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LMX2594: Modulating Reference Input

David Langan
Intellectual 270 points
Part Number: LMX2594
Other Parts Discussed in Thread: LMX2492,

The LMX2494 is potentially an excellent alternative to the LMX2492 + external VCO.  However, it does not have all of the modulation capabilities present in the LMX2492.  My question is:  How feasible would it be to modulate the reference input to the PLL as an alternative to using the internal modulation circuitry?

For example,  a LFM chirp from say 150 to 200 MHz could be generated with an FPGA + DAC and then passed to the LMX2494 reference input.  The LMX2494 would then act as a frequency multiplier.  A value of 50 would theoretically result in an output sweep from 7500 MHz to 10,000 MHz.  Modulating on the FPGA would provide a great deal more flexibility and perhaps FSK and PSK functionality could be implemented as well. I'm curious to know if TI designers believe there would be any impediments to such a scheme.  I would be interested in sweep rates on the order of 5 MHz/us at the RF output or equivalently 0.1 MHz/us at the reference input.

Many Thanks!

David

  • 0
    TI__Expert 5370 points
    Dear David,

    this is an excellent question. There are a few thing to consider:

    1) Modulating the reference can be done. In order for the PLL to "follow" the reference you will need enough loop bandwdith on the PLL.
    2) While LMX2594 can produce ramp, it has to re-calibrate along the way because the tuning range of a calibrated VCO is about 60-70 MHz. The ramping function controls that however you would not be able to do that with the modulated reference which is not practical. You could do it if modulating less than 60-70 MHz.
    3) You would need a clean clock for the DAC to produce a low phase noise as the reference noise would multiply up inside the lop badnwdith.

    Hope this helps,
    Regards, Simon.
  • 0 in reply to Simon Damphousse
    Intellectual 270 points
    I really appreciate this response. The DAC would definitely need to be clean to avoid phase noise issues after multiplication. Can you elaborate a bit more with regards to the tuning? I was under the impression that the VCO had a range of 7500 to 15000 MHz. If the internal VCO is truly limited to ~60 MHz could I perhaps employ the scheme using the LMX2492 and an external VCO with wider bandwidth?

    David
  • 0 in reply to David Langan
    TI__Expert 5370 points
    Dear David,

    there are 7 VCO on this device and each of them has a corse tuning. Before the PLL can be engaged, The calibration (consistuing of VCO selection corse tuning etc) must be done and this prevent long and accurate chirp requied in FMCW applications. As for using LMX2594 with an external VCo, it is not possible on this device.

    Regards, Simon.