Moku:Go's Spectrum Analyzer
Lab study of Amplitude Modulated signal with Moku:Go
Application Note | March 22, 2021
Introduction
Moku:Go combines 8 lab instruments in one high-performance device, complete with integrated power supplies. This application note discusses Moku:Go's Spectrum Analyzer and Waveform Generator, and the advantages of a hybrid, swept signal spectrum analyzer, particularly for educational lab use investigating Amplitude Modulated (A.M.) signals.
Spectrum Analyzers
Spectrum analyzers are essential test and measurement instruments in any electrical engineering lab, used to display and analyze signals in the frequency domain. Compared to the Fast Fourier Transform (FFT) function available in some oscilloscopes, frequency-swept spectrum analyzers typically provide better spectral resolution while maintaining a large frequency span. However, conventional spectrum analyzers can be slower than the FFT-based approach, especially at finer resolutions. Moku:Go's Spectrum Analyzer instrument uses a hybrid technique that balances speed and frequency resolution by combining a frequency mixer with an FFT. This guide uses Moku:Go's Spectrum Analyzer to investigate an A.M. signal and examine its sidebands using the available frequency range and resolution bandwidth.
Generating A.M. Signal
An amplitude-modulated signal consists of a carrier signal plus a modulating signal. For this study, a carrier of 15 MHz is modulated by a 4 kHz signal, a common example of an A.M. radio carrier carrying audio bandwidth signals.
The Moku:Go Waveform Generator interface is shown. It is configured to output a 15 MHz sine wave carrier with 4 kHz modulation at 1.000 Vpp. A second output is configured for a 500.0 mVpp ramp signal.
An A.M. signal features a large carrier component along with sidebands. The bandwidth of these sidebands corresponds to the bandwidth of the modulating signal. Therefore, for a 4 kHz modulating signal, sidebands are expected at 14.996 MHz and 15.004 MHz, offset from the 15 MHz carrier. An FFT-based spectrum analyzer, while fast, might not resolve these fine 4 kHz details at the carrier frequency, making it difficult for a meaningful lab investigation of amplitude modulation.
Spectrum Analyzer Results
The Moku:Go Spectrum Analyzer is shown in operation, displaying a signal centered around 15 MHz. The hybrid approach, utilizing a frequency mixer on Moku:Go's FPGA combined with an FFT, clearly reveals the sidebands offset by 4 kHz. The minimum resolution bandwidth (RBW) is indicated as 481.3 Hz. The frequency span covers approximately 14.850 GHz to 15.091 GHz. Averaging is set to 90/100.
In a lab setting, students can modify the modulating signal and observe how the sideband positions change, relating these observations to theoretical concepts. Furthermore, Moku:Go's Waveform Generator can modulate the carrier using signals from its analog inputs. This allows for experiments where audio inputs, such as music or voice recordings, can be used to modulate the carrier, and the resulting sideband bandwidth can be observed.
Benefits of Moku:Go
For the Educator & Lab Assistants
- Efficient use of lab space and time.
- Ease of consistent instrument configuration.
- Focus on electronics principles rather than instrument setup.
- Maximizes lab teaching assistant time.
- Supports individual labs and individual learning.
- Simplifies evaluation and grading via screenshots.
For the Student
- Individual labs at their own pace enhance understanding and retention.
- Portability allows choice of pace, place, and time for lab work (home, campus, or remote collaboration).
- Familiar Windows or macOS laptop environment provides access to professional-grade instruments.
Moku:Go Demo Mode
The Moku:Go app can be downloaded for macOS and Windows from the Liquid Instruments website. The demo mode operates without requiring any hardware and offers a comprehensive overview of Moku:Go's capabilities.
Questions or Comments?
Contact Liquid Instruments at support@liquidinstruments.com.
