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I would like to be able to measure the instantaneous frequency of my FM modulated signal.

Question :

I would like to be able to measure the instantaneous frequency of my FM modulated signal.

Answer :

Essentially, this graph is called a time trend of frequency. Essentially it is a graph of the demodulated signal. This is something that can be done with the help of TDSJIT3.

This can be demonstrated using a signal from an AFG3000 to generate an FM sine wave. The modulation profile can be anything, but in this example has been chosen to be a sine profile. I then used JIT3 to measure the frequency, and then created a time trend plot to plot frequency for the entire record.

The modulated sine wave is 1MHz, and deviates 0.5MHz, at a 10kHz modulation frequency. This means that acquiring the signal at 100usec/div, there will be 10 cycles of the demodulated sine captured by the oscilloscope. JIT3 will measure a MAX of about 1.5MHz, and a MIN of about 500kHz. See Figure 1 for the measurements and the time signal.

Figure 1

Figure 1: Modulated FM sine wave and JIT3 Measurements

Notice in Figure 2 that 10 full cycles of the demodulated signal have been captured, just as we wished.

Figure 2

Figure 2: Time Trend plot of the demodulated signal


This FAQ Applies to:

Product Series: tds5000 tds5000 tds5000

Product:

FAQ ID 52871

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