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LMX2541: Maximum Loop Bandwidth/Baud Rate?

Calvin Pitman
Prodigy 100 points

Part Number: LMX2541
Other Parts Discussed in Thread: LMX2594

Hi,

I'm working on using the LMX2541 for an FSK implementation. I am attempting to use a baud rate of 1 Mbps, so I have consequently made the bandwidth at about 2 times that, centered at about 978MHz. When I attempt to run it, there's output in the range of +-7MHz approximately, resulting in an output similar to that of a bandpass filter in that range. Is this because the LMX2541 is unable to run at that baud rate, or is there another reason for it? Is there a max baud rate? When we run the same code on an older revision of the board we are testing, it seems to run fine as long as we do not run it at a high baud rate.

Please let me know if there is any other information needed in order to properly answer this question.

Thanks!

  • 0
    TI__Mastermind 28031 points
    The LMX2594 divides the VCO frequency range into several different bands. I can't remember how wide each of these is, but +/- 7 MHz sounds pretty close to it. This is what I think could be what is limiting you.
  • 0 in reply to Dean Banerjee
    Prodigy 100 points

    Hi Dean, I'm actually using the LMX2541, not the LMX2594.  Is there a difference in the way that these parts work in this aspect? My bandwidth should be smaller than 14MHz, though, so I don't see how that would be limiting me.So should I be able to use a baud rate for FSK at speeds of 1 Mb/s,  as long as my filter is large enough? There is no absolute max for speed? 

  • 0 in reply to Calvin Pitman
    TI__Mastermind 28031 points

    THat was a typo, I meant LMX2541.

    What loop filter bandwidth are you using?  Also, with this device, it has an integrated 3rd and 4th pole.  Realise that these can restrict your loop bandwidth.   So hopefully your design tool accounts for this.   If you are looking for a tool, the TI PLLatinum Sim is an excellent tool for this.


    Regards,
    Dean

  • 0 in reply to Dean Banerjee
    Prodigy 100 points

    We are attempting to use a 2MHz bandwidth, in order to generate an FSK signal capable of transmitting at a frequency of 1Mb/s. Fcenter = 978MHz, Fdev = 312.5kHz.

    How should the TI PLLatinum tool be used for FSK? I can't seem to find any options for it? Should I just design for my center frequency and a 2 MHz loop bandwidth?

  • 0 in reply to Calvin Pitman
    TI__Mastermind 28031 points

    Calvin,

    Yes, you can use PLLatinum Sim.  However, for a 2 MHz loop bandwidth, be aware that what you get might be much lower.  PLLatinum Sim has  0 pF for the VCO input capacitance, but it could be higher.  We haven't characterized this, but it'ts probably 20 - 100 pF, not 0 pF.  So when you design a wide loop bandwidth, you could end up with capacitors that are much smaller than this and get swamped by this.

    PLLatinum Sim does model the impact of VCO input capacitance.

    Regards,

    Dean

  • 0 in reply to Dean Banerjee
    Prodigy 100 points

    Dean,

    Ok, I will have to look into that. The other problem is that when we have our 2MHz filter on the board, we are not able to get a clean output from the LMX2541. Before we even begin sending data to the DATA pin for shift keying right after initialization of the registers, the output, as I mentioned earlier, is very low power in the range of 970-985MHz approximately. However, when lowering the loop bandwidth down to 65kHz, the output is a clean signal at about 978 - Fdev initially but cannot handle the fast baud rate, causing the output to become a lump of frequency around the 978 area of frequencies. We initialize the ICs at the same data baud rate, 1Mb/s. Do you know what could be causing this weird output with the larger bandwidth?

  • 0 in reply to Dean Banerjee
    Prodigy 100 points
    You mentioned earlier that the frequency range of the VCO is divided into several bands around 14MHz wide. What might cause the output to fill an entire band? We aren't switching very far from frequency to frequency, only about 625kHz, so I don't see why we would be causing issues in a 14MHz band.

    Additionally, the modulator seems to shut off after being on for some time, usually, anywhere from 10 seconds to 1minute after initialization. What might cause that?
  • 0 in reply to Calvin Pitman
    TI__Mastermind 28031 points
    Calvin,
    The first thing that comes to mind is filter instability. If you are using the internal loop filter poles on the LMX2541, try switching these out and see if it helps. If there is a loop filter issue, then taking these poles away often fix stability ussues.

    Regards,
    Dean