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CC1120: CC112x: -129dB sensivity , howto ?

GGA
Genius 3985 points
Part Number: CC1120
Other Parts Discussed in Thread: CC1125

Using CC1120 for years now with up to 4kBit data rate, we never achieved to reach better sensivities than ~120dBm.

Even when I reduce data rate down to 300b, with DEV of 1kHz, I never got better deals than -123 (both 1120 & 1125)

Looking at the data sheets it says

1120: "300 bps with coding gain (using a PN spreading sequence with 4 chips per data bit) DEV = 4 kHz, CHF:10k  : –127dBm"
I didn't find out how to setup the chip accordingly, what parameters are to set / where in SmartRF do we find it?

1125: it just states -129dBm without any hint to spreading, does it mean that 1125 reaches -129 even without spreading?

Did you made any measurements you could share?

Thanks

  • 0
    TI__Guru**** 317180 points
    CC1125 allows a narrower RX BW than CC1120 which makes it possible to achieve -129 dBm using a very low datarate and RX BW without spreading. Note that to be able to achieve a stable system using a RX BW equal to 3.8 kHz a very accurate TCXO has to be used.

    DSSS PN mode is described in 5.2.5 in www.ti.com/.../swru295e.pdf and it should be possible to set this register in SmartRF Studio for test purposes.

    Not sure what you mean by measurements since the numbers in the datasheet are a result of measurements on 30+ devices.
  • 0 in reply to TER
    Genius 3985 points
    TER,
    thx. for quick reply.
    To understand it right:
    -129 can be achieved with CC1125 'out-of-the-box' without spreading etc.
    -127 can be achieved with CC1120 with spreading only?

    You mention 'very accurate TCXO': how accurate?
  • 0 in reply to GGA
    TI__Guru**** 317180 points
    Required RX BW/ required spec of the xtal can be calculated according to:
    Required RX filter bandwidth can be approximated as: Signal BW + 4*ppm xtal*Frequency of operation
  • 0 in reply to TER
    Genius 3985 points
    OK,
    sorry to insist on the other question:
    -129 can be achieved with CC1125 'out-of-the-box' without spreading etc.
    -127 can be achieved with CC1120 with spreading only?
  • 0 in reply to GGA
    TI__Guru**** 317180 points
    Yes, since the CC1125 can have a RX BW that is > 2x narrower than CC1120
  • 0 in reply to TER
    Genius 3985 points

    TER, a last one on that:

    a. is there a function that could give me the sensivity as f(RXBW)? Means e.g. starting at -129 for 3.8KHz going up with RXBW/going down in sensivity. Data sheet delivers some edge points only, not sure if I can interpolate.

    b. a function for "Deviation as f(Datarate)"

    c. sensivity as function of RXBW: can I transform your values to transparent legacy mode? Or is it limited to paket mode?

    thx

  • 0 in reply to GGA
    TI__Guru**** 317180 points
    change in sensitivity: log10(change in rx bw)

    Copy/ paste from an old E2E post:

    Theoretically, there is an optimum separation/datarate setting if you simultaneously minimize the receiver filter bandwidth. Every halving of receiver filter bandwidth increases sensitivity with 3 dB whereas sensitivity vs separation/datarate decreases with about 1.5-2.5 dB per halving down to a certain limit where the loss increases very fast. In our experience a modulation index (=separation/datarate) = 1 is a good design compromise.

    Using a mod index of less than 1 is not a problem (153 kHz separation and 153.6 kbps data rate)

    Assume the RX filter BW is fixed. The data rate is 100 kBaud
    With 50 kHz deviation (= 100 kHz separation) the mod index = 1. The sensitivity is -Y dBm
    With 25 kHz deviation (= 50 kHz separation) the mod index = 0.5. The sensitivity is (-Y + 1.5/2.5) dBm

    If you lower the modulation index (i.e. separation) much below 0.5 the sensitivity you will take a hit on sensitivity.

    To find the optimum separation/data rate setting vs RX filter BW for your application you also need to take into account the crystal tolerance and do some real measurements. I suggest you use the settings you get from SmartRF Studio if you are to operate at 153.6 kBaud.