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CC1101+CC1190: 26 or 27 MHz crystal?

Christian Mallas
Prodigy 20 points
Other Parts Discussed in Thread: CC1101, CC1190

Hi!

I'd like to use the CC1101+CC1190 (following the TI ref design) for an 868.3 MHz application (EU) and I'm confused about which crystal (26 or 27MHz) to use in order to conform with EMI requirements.

In the AN094, TI recommends 27MHz with a sensible explanation: The 864MHz spur of the 27MHz crystal is farther from 868.3MHz than the 871MHz spur of the 26MHz crystal. (As far as I understand, the critical value would be 867.5 MHz)

But in the CC1101 datasheet is written: "For compliance with [...] EN 300 220 [...] it is recommended to use a 26 MHz crystal for frequencies below 869 MHz and a 27 MHz crystal for frequencies above 869 MHz."

What is the explanation for the recommendation in the CC1101 datasheet and which is the best choice for CC1101+SAW filter+CC1190 @ 868.3MHz: a 26 or a 27 MHz crystal?

Thank you in advance

Christian Mallas

  • 0
    TI__Mastermind 39860 points

    The CC1101-CC1190 ref design was developed for operation in the 869.4 - 869.65 MHz frequency band since this allows up to +27 dBm  output power. When operating at 869.525 MHz (middle of the band), we recommend 27 MHz crystal. Spurs will then be at 864 MHz and 875.05 MHz.(26 MHz: spurs will be at 871 MHz and 868.05 MHz). The spurs will to some extent be attenuated by the narrowband SAW filter to comply with the -36 dBm limit.

    CC1101 general note: The N x crystal frequency/2 spur should be more than approx 2.5 MHz away from the carrier for the spur to be below -36 dBm.  The further away in Hz the spurs are from the carrier the lower they will be in amplitude. For CC1101 stand-alone and operation at 868.3 MHz you can use either 26 or 27 MHz crystal. We use 26 MHz in the reference design.

    For a CC1101-CC1190 design at 868.3 MHz (note: you are not allowed more than +14 dBm at this frequency) you should consider 27 MHz as this puts the spurs at 4.3 MHz offset whereas a 26 MHz crystal puts the spurs at an offset of 2.7 MHz.

  • 0 in reply to Sverre
    Prodigy 20 points

    Thank you for your prompt answer and your excellent support!