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CC1120 Zero-IF setup

John Mach
Prodigy 50 points
Other Parts Discussed in Thread: CC1120, CC1120EMK-868-915

Hi. Are there any other details available on setting up the CC1120 for Zero-IF operation beyond what is contained in sections 5 and 9 of the User Manual (SWRU295, 30.06.2011)?


We would like to be running the transceiver following mode: 200 kbps 4GFSK, 83 kHz deviation (outer symbols), 200 kHz channel filter, zero IF.

Thanks,
John.

  • 0
    TI__Guru**** 317180 points

    If you use SmartRF Studio, a 200kbps, 4GFSK, 83kHz deviation case is defined in the "Typical Settings" section. From Studio you will get all register settings you need. Do you need any information other than this?

  • 0 in reply to TER
    Prodigy 50 points

    Thank you for the reply. I’m trying to do blocking and selectivity tests around 915MHz. I’m starting with the Smart RF Studio 7 (v 1.5.0) default settings at 950MHz (PG2.1,950MHz, Bite Rate: 200kbps, 4-GFSK, max throughput). I then simply adjust the carrier frequency to the 915MHz range. The FREQ_IF_CFG register is indeed set to 0x00 indicating Zero-IF configuration. DCFILT_CFG is set to 0x04 (dcfilt_freeze_coeff=0x00, dcfilt_bw_settle=0x00, dcfilt_bw=0x04).

    I have a CC1120EMK-868-915 jammer setup at 915MHz. I have a pair of TrxEB/CC1120EMK-868-915 setup for transmission/reception where I vary the carrier on either side of the jammer.

    I am running the test at two different frequencies: 915.429688MHz and 914.570312MHz. In each case, I’m getting around 33db of blocking from the jammer before encountering a significant percentage of errors. Although not specified at 915MHz, I’m expecting close to 40dB blocking based on the CC120 datasheet specs at 868MHz and 950MHz.

    Can you please provide some insight into this?

  • 0 in reply to John Mach
    TI__Guru**** 317180 points

    I did a quick measurement in the lab on a EM on the two frequencies you are measuring on with the same settings and got 41dB on one side and 44dB blocking on the other side. The measurements I did was done with the wanted signal 3dB above the sensitivity limit.

    How do you measure the signal levels in your setup and which signal levels do you use? Is this conducted measurements?

  • 0 in reply to TER
    Prodigy 50 points

    Hi. Thanks for the response. I did some initial testing with everything connected with coaxial cables and coaxial attenuators.

    The intended transmitter was set to a 14dBm output. I put 60dB of fixed attenuation off the EVB sma connector and fed this to a power combiner (mini circuits ZFSC-2-5)

    The jammer transmitter was also set to a 14dBm output. I had a variable attenuator off the EVB sma connector and fed this to the other side of the power combiner. Our jammer uses the CC1120EMK that is being controlled by a Microchip PIC32 controller. It is sending about 3ms of data every 5ms (60% duty cycle).

    I had another 20dB of fixed attenuation from the output of the combiner to the input of the receiver board.

    In both the high and low side cases, 27dB of attenuation was the threshold for a significant packet error rate (>5%) 60dB-27dB=33dB.

     

    We did some subsequent testing at a local RF lab where we could measure and record actual data. We first determined the sensitivity limit as you did and increased the intended transmitter signal by 3dB.

    We varied the jammer attenuation until we saw significant packet error rates (>5%). Our results were very similar to the initial testing. 32.84dB below the jammer, 33.03 above the jammer. Plots are included below.

     

    sensitivity limit +3dB at 914.6MHz

      

    Intended transmitter below the jammer. 32.84dB rejection

     

    sensitivity limit +3dB at 915.4MHz

     

     Intended transmitter above the jammer. 33.03dB rejection.

     

     

    Noise floor.

     

     

    Please let me know your thoughts.

    Thanks,

    John.

     

  • 0 in reply to John Mach
    TI__Guru**** 317180 points

    The measurements I did was with a unmodulated jammer.

    Are you sending with the same datarate both on the transmitter and the jammer? If so, try with an unmodulated jammer and see if you get closer to my numbers.

  • 0 in reply to TER
    Prodigy 50 points

    Thank you for your comments. Yes, the jammer datarate, modulation and related settings were the same as the transmitter/receiver pair. We are most concerned with modulated jammers. Our goal is to have co-located systems and adjacent channel jamming is an issue.

    One last question on this topic. What are your thoughts on the spectral profile of the jammer? There are three distinct peaks, whereas the intended transmitter profile is relatively flat.

    Thanks again for your help!

    John.

     

  • 0 in reply to John Mach
    TI__Guru**** 317180 points

    I did a quick test with two EMs set on the frequencies and settings you are using, attached the two outputs to a splitter and then the output to a spectrum. If I used cont. TX, random data on both EMs and using max hold on the spectrum both the main signal and the jammer spectral profile looked equal. In my setup I sat the signal to send with 10dBm and the jammer with -15.5dBm. 

  • 0
    Prodigy 10 points

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