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WEBENCH® Tools/LMX2492: Need an FMCW PLL capable of operating from 300MHz through 700MHz.

James Canyon
Prodigy 110 points
Part Number: LMX2492
Other Parts Discussed in Thread: LMX2491

Tool/software: WEBENCH® Design Tools

Hello.

I am attempting to use the 2492 as a FMCW frequency source controlling a VCO in a saw tooth pattern from 300MHz to 700MHz.  The device is advertised to work from 500MHZ, but I am able to get it to work down to 400MHz.  Can you suggest an alternate synthesizer chip to use?  Do you think I could lock using the second harmonic?  ie 600MHz through 1400MHz?

Thanks very much for your help.

Jim

858 204 9422

  • 0
    TI__Guru** 108130 points
    Hi Jim,

    It is not safe to operate at below 500MHz, as we cannot guarantee it will work. Furthermore, we have a min. N divider requirement as shown in Section 8.3.4 in the datasheet, as a result, Fin cannot be too small.
    Simply using the 2nd harmonic may not work well. First of all, the signal strength may be too small for LMX2492. Secondly, the 2nd harmonic of 300MHz falls into the fundamental band, you cannot use a simple high pass filter to separate fundamental and harmonic signals. I think you may end up require a well design frequency doubler.
    Furthermore, what is the Kvco of your VCO? I think you need > 5V charge pump voltage to tune the VCO for 400MHz range. That means you need to use active loop filter.
    Currently we don't have a chip that can support automatic frequency ramping below 500MHz.
  • 0
    TI__Mastermind 28011 points
    Jim,

    There is significant margin on the lower spec and it accounts for process, voltage, and temperature variation, so it doesn't suprise me that it works at 300 MHz. That being said, the spec is 500 MHz.

    If you want this lower frequency, you might consider the LMX2491, which is a lower frequency version, although it also has a lower spec of 500 MHz. To meet the spec, I would consider using a higher frequency VCO with divide by 2. Send the higher frequency to our chip and send the divide by 2 to your chip. This actually could improve performance too. Some silicon VCOs include a divide by 2. The challenge will be to find a VCO that can tune such a broad range (more than a factor of 2).

    Regards,
    Dean