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Correct parts to change to get TSW38J84 to use external reference.

Gordon Hardman1
Intellectual 540 points
Other Parts Discussed in Thread: LMK04828, TRF3722, DAC38J84

I am using the TSW38J84 with the TSW14J56. I want to set it up to use an external reference, option 3 in the data sheet:

"3. Clock Generator using External Reference
The LMK04828 is used a clock generator using an external reference provided to the CLKIN SMA
connector (J17). For this option, R177, C206, and C121 should be uninstalled and R185, R186, and
C92 should be installed."

This is from page 15 of "SLAU593A–November 2014–Revised December 2014". I have the EVM schematic, it is for board rev B, and dated April 3, 2014. The board itself is marked rev B, 2014. I also have the assembly drawing for rev B. From what I can see, it appears to be from 2013.

On the schematic, I can find R177. I have not found it on the board. On the assembly drawing, doing a search for R177 produces "No matches found". The same is true for C206 and C121, except I have found C206 on the board. The silk-screen of the assembly drawing appears to be missing C206.

Searching for R185 and R186 on the schematic results in no matches as well. It does find C92. Searching on the assembly drawing produces no matches for R185, R186 or C92.

All the dates on the documents appear to be from 2014, except for the assembly drawing, which is earlier, but unzipped from the same package as the schematic. But I really can't figure out from them how to set up to use this option. It appears to be a disagreement (or multiple ones) between the various documents and the board. If there is a concise set of notes on how do do this, it would help. Especially if it had a drawing showing the parts!

I did not find anything on the forum that seemed to address this.

  • 0
    TI__Guru* 78124 points

    Gordon,

    let me look into this.

    -Kang

  • 0
    TI__Guru* 78124 points

    Gordon,

    The TSW38J84 User's Guide had typos regarding the R185 and R186. These two resistors should be R23 and R24. I have marked the correction on the attached user's guide page. I have also noted the locations of the resistors/capacitors that need to be changed on the schematic.

    TSW38J84 External Reference for PLL2.pdf

    -Kang

  • 0 in reply to Kang Hsia
    Prodigy 100 points
    Kang,

    Thanks. I will make these changes tomorrow and test and let you know!

    Gordon
  • 0 in reply to Gordon Hardman45
    Prodigy 100 points
    Kang,

    That seems to work. I connected a signal generator at 122.8MHz and +7dBm, and the output spectrum looked identical. I am wondering how to set the GUI for different data rates. I am trying to get the DAC to run at 216MSPS.

    I set EVM Clocking Mode to "External Clock) or I can't get any other frequencies.

    I set TRF3722 Ref Frequency also to 108.

    Suggested FPGA clock is 108 MHz.

    I set HSDC Pro to 216M.

    I get no error, and I see a small carrier at 999 MHz (I am trying to run near 1GHz).

    But I see no data spectrum.

    I think I must still have something wrong?
  • 0 in reply to Gordon Hardman45
    TI__Guru* 78124 points

    Gordon,

    If the TSW38J84 EVM is configured as "External Clock mode", the LMK04828 is operating as a clock distribution circuit. You will need to provide a reference clock that is equal to the DAC38J84's final DAC sample rate (device clock after interpolation stages).

    For instance, if the data rate for each input channel is 108MSPS, with 2x interpolation, the final DAC sample rate is 216MSPS. You will need to provide a 216MHz clock to the J17 connector.

    HSDC PRO input data rate should be set as 108MSPS (before interpolation). The interpolation is done at the DAC side. Picture below is from the DAC3xJ8x GUI, but the same applies to the TSW38J84 GUI. 

    The TRF3722's reference input comes from the LMK04828's OSCOUT port. By default, the OSCOUT is a copy of the OSCIN, which is the 122.88MHz VCXO on the board. To set it to 108MHz, you may set the OSCOUT to output a copy from DCLKOUT6

    Be sure to set DCLKOUT6 to /2 mode to provide 216MHz/6 = 108MHz clock to the TRF3722.