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Why AD9864 output is noise when input IF signal is -90dBm

Our product is using AD9864, by out test, when the input IF signal with level of -90dBm directly into AD9864, and the DSP capture AD9864 SSI output IQ data then draw it with Matlab, the data is almost noise, we can't see valid signal. For -70dBm input or -30dBm input, the IQ data looks ok.

So, what's the possible reason for my problem, hardware or AD9864 configuration issue? Could anyone help me? Also I want to know what's the smallest input level that AD9864 can accept.

Here is my AD9864 register setting. And my IF input freq is 73.35MHz, LO freq is 71.1MHz.

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  • Hello,

    A few comments:

    1) Table 24 shows an example SPI sequence for your exact same conditions (excluding the decimation factor and SSI port setting).  Are you following the same step-by-step procedure?

    2) With regard to the SSI port settings, it seems like you are operating with 24-bit data and AGC data resulting in 64-bit frame.   With a decimation factor of 960 for CLK=18 MSPS, the Frame rate is 18.75 KSPS.  The minimum clock rate for SSI port must be greater than 1.2 MHz (=64 x 18.75 KSPS).   Your CLKOUT frequency is set to be also 1.2 MHz.

    Wondering if increasing CLKOUT to 1.5 MHz  (SSIORD=0xC) provides some extra margin for AD9864 to push out data within one frame.

    3)  If this does not work..............try using settings shown in Table 24 of datasheet for decimation factor and SSI port.
    If this does not work............can you provide us with a file consisting of the captured IQ data.

    4) The AD9864 has a very low NF (<9 dB) for narrowband signals below 10 KHz.   The integrated noise in 10 KHz would be -125 dBm hence device should be easily able to recover -90 dB input signal that has BW<10 KHz.

    Regards.

Reply
  • Hello,

    A few comments:

    1) Table 24 shows an example SPI sequence for your exact same conditions (excluding the decimation factor and SSI port setting).  Are you following the same step-by-step procedure?

    2) With regard to the SSI port settings, it seems like you are operating with 24-bit data and AGC data resulting in 64-bit frame.   With a decimation factor of 960 for CLK=18 MSPS, the Frame rate is 18.75 KSPS.  The minimum clock rate for SSI port must be greater than 1.2 MHz (=64 x 18.75 KSPS).   Your CLKOUT frequency is set to be also 1.2 MHz.

    Wondering if increasing CLKOUT to 1.5 MHz  (SSIORD=0xC) provides some extra margin for AD9864 to push out data within one frame.

    3)  If this does not work..............try using settings shown in Table 24 of datasheet for decimation factor and SSI port.
    If this does not work............can you provide us with a file consisting of the captured IQ data.

    4) The AD9864 has a very low NF (<9 dB) for narrowband signals below 10 KHz.   The integrated noise in 10 KHz would be -125 dBm hence device should be easily able to recover -90 dB input signal that has BW<10 KHz.

    Regards.

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