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LMX2581 Minimum Oscillator Input Frequency

Other Parts Discussed in Thread: LMX2581, LMX2531

Hello,

I am attempting to drive the LMX2581 with a 1 MHz oscillator input frequency. The device does not seem to be able to use a frequency this low, the lowest frequency I can get it to work at is 3.5 MHz. I realize the datasheet states the minimum oscillator input is 5 MHz, however, I have used other devices in this family such as the LMX2531 and had no issue running it with a 450 KHz oscillator input frequency (provided I provide the proper clock slew rate).

Is there a way to get the LMX2581 to operate with a 1MHz oscillator input?

Thanks,

  • To further:

    When I apply a 1 MHz square wave to the oscillator input pin and configure the LD pin to output R/2 I get a clean 500 KHz output. Wouldn't this indicate that the R divider is not miscounting? Would this also not indicate that the input pin is not self oscillating?

    When I reconfigured the LD pin for N/4 I get a 126 KHz signal out. The phase detector frequency is set to 1 MHz, N count 3604, num 0, den 100, to give an expected output of 3604MHz. So I'd expect to see 901 MHz for N/4 on the LD pin...

  • For the Oscin pin, one issue with low frequency is slew rate, but maybe that's ok for a square wave.  However, I do wonder if maybe this may present some issues for the internal vco cakibration

    If so, there's not too many workarounds for that.

    Dean

  • Hi Dean

    Is there anyway to disable the vco calibration and manually set the vco based on the desired output frequency or any other possible work around to make the device function with a 1 MHz input?