DDS AD9953 output is too low

Hi,

I using AD9953 for a single tone generator.

My circuit is working when i changed clock source from singgle ended PLL to x-tal oscillator.

But output is too low, the output level is -52dBm with -75dBm white noise.

My expectaion is that over -10dBm which is shown in datasheet.

My configuration of resister is that. CFR1=0x00 00 02 00, CFR2=0x00 02 30.

How can i debug my circuit to increase output level?

best regards,

Ryu.

  • I can see on your schematic something wrong. The quartz crystal is connected to two capacitors C153 and C139. Depending on the quartz impedance and internal circuitry of the chip, the correct values are about some pf to some 10 pF. The indicated value, 0.1uF is surely far too high to get a proper oscillation.

    But I cannot say if there is anything else wrong ...

    Cordially

  • Hi,

    I'm missed the value of capacitors.

    The actual value of C153 and C139 is 18pF and C173 is opend.

    The SINK_OUT and CRYSTAL_OUT are fine.

    But Output is still too low. it is only -52dBm.

    best regards,

    Ryu.

  • Hi, Two things to check on.

    1) For Crystal Mode operation pin 11 should be pulled high. Note pin 11 (CLKMODESELECT) is an 1.8V logic input, not 3.3V like the other digital inputs.

    2)Make sure the pulse width of the IO_UPDATE is wider than the SYNC_CLK. If not, it want program the desired value in.

    Note, the SPI ports is used to program data into a bufferred register. The IO_UPDATE transfers the data from the buffer register to the active register.

  • Hi,

    I pulled up CLKMODESELECT and checked the pulse width of IO_Update.

    There is no problem.

    The device is controlled by serial command and frequency is changed by command.

    But still out put level is too low which is under -52dBm.

    best regards,

    Ryu.

  • Hello,

    Per the postings and schematic, I'm not sure why the output is at such a low level.

    Possible causes:

    1) Cold solder joint possibly at the DAC output pins or AVDD pins or PWR down pin.

    2) Transformer output shorted to ground.

    3) Possible bad AD9953 part or transformer.