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Following image is from a measurement I did with a spectrum analyzer, configured to save the highest peaks.

GFSK Signal

The channel center is at 2.402 GHz and has a BW of 2MHz (2.401 - 2.403 GHz). The signal modulation is GFSK. I do know that, depending on the data, for 0 the offset from the center frequency is -f_o and for 1 it's +f_o (frequency_offset).

Am I assuming right, that the peaks @ 2.401 and 2.403 are transmitted 0's and 1's?

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The peaks at 2.401 and 2.403 are spurious energy, you can see that they're more than 30dB down on the central 'pudding', which is where 99% of the modulation energy is.

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When you consider the signal Bandwidth and slew rate of the VCO, how much of the time it spends at 100% input voltage and the deviation ratio of FM/data rate shapes the spectrum in between and thus the the end points may be reached in a small portion of the time.

Examine the amplitude with zero f sweep say 20 MHz off centre to get an AM response of the FM with no video filter and 10MHz resolution and see what your data looks like on XX us / div. This is a poor man’s FM discriminator on the slope of the filter. then examine phase shift, ISI and group delay when centred.

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  • \$\begingroup\$ I will try it tomorrow, when I'm at the lab again. Altough I have to admit that it is somehow difficult to understand :D. Thanks. \$\endgroup\$ – OcK Nov 7 '17 at 17:25
  • \$\begingroup\$ Was it random data? Or just a filtered square wave with round edges at a rate > 500kHz. Maybe the VCO BW is limited \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Nov 7 '17 at 18:05
  • \$\begingroup\$ Obviously if the input was a lower data rate there wouldn be no spectrum in between large twin peaks \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Nov 7 '17 at 18:09
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At some point the very circuits used to build the GSM signal will limit the trash floor.

Finite carrier suppression or sideband suppression may produce these lobes.

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