **9-bit PWM**
- Balanced compromise
- Harmonics begin entering mid-frequency range
- **10-bit PWM**
- Better resolution (1024 levels)
- Carrier dangerously close to audio band
- Requires higher-order filtering (LC, multi-stage)
The key takeaway is not theoretical—it is measurable:
**Resolution and filtering complexity are tightly coupled design parameters.**
Time-domain observations reinforce this. Lower carrier frequencies (higher resolution) introduce visible distortion after reconstruction, especially if filtering is insufficient.
## Why Red Pitaya Gen 2 Matters Here
This type of experiment highlights where Red Pitaya STEMlab Gen 2 fits in an engineering workflow:
- Combines **oscilloscope, spectrum analyzer, and signal generator** in one device
- Provides **125 MS/s ADC/DAC** for mid-frequency analog work
- Enables **FPGA-level customization** for DSP pipelines
- Supports **SCPI, Python, and browser-based control**
For DSP, embedded, or FPGA engineers, it effectively bridges the gap between development board and lab instrumentation.
## Practical Implications for Engineers
If you are working on:
- Class-D amplifier design
- Audio DSP pipelines
- FPGA-based signal processing
- Mixed-signal validation
this experiment reinforces a few operational realities:
- Simulation alone is insufficient for PWM systems
- Spectral verification must include real switching artifacts
- Hardware-in-the-loop testing exposes filter limitations early
- Tooling flexibility (FPGA + measurement) reduces iteration time
## Read the Full Technical Breakdown
This summary only covers the key insights. The original article goes deeper into:
- Verilog PWM implementation
- Spectrum plots across resolutions
- FPGA workflow and deployment
- Detailed measurement methodology
Read the full article here:
[Red Pitaya Gen 2: Analysis of an audio signal modulated using PWM | controlpaths.com](https://controlpaths.com/2026/03/15/red-pitaya-gen2-audio-pwm-signal/)
---
## FAQ
### **What is PWM audio modulation used for?**
Primarily in Class-D amplifiers, where efficiency is achieved by switching instead of linear amplification.
### **Why does PWM resolution affect frequency spectrum?**
Because resolution determines the division of the system clock, directly setting the carrier frequency and harmonic spacing.
### **Can Red Pitaya replace a spectrum analyzer?**
For many applications up to ~50 MHz bandwidth, yes. It provides sufficient resolution and flexibility for embedded and DSP work.
### **Why not just simulate PWM behavior?**
Simulation cannot capture analog effects such as switching noise, non-ideal filtering, and real hardware constraints.
### **Is this relevant beyond audio systems?**
Yes. The same principles apply to motor control, power electronics, and any PWM-based control system.
### About the Red Pitaya Team
The Red Pitaya Technical Editorial Team is a cross-functional group of technical communicators and product specialists. By synthesizing insights from our hardware developers and global research partners, we provide verified, high-value content that bridges the gap between open-source innovation and industrial-grade precision.
Our mission is to make advanced instrumentation accessible to engineers, researchers, and educators worldwide.
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