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CC1120: CC1120 4GFSK Whitening option Question

Chanho No
Intellectual 790 points
Part Number: CC1120

Hello,

I have tested with CC1120DK (TrxEB+CC1120EMK).

 - SmartRF Studio Setting : Generic 434Mhz / Bit rate:1.2kbps, 4-GFSK, RX BW : 25 kHz (434)

I have only changed 2-FSK to 4-GFSK on default setting.

If the specific repeated characters('aaaaaaaaaaaaaaaaaaaaaaaa') trasmited, the CRC error was occured.

1. Random data trasmit --> CRC is OK

2. Text  data('aaaaaaaaaaaaaaaaaaaaaaaa') trasmit --> CRC error

3. Text  data('aaaaaaaaaaaaaaaaaaaaaaaa')  with Whitining option trasmit --> CRC is OK.

If the modulation was set with 2-FSK or 2-GFSK, there was no problem.

But, if specific repeated characters('aaaaaaaaaaaaaaaaaaaaaaaa') was transmited with 4-FSK or 4GFSK modulation, the CRC error was occured.

Must we use Whitening option on 4GFSK modulation ?

Could you please explain cause of error  and about Whitening option?

Thanks.

  • 0
    TI__Mastermind 36375 points

    Chanho No,

    That is a nice test and it explains very nicely the reason for using data whitening in wireless transmission of data. What is going on here is that the receiver is trying to figure out if the transmitter is sending a "00", "01","10" or a "11" by looking for 4 distinct frequency components (using a frequency discriminator), but at the same time we are running a frequency error compensation algorithm that tries to reduce the frequency error between TX and RX to 0. This is done by looking for the "average" energy in the frequency domain. Now imaging that you are sending all "11" or in your case a highly repetitive pattern. The frequency error compensation circuit will see this an bias towards one side and start compensating the center frequency of the RX node. This compensation then results in the RX starting to make incorrect decision on the payload and you see that as errors.

    The datawhitening option is just a pseudo random number generator that is XOR'ed with the payload before it is sent out, this ensures randomness and therefore keeping the frequency compensation circuit from making mistakes. There is no drawback with using data whitening, there is no additional data being sent over the air. Its basically transparent to the upper layer protocols.

    There are two options,

    • Use datawhitening
    • disable the frequency compensation circuit after the syncword (I do not recommend this, but it is possible) 

    Regards,

    /TA