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CC1125: CC1125 receiver selectivity problem - ETSI 300 113 compliance issue

Johnny Wang
Prodigy 120 points
Part Number: CC1125


Hi All,

Met another issue with CC1125. I am using CC1125 as core device in a data link device and need to meet ETSI 300 113 spec. But the Rx selectivity is way lower than spec. Below is the configuration.

Modulation: 4GFSK

Symbol rate: 7.6K

Modulation Dev: 3.4K

this configuration is used for 25KHz channel that requires 70dBc adjacent channel selectivity for receiver per ETSI 300 113 standard, but what I measured is only 52dB. The circuitry has no external crystal or saw filter as channel filter in that impossible to find an appropriate 25KHz filter at UHF or 800MHz band. The selectivity is mostly due to the receiver backend CC1125 so some questions raised as below:

According to CC1125 datasheet, I found that it seems can only achieve 57dB selectivity.

Question is:

1. If the CC1125 is been limited internally on the selectivity performance, due to internal ADC resolution or digital filter limitations.

2. Is there any method to improve it ?

Used to meet some Tx transient spurious issue on the CC1125 and now we are little struggling on the receiver selectivity. If CC1125 cannot meet 70dB spec then we have to give it up and moving to new candidates or new receiver architecture. 

Any one has same experience or any idea may help.

Thanks

  • 0
    TI__Mastermind 39860 points
    CC1125 will not pass EN 300 113.

    In the past I have managed to get CC1125 to pass selectivity and blocking requirements for operation in the 400 MHz band using 12.5 kHz channels (1.2 kbps data rate, +/-1.2 kHz deviation, 5 kHz RX filter BW, 2GFSK modulation).
    EN 300 113 requirements:
    - Selectivity and blocking should be measured with a wanted signal of -107 dBm
    - Adjacent channel selectivity should be -47 dBm (or higher). This is for +/-12.5 kHz offset
    - Blocking should be -23 dBm (or higher). This is for 1, 2, 5, and 10 MHz offset.
    The problem, however, is the intermodulation response, where CC1125 was approx 12 dB short of the requirements.

    At 868 MHz the phase noise will be higher than at 433 MHz so this will degrade selectivity and blocking
    In the measurements done at 4xx MHz the alternate channel rejection (i.e 25 kHz offset) was -43 dBm. For a 25 kHz channelized system the requirement is -37 dBm (or higher)
    4GFSK requires higher SNR than 2GFSK and this will affect selectivity
  • 0 in reply to Sverre
    Prodigy 120 points
    Ok, thanks a lot. So it looks like I have to find another transceiver to replace the CC1125 then.