Modulation Analyzer
What is FM-De-emphasis?
What is the fm De-emphasis used for in modulation analyzer ?
I am trying measure FM residual of a signal generator.
what is difference between 75us & 750us
FM de-emphasis is the opposite of FM pre-emphasis (not FM emphasis, there is no such thing!). In an FM system the amplitude of the higher frequencies is boosted before being applied to the modulator. This is because the noise in an FM system is parabolic; that means that if you increase the modulating frequency, the noise goes up as the square of the increase. For instance, there will be more noise at 10 kHz compared to the noise at 5 kHz. So pre-emphasis is used to boost the amplitude of the higher frequency signals to improve the overall signal to noise ratio.
Now, on the receiving end (your modulation analyzer is just a fancy receiver) you need to use de-emphasis to restore the higher frequencies to their original amplitudes. That’s what de-emphasis does, so your overall system audio response will be flat.
About the time constants, 75 us vs. 750 us… FM broadcasting uses a pre-emphasis time constant of 75 microseconds. This represents a single pole highpass (RC) network with a time constant of 75 us, or 2122 Hertz cutoff frequency. Signals below 2122 Hz are modulated flat and signals above 2122 Hz are modulated with a rising response of 6 dB/octave (i.e., a 15 kHz signal has twice the amplitude of a 7.5 kHz signal, and 4 times the amplitude of a 3.75 kHz signal and so forth). In the receiver there is a single pole lowpass filter with a cutoff frequency of 2122 Hz to restore the amplitude of the pre-emphasized frequencies. Land mobile radio (police, fire, other public safety services) use 750 us time constants rather than 75 us.
As far as measuring residual FM of a signal generator, you most likely want to measure that flat (no de-emphasis) and you usually want the measurement in rms rather than peak devaition.
Line 6 M9 – Frequency Analyzer, Chorus’, Phasers
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