433mhz waveform. FSK? GFSK? GMSK?

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Joined: Fri Sep 07, 2018 5:00 am

433mhz waveform. FSK? GFSK? GMSK?

Post by polygonfuture » Fri Sep 07, 2018 5:09 am

Hi all,

I am working on a fun creative project capturing and decoding 433mhz RF signals from a Si44631B chip. Looking at the datasheet, the device can modulate in (G)FSK, 4(G)FSK, (G)MSK, and OOK.

I captured my 433mhz signal and noticed something funny about the wave form. It appears to have two transmissions at slightly different frequencies as shown below:
RTL-SDR_433mhz_fft_display.JPG (96.94 KiB) Viewed 225 times

This is a view of the wave-form in Audacity:
modulation_fsk_gfsk_gmsk.JPG (161.79 KiB) Viewed 225 times
wave_form_closeup.JPG (58.11 KiB) Viewed 220 times

Judging by the wave form in audacity, I believe the modulation is either GFSK or possibly GMSK.

1. What does everyone think? Am I really far off base here?

2. Why does the FFT waveform display show such a strange wave-form (with two 'horn' peaks)?

Thanks so much for all of your help. Excited to keep learning more about RF.

Posts: 12
Joined: Wed May 23, 2018 8:27 pm

Re: 433mhz waveform. FSK? GFSK? GMSK?

Post by Airwind » Sun Sep 09, 2018 9:38 pm

Looks like 2FSK to me. The two peaks are the two alternating frequencies.

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