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CC1120: Center Frequency Question

cc1120user
Prodigy 100 points
Part Number: CC1120
Other Parts Discussed in Thread: CC1020, CC1175,

I have a 2-FSK signal with a baud rate of 9600 kHz with a 32 MHz reference crystal.

 - How far off my center frequency can my received signal be?

- Which filter is best for performing demodulation of a CC1020 2-FSK 9600 baud signal (Gaussian, Root Raised Cosine, Root Cosine) ?

  • 0
    TI__Genius 12395 points

    Hi,

     - How far off my center frequency can my received signal be?

    This is influenced by several factors such as the PHY/settings you use (including frequency band and RX Filter Bandwidth) for RX/TX, the crystal accuracy, and the antenna design - so, it depends on your desired application and your design.

    We recommend using the predefined settings in SmartRF Studio 7 as a starting point for your settings.

    If you haven't already, I recommend looking at SWRU295E (CC112X/CC1175 Low-Power High Performance Sub-1 GHz RF Transceivers/Transmitter User’s Guide): https://www.ti.com/lit/ug/swru295e/swru295e.pdf for a more detailed description of the different parameters that can affect this. Section 6.1 discusses RX Filter Bandwidth.

    - Which filter is best for performing demodulation of a CC1020 2-FSK 9600 baud signal (Gaussian, Root Raised Cosine, Root Cosine) ?

    The chip only directly supports Gaussian filtering; this has no impact on performance compared to FSK so we recommend using GFSK over FSK for your modulation format.

    From Section 5.10.1 - FSK Modulation Formats of the CC1020 Low-Power RF Transceiver for Narrowband Systems datasheet (Rev. I): https://www.ti.com/lit/ds/symlink/cc1020.pdf : GFSK is recommended for narrowband operation.

    For more details on this, please see the linked datasheet.

    The CC1120 User's Guide also gives some additional information: In ‘true’ 2-FSK systems with abrupt frequency shifting, the spectrum is inherently broad. By making the frequency shift ‘softer’, the spectrum can be made significantly narrower. Thus, higher symbol rates can be transmitted in the same bandwidth using GFSK.

    Regards,

    Zack

  • 0 in reply to ZC
    Prodigy 100 points

    Hi Zack,

    I have a signal that has a 99% signal bandwidth of 19.6 kHz. My RX filter bandwidth is 30 kHz.

    Assuming that my signal is within the bandwidth of my antenna and that my crystal is exactly at the nominal frequency, how far off can the center frequency of my receiver be from the center frequency of my received signal?

    Based on my current understanding, as long the receiver captures 99% of the signal bandwidth, I should be good. Given that my RX filter bandwidth is 30 kHz, I expect to be able to comfortably receive my signal given that 19.6 kHz fits within the 30 kHz bandwidth.

  • 0 in reply to cc1120user
    Prodigy 100 points

    Asked another way, will I still be able to successfully demodulate my signal as long as my 99% received signal bandwidth falls somewhere within the 30 kHz bandwidth? (Whether the high or the low part of the bandwidth?)

  • 0 in reply to cc1120user
    TI__Genius 12395 points

    Hi,

    Thank you for clarifying.

    Assuming no crystal drift, then yes you should be able to successfully demodulate your signal.

    However, taking the frequency error of the transmitter and receiver into account (which should be done), you can calculate your required RX BW using:

    RX BW > Signal Bandwidth + (4 * XTALppm * RF Frequency of Operation)

    SWRA122D (CC11xx Sensitivity Versus Frequency Offset and Crystal Accuracy) may be helpful if you need more clarification on this: https://www.ti.com/lit/an/swra122d/swra122d.pdf

    Entering your desired settings into SmartRF Studio 7 will check if your wanted combination of Symbol Rate and RX BW is possible.