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Help selecting PLL/synth for VHF transmitter

Steve Palmer
Intellectual 365 points
Other Parts Discussed in Thread: LMX2571, LMX2541, LMX2581

I'll looking for a PLL/synth that I can use to generate 140-180 Mhz output with 2.5 Khz steps.  To help keep spurs low, I think I should use a non-fractional PLL (or avoid N-Fractions in one).  Can someone help guide me to candidate parts?  I only need one VCO or output and plan to drive with VCTCXO.

Thanks very much.

  • 0
    TI__Mastermind 28001 points
    Steve,
    I would recommend the LMX2571. This part can easily generate these frequency range of 140-180 MHz.
    In regards to fractionality, there is a misconception that fractional spurs are worse than integer spurs. In truth, people demand much more of fractional PLLs by driving narrrow channel spacing, as is your 2.5 kHz.
    The LMX2571 has a VCO that is around 5 GHz, so this helps with spurs because your channel spacing at the VCO will be much wider and the spurs will be easier to filter. It is a fractional part, but as for all our devices you can always shut the fractionality off and run in integer mode if you so choose. Including hte PLL and VCO, the current consumption is 39 mA. It has an excellent PLL figure of merit of -231. We do have other synthesizers like the LMX2541 nad LMX2581 as well that can also achieve this frequeny range.
  • 0 in reply to Dean Banerjee
    Intellectual 365 points
    Thanks for your guidance. We've been looking at the LMX2571. Very cool PLL with the FAST SPI for powerful modulation!
    Is there an easy way to estimate spurs? I need to be about -60 dB below the output. I plan to use this to drive an RF PA for a small transmitter. NBFM modulation will be provided by a VCTCXO.
    I contacted support about a week ago regarding the EVM and the software to drive it. I guess it's "coming soon." Can you shed light on this?
    Thanks!