In WaveForms 3.9.1, what does the FM Modulation Index setting actually set?
The screenshot explains why this is unclear. On the Electonics Explorer board a single jumper wire connects AWG1 to the Scope 1 AC input. Frequency Modulation (FM) Modulation Index is defined as (peak frequency deviation) / (modulation frequency) or delta_f / f_mod. A value of 2.405 is the first Bessel null, meaning on a spectrum analyzer the carrier level goes to zero as shown in the image. This is a common test scenario. So given that f_mod = 987.6 Hz, delta_f = 987.6 * 2.405 = 2.375 kHz. I found the carrier null by small changes to the Amplitude/Index FM setting in WaveForms, and 20.3% nulls the carrier lower than either 20.2% or 20.4%. What logic makes a setting of 20.3% result in 2.375 kHz FM frequency deviation? I thought it might be "peak frequency deviation = 20.3% of the carrier frequency" but that would be 0.203*12.34 kHz = 2.505 kHz so that doesn't work. Both Factory and my own calibration settings produce the same result.
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Rick314
In WaveForms 3.9.1, what does the FM Modulation Index setting actually set?
The screenshot explains why this is unclear. On the Electonics Explorer board a single jumper wire connects AWG1 to the Scope 1 AC input. Frequency Modulation (FM) Modulation Index is defined as (peak frequency deviation) / (modulation frequency) or delta_f / f_mod. A value of 2.405 is the first Bessel null, meaning on a spectrum analyzer the carrier level goes to zero as shown in the image. This is a common test scenario. So given that f_mod = 987.6 Hz, delta_f = 987.6 * 2.405 = 2.375 kHz. I found the carrier null by small changes to the Amplitude/Index FM setting in WaveForms, and 20.3% nulls the carrier lower than either 20.2% or 20.4%. What logic makes a setting of 20.3% result in 2.375 kHz FM frequency deviation? I thought it might be "peak frequency deviation = 20.3% of the carrier frequency" but that would be 0.203*12.34 kHz = 2.505 kHz so that doesn't work. Both Factory and my own calibration settings produce the same result.
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