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LMX2541: FSK Output at Fcenter

Calvin Pitman
Prodigy 100 points
Part Number: LMX2541
Other Parts Discussed in Thread: LMX2571

I am using the LMX2541's FSK function to output a signal that is +-312.5kHz from 978MHz. I am attempting to use a bit rate of 1Mbps with a bandwidth of 1MHz. However, at that speed, there is a spike on our spectrum analyzer at 978MHz, which is Fcenter. When the bit rate is reduced to 500kbps, the spike at 978MHz is much smaller and the peaks at 978.3125MHz and 977.6875MHz are much clearer. Why is this spike appearing? How can we decrease its size while still using a 1Mbps bit rate? 

Thanks,

Calvin

  • 0
    TI__Mastermind 27831 points
    Sounds to me that either this is a measurement issue or the loop bandwidth is cutting off your modulation. Typically, the loop bandwidth sholud be twice the modulation rate. Also, ensure that your banwidth is really 1 MHz. Sometimes the added 3rd and 4th poles can pull in the bandwidth
  • 0 in reply to Dean Banerjee
    Prodigy 100 points

    Hi Dean,

    When we increase our bandwidth to 2MHz, the signal completely disappears. While using a 1MHz loop bandwidth, the signal is still there.  We are not using the 3rd and 4th poles, and these have been set to 0 in software. 

    Do you know what might be causing the signal to disappear when we increase our loop bandwidth?

  • 0 in reply to Dean Banerjee
    Prodigy 100 points

    We have now tried increasing the loop bandwidth to 2MHz and 3MHz. We are able to get a signal, but when we connect it to the spectrum analyzer, the signal is not very clean, as shown in the image below.

    Do you have any ideas for how we could clean up this signal? We would like there to only be peaks at 978+-312.5kHz. At lower FSK bit rates such as 500kHz, the signal is cleaner.

  • 0 in reply to Calvin Pitman
    TI__Mastermind 27831 points
    David,

    I think of it this way:
    If your loop bandwidth was 0 Hz, then would get just the center frequency.

    If your loop bandwidth was infinite, and we disregarded stability issues, you would get a spectrum with a center and two high spurs, very similar to a fractional spurs situation.

    For the LMX2541, you can not do FSK modulation that crosses an integer boundary. Are you doing this?

    Also consider the LMX2571. This has better PLL noise, lower current, and also supports higher level FSK.


    Regards,
    Dean