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Required SNR of CC1120

Abdulhussain Songerwala
Prodigy 110 points
Other Parts Discussed in Thread: CC1120, CC1120EMK-420-470

I am using the CC1120 in my application where I'm using BFSK with frequency deviation of 9.6kHz, receiver bandwidth of 25kHz and baud rate 1.2kbps. I wanted to evaluate what is the required SNR for CC. To evaluate this I added AWGN noise via the Signal Generator. The settings for noise were 1.92kHz(1.6 times the baud rate as per generators manual) Flat Noise Bandwidth. Error free packets were received at carrier to noise ratio of 9dB. Is this correct or have I made some mistake in the settings? Ideally, what should the required SNR be and what is the accepted method of evaluation of the same?

  • 0
    TI__Mastermind 39860 points
    SNR is approximately 10 dB (depends on test case - modulation format, data rate, deviation,...). If you have to signal generators and a combiner you can inject a jammer at the wanted frequency which is 6 d dB (say) above the sensitivity limit and then increase the wanted signal until you measure 1% BER. The difference between the two is a measure of the SNR.
  • 0 in reply to Sverre
    Prodigy 110 points

    First of all thank you for your reply. We have a Signal Analyzer, Signal Generator and a Vector Network Analyzer. Can the VNA be used to add the noise and a T connector as a combiner? The power of noise should be kept 6dB above sensitivity of the overall reception link right?

  • 0 in reply to Abdulhussain Songerwala
    TI__Guru**** 317180 points
    Required SNR is a more a theoretical number. We normally measure sensitivity which is more useful for our customers. For what purpose are you trying to measure SNR?
  • 0 in reply to TER
    Prodigy 110 points
    I am trying to receive data from Digital Sats using Serial Synchronous Mode of CC1120. I have checked the received power on a Signal Analyzer and I am above the sensitivity level. Yet we are unable to decode the data correctly. Hence we were wondering if the SNR obtained is too low.
  • 0 in reply to Abdulhussain Songerwala
    TI__Guru**** 317180 points
    - Note that if you use serial sync mode 'out of the box' (default register settings) the chip will look for a sync word
    - How do you define 'unable to decode correctly'? Do you receive just garbage?
  • 0 in reply to TER
    Prodigy 110 points

    Yes we have changed the settings - preamble and sync word both are disabled. The packets we are trying to receive are in AX.25 format. There are several leading flags followed by data. We are unable to find more than one leading flag and no data is found either

  • 0 in reply to Abdulhussain Songerwala
    TI__Guru**** 317180 points
    If you try transparent mode, do you see the expected data?

    Serial sync mode works best if a sync word is used. Try to set this to some of the lead flags. (I'm not familiar with the ax.25 packet, if you have a link I can give you a more detailed suggestion)
  • 0 in reply to TER
    Prodigy 150 points
    Hello TER

    Generally Sensitivity of any Radio receiver is defined at a specific Signal to Noise Ratio, for example, my analog receiver has sensitivity of 0.11 uV at 10 dB SNR. Are there plots for Sensitivity at given SNR available for CC1120, or vice-versa?

    These are my CC1120 configurations:
    BFSK
    Freq: 437.025 MHz
    Deviation: 9.6 kHz
    Bit Rate: 1200 bps
    Receiver Bandwidth: 25 kHz

    The CC1120EMK-420-470 modules which I have, have been tested for their sensitivity without SNR considerations, which results in -117 dBm. However, the minimum required SNR at RX power of -80 dBm is found to be 17 dB, which also remains the same for -50 dBm RX power. Below this SNR, CC1120 is not able to decode.
  • 0 in reply to Vishal Desai
    TI__Guru**** 317180 points
    No, we don't have SNR plots. The required SNR will be dependent on a lot of different settings so it's fairly difficult to generate a generic table. How have you measured the SNR given -80 dBm at the input?
  • 0 in reply to TER
    Prodigy 150 points
    The digital packet was generated using Signal Generator, with Additive White Gaussian Noise Generation settings: Flat Noise Bandwidth 25 kHz (same as my CC channel bandwidth), and by setting Carrier to Noise Ratio Manually. The C/N Ratio was verified on Spectrum Analyzer.
  • 0 in reply to Vishal Desai
    TI__Guru**** 317180 points
    Ok,
    the result is as expected in the way that you need the same SNR independent of the RF signal level. Do you here define SNR as the distance from the carrier to the noisefloor? Note that is is the total amount of noise in the channel that is important. What is you goal with these measurements?
  • 0 in reply to TER
    Prodigy 150 points

    1. SNR in the instrument is defined as the signal power to the total noise power introduced in the channel. 

    2. Our Satellite system uses CC1120. We have designed our Link Budget in which we estimate the SNR present at CC1120. Earlier, we had considered SNR of CC1120 as 5 dB, with sensitivity of -117 dBm, as per settings posted here earlier. We wanted to verify if the minimum SNR required is indeed 5 dB.

    Please suggest me alternative tests for this, if at all AWGN method doesn't seem convincing.