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CC110LEM-433-RD: Fdev doesn't change while the setting is changed in SmartRfStudio

Part Number: CC110LEM-433-RD

433M, 250kbps

If I config the "Deviation" setting to 127kHz/114kHz/101kHz, the actual Fdev in spectrum is not changed. See the picture below.

Why?

  • If you look at a wider span you will see that the spectrum changes with deviation (for the same data rate). The spikes you see in your plot are set by the data rate. That is, if you keep the deviation and change the data rate the spikes will move. To convince yourself that the spectrum does change you can look at a wider span, measure the 99% occupied bandwidth, or look at the signal in the time domain (frequency vs time) and not amplitude vs frequency as in your plot.  

  • So how can we measure Fdev using spectrum analyzer? And I seemed not be able to find frequency vs time in the SA. I could only see amplitude vs time. Please give me some more details.

  • A question slightly on the side: Why do you actually want to measure Fdev? The deviation is given by 5.17.1 in www.ti.com/.../cc110l.pdf and as can be seen from the equation the deviation is set digitally meaning that the accuracy will be better than you manage to measure with a SA.
  • I find a way to measure the Fdev using SmartRfStudio:

    1. Select "Continuous Tx"->"Unmodulated", and I could see the center frequency in SA.
    2. Select "Modulated" and "Synchronous serial mode", and I could see another CW. The frequency difference with center frequency should be the Fdev.

     

     

  • "Continuous Tx"->"Unmodulated" will set up the chip in OOK mode and transmit the carrier frequency

    "Continuous Tx"->"Modulated" synchronous serial mode will set up the chip in GFSK mode (assuming this is what you enter under Modulation Format) and transmit the carrier frequency - frequency deviation if the data input is pulled low (e.g. connect to GND) and transmit the carrier frequency + frequency deviation if the data input is pulled high (e.g. connect to VDD). If you leave the data input floating on the TrxEB carrier frequency - frequency deviation will be measured.