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WL1837MOD: Burst Power too low in CCK and OFDM mode

Oliver Ge
Prodigy 100 points
Part Number: WL1837MOD
Other Parts Discussed in Thread: WL1837

I am using CMW500 to measure TX power in signaling mode. The burst power for DSSS 1Mbps is about 16dBm as expected, however the burst power in CCK mode is only 8.5dBm for all channels. Any reason what'd cause this issue? I am using the FCC_CE_JP ini file.

  • 0
    Prodigy 100 points

    Making a correction here:  It is at OFDM mode (all data rates) instead of CCK I am seeing 8.5dBm power. Sorry. Settings in ini is as following

    PwrLimitReference11ABG      = 55 

    PerChanPwrLimitArr11ABG = 78 88 88 78 88 88 78 88 88 78 88 88 78 88 18 78 88 88 78 88 82 78 88 88 78 88 88 78 88 88 78 88 88 78 88 88 78 88 88 FF FF FF F8 FF FF F8 FF FF F8 FF FF F8 FF FF FF FF FF FF FF FF FF FF FF FF FF FF F7 22 22 FF FF FF F7 22 22 FF FF FF F7 66 66 FF FF FF F7 66 66 F4 44 55 F4 44 55 F4 22 22 F4 22 22 F6 44 44 FF 44 44 F7 66 66 F7 66 66 F7 66 66 F7 66 66 F7 66 66 F7 66 66 F7 66 55 F7 44 55 F4 FF FF FF 33 22 FF 33 22 FF 66 66 FF 44 66 F6 FF FF FF FF FF

    PerChanBoMode11ABG              = 40 00 04 30 00 00 00 00 00 00 00 40 19

  • 0 in reply to Oliver Ge
    TI__Mastermind 35795 points
    Hi ,
    Are you getting desired results in non-signaling mode when you run calibrator tool ?
    processors.wiki.ti.com/.../WL18xx_TX_Testing

    Thanks
    Saurabh
  • 0 in reply to S
    Prodigy 100 points
    Hi Saurabh,

    Yes, my results in non-signaling mode were close to TI's datasheet. For example, 13dBm at 54Mbps for 802.11g.

    To elaborate my signaling measurement, bursts that are captured by CMW500 that match proper payload has low power, but the ACK frames have higher power.

    Thanks,
    Oliver
  • 0 in reply to Oliver Ge
    TI__Mastermind 35795 points
    Hi Oliver,
    In PLT mode ( non-signalling ) and normal mode, WL18xxMOD uses same configuration file i.e. power limits /channel / modulation group are the same - excluding any regulatory settings passed by Linux CRDA. Are you able to share more details of test on CMW500 ? How is it measuring power ? duty cycle ?, any plots that you can share ?

    Thanks
    Saurabh
  • 0 in reply to S
    Prodigy 100 points

    Hi Saurabh,

    I am having issues capturing frame at the correct data rate in CMW500 signaling mode. I am working with R&S on this.

    In the meantime, I did some more digging on PLT mode. It turned out the burst power is also a lot lower than expected. I have been measuring channel power using VSA. Is the TX power in WL1837MOD burst power or channel power? I attached a screen shot of burst power in the following PLT settings:

     calibrator tiw_sta0 wl18xx_plt tune_channel 6 0 0

    calibrator tiw_sta0 wl18xx_plt set_tx_power 17100 0 0 6 0 0 0 1 0 0 0 0

    calibrator tiw_sta0 wl18xx_plt start_tx 400 11 1000 0 0 1 0 0 00:11:22:33:44:55 11:22:33:44:55:66 0

    Thanks,

    Oliver

  • 0 in reply to Oliver Ge
    Prodigy 100 points

    Attached a few plots from CMW500, power & settings. The amount of symbols matches the payload length (1000). What I found in signaling mode is, ACK bursts have a lot higher power than the longer bursts with payload. So if we are talking about the peak envelope power that includes both ACK and bursts with payloads, then the power matches the datasheet better.

    Thanks,

    Oliver

  • 0 in reply to Oliver Ge
    TI__Genius 12900 points
    Hi Oliver,

    Glad you were able to solve the issue. I will close the thread for now. If you are still having an issue, feel free to re-open the thread. If the thread is locked, please open up a new one!

    Thanks,
    Riz
  • 0 in reply to Rizwan Murji
    Prodigy 100 points
    Hi Riz,

    Actually my last question has not been answered yet. For my power settings, is it normal (correct) that my burst power is around 8dBm, but peak envelop power being close to 16dBm? Is the TX power in WL1837 datasheet burst power or peak power?

    Thanks,
    Oliver
  • 0 in reply to Oliver Ge
    TI__Genius 12900 points
    Hi Oliver,

    I hope all is well. The numbers in the datasheet are average numbers, not peak numbers. For the ACK this is done at 1DSSS so we expect higher numbers for that. Depending on the modulation you are using 8dBm is low. For 54 OFDM this should be closer to the 12dBm mark.

    We had another customer will the same issue using the CMW500. The method they used to measure the power in active/signaling mode was as follows:

    1) Connect the WL1837MOD to the AP (conducted) and then couple the RF signal to the Litepoint.
    2) run iperf at different attenuation levels in order to land at different modulation rates.
    3) run the modulation rate outputs by iw dev wlan0 station dump. This will provide you the rate that is being used to match to the Litepoint.

    In doing so the output powers match that of what was done using calibrator in test mode. This indicates that there is something (setup or measurement related) that is being seeing with the CMW500. Unfortunately we do not have the CMW500, but validated the method above.

    Note that the Litepoint is an all in how measurement box that you can measure the burst power only (you can see the waveform/duty cycle so you can be sure of the measurement. You may also use a spectrum analyzer to do the measurement, but you will need to do duty cycle correction in order to get the burst power. Basically this is setting the spectrum analyzer to zero span and determining the duty cycle of the signal. For example if the duty cycle is 50%, then you would add 3dB to the measured data.

    I hope this helps.

    Thanks,
    Riz
  • 0 in reply to Rizwan Murji
    Prodigy 100 points

    Hi Riz,

    Thank you for the detailed explanation. This helps a lot. You are correct - I am seeing large variations in CMW500 burst power. I will couple the TX signal to a spectrum analyzer to verify the power and duty cycle.

    I will mark this as resolved. Appreciated your support!

    Oliver