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TRF3765: Getting phase-coherent behavior of multiple frequencies out of TRF3765

Danny R.
Prodigy 75 points
Part Number: TRF3765
Other Parts Discussed in Thread: LMX2572, , LMX2594, LMX2582, LMX2820

Hello, we're aiming to get multiple carrier sine-frequencies on multiple channel, but these meet every N*Ts time again. Here is a graph, what exactly the goal is:

We got ourself fractional PLLs (LMX2572) and it went terribly. We think the dividers were responsible for not getting a clean in-phase behavior.
The desired operating frequency is the ISM Band between 2.4 GHz and 2.5 GHz

P.S. we have a clean clock distribution to all PLLs with matched impedance and length-measured routing on the PCB.

We're thinking now about taking the TRF3765, since this is Integer-based PLL and the only dividers are:

1. Two frequency dividers in the forward path to LO: Reference & RF divider. 1/2/4/8
2. In the closed-loop three additional dividers appear: Additional RF, prescalar & NF divider.

Is the phase-coherent behavior guaranteed, if we use multiple TRF3765s, that have different frequencies on different channels like shown before in the picture?
Can you still guarantee the phase alignment for such a setup?

I also checked a second part (LMX2594): This is similar to the LMX2572, but on the EVM board, you're showing a phase-sync behavior for multiple channels, that have the same frequency.
Could you maybe help us with our part choice?

Edit: I found also the LMX2582. This might also be the most suitable, and most likely the most affordable, part to use. If we keep R-Divider and Post-R-Divider at 1 and only used the Output Dividers (Prescalar and N-Divider) for the calibration, we should be fine with the phase-sync.

  • 0
    TI__Guru* 78744 points

    Hi,

    I was going to make the same suggestion for the TRF3765 where you keep the R-divider = 1 (marker 1) to ensure the PFD (marker 2) going to the various TRF3765 are the same phase to ensure the output phase of the PLL to be the same (marker 3). However, you will also need to ensure the output divider to be set to /1 only as setting to other divider without a synchronization circuit will result in varying phase through the divider logic. The last variable is the output driver (4) which may have temperature and phase variations from one device to another. 

    I am assigning this to CTS team to cover the LMX2582. 

  • 0 in reply to Kang Hsia
    TI__Mastermind 18095 points

    Danny:

    On the TRF3765, there is no option to synchronize the output dividers across multiple devices.  In your case, it looks like the frequencies are binary multiples of an initial frequency.   If the frequency is always in the fundamental range (i.e. 2.4 - 4.8 GHz), then this is not an issue and the (same) output channel between devices will be properly synced.  In this scenario it would only allow frequencies of 2.4 and 4.8 GHz.  Further note the output of each of the four channels are not phase synced to each other; there is a fixed delay between each output.

    It seems like one of the LMX parts which include a phase sync pin would be a better choice.  A device like the LMX2594 (that you mentioned) is suitable as would be the LMX2820.  I do not think the LMX2582 has that feature.

    --RJH

  • 0 in reply to RJ Hopper
    Prodigy 75 points

    Hello, thanks for the quick answer.

    Won't Integer PLLs allows output synchronizations by nature because the output divider is in the loop? Is the PLL-Sync Feature needed in this case, if you are operating in the integer-mode?

    Why wouldn't it work with the LMX2572? It has an integer-mode and a Sync-feature as well. We want to use this for mutliple channels (8+), so we are also checking for a good priced PLL.

  • 0 in reply to Danny R.
    TI__Mastermind 18095 points

    Danny:

    The output divider is not "in the loop".  The PLL loop does have its dividers (i.e. R-div and N-div) for properly locking the reference frequency to the desired VCO (fundamental) frequency.  Once locked, the output dividers provide additional range outside the fundamental VCO range to get to lower frequencies.  These output dividers need to be synced to ensure the phase is properly aligned to multiple outputs.  If you are not using the output dividers (i.e. only using the VCO within its fundamental range), then I agree that no "syncing" is required.  Yes, the LMX2572 does have a sync feature so that can also be a suitable choice.

    --RJH