Characteristic S-Curve of an FM Demodulator Using the GW Instek MPO-2000 Oscilloscope 

In signal processing, understanding the performance of frequency modulation (FM) demodulators is critical. The characteristic "S" curve provides valuable insights into the demodulator's response to varying input signals. This article explores how to analyze the "S" curve of an FM demodulator using the GW Instek MPO-2000 oscilloscope, a versatile tool for advanced signal analysis. 

What is the S-Curve?

 The S-curve illustrates the relationship between input frequency deviation and output voltage of the FM demodulator. It highlights how the demodulator responds to frequency changes, providing insights into its linearity, sensitivity, and overall performance. A well-defined S-curve indicates a properly functioning demodulator capable of accurately tracking frequency variations. 

 Using the MPO-2000 for S-Curve Analysis

The GW Instek MPO-2000 features built-in Python scripting capabilities for easy measurement automation. The demonstration utilizes an MC1496 Modulator/Demodulator IC, a resonant circuit, and a variable inductor (L1) for frequency tuning. The circuit is powered by a +12V and -12V supply from an experimental box, while the sine wave signal for demodulation is generated by the MPO-2000's AWG channel, with output voltage measured via Channel 1 of the oscilloscope.

The process consists of two parts: first, generating a 4.5 MHz carrier frequency with a 1 kHz FM signal and adjusting L1 for optimal output; second, measuring the S-Curve using a point-by-point method, generating sine waves from 4.5 MHz to 5.5 MHz in 100 kHz increments.

The Python script sequentially controls the AWG to produce multiple frequencies, measuring and calculating the average waveform (using Channel 1 of the oscilloscope or the DMM on the MPO-2000). After each frequency is set, the script displays the characteristic curve on the MPO-2000's LCD and records measurements in a .csv file.

The figure above shows an S-Curve with 101 test points spaced 10 kHz apart, completed under 60 seconds. While the curve exhibits some linear segments, it significantly improves upon the original tabular data representation and reduces manual effort.

Analyzing the characteristic S-curve of an FM demodulator using the GW Instek MPO-2000 is straightforward and yields valuable insights into device performance. By following the outlined steps, engineers can effectively evaluate and optimize FM demodulators for their specific applications. The MPO-2000's advanced features make it an indispensable tool in signal processing, ensuring reliable and accurate measurements.

References:

MPO-2000 Python API programming example source code: https://github.com/OpenWave-GW/Python_APP
MPO-2000 Python API on-line article: https://python-app.readthedocs.io/en/latest/index.html#
Python Tutorial and Application Handbook (200 pages): https://www.gwinstek.com/zh-TW/products/downloadSeriesDownNew/23931/2400
MPO-2000 product introduction YT clip: https://www.youtube.com/watch?v=BNPRGuqg0ew&t=5s
MPO-2000 Python editing tools – WebIDE introduction YT clip: https://www.youtube.com/watch?v=mK6hKHGahK8&t=47s
MPO-2000 CAN FD, FlexRay, USB 2.0, USB PD, I2S bus decode YT clip: https://www.youtube.com/watch?v=6GCXpvencKg&t=12s

Overseas Sales Department
Good Will Instrument Co., Ltd
No. 7-1, Jhongsing Road, Tucheng Dist.,
New Taipei City 23678, Taiwan R.O.C.
Email: marketing@goodwill.com.tw