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LMX2582: Performance Difference - Sweep Rate

Peter Baxter
Intellectual 400 points
Part Number: LMX2582
Other Parts Discussed in Thread: LMX2571, LMX2581, LMX2485E, LMX2572

I want to sweep a PLL VCO from  50MHz to 90MHz.

Instrumentation type application.

Frequency of Phase Detector probably 100MHz.

For Ref Frequency Input - I prefer to use 10MHz TXCO but will consider 20MHz crystal.

The LMX2571 appears to be quite suitable.

But is the LMX2581 family vastly superior?

Is there more popular part I should consider?

Does TI have sub 20MHz - 100MHz active mixers that go well with these parts?

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The LMX2571 data sheet suggests you can step from one frequency to the next in about 1.5ms.

Is this just switching time between two pre-set Frequencies.

Or is it the time that a new frequency can be entered, CALed and be stable.

So 1000 points could be swept in 1.5 seconds.

Thanks

  • 0
    TI__Guru** 111890 points
    Hi Peter,

    I believe you need a linear sweep between 50MHz and 90MHz.
    Depending on your system performance and software requirement, there are couples of choice:
    (1) LMX2485E + external VCO. With this solution, you need to continuously program the device to change the output frequency from one point to another.
    (2) LMX2571/2581/2582. Continue programming the device is required in order to sweep the output frequencies.
    (3) LMX2572. The device supports frequency ramping. That is, you define the start and stop frequencies, then it will ramp automatically without continue programming. Since the internal VCO is operated at 3200MHz to 6400MHz, multiple VCO calibrations are required during ramp. So you won't get a linear ramp, you will get a step-wise ramp. Please see datasheet section 8.1.7 for details.

    LMX2571 can switch (toggle) between two predefined frequencies very fast. The 1.5ms assumes external VCO was used and the loop bandwidth is 2kHz. If internal VCO is used, the switching time is in nano-second range.
  • 0 in reply to Noel Fung
    Intellectual 400 points
    Excellent Information - Thanks