Post Go back to editing

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.

Parents
  • Hello,

    It now appears that you are capturing data correctly.  

    Referring to figure 67 of the AD9864 Rev A datasheet,  the BW=10 KHz curve suggests that you should achieve an SNR-dBFS of around 94.1 dBFS when the VGA attenuation is set to 0 dB.  Note that this assumes no contribution from any prior driver stage in front of the AD9864.   The "IF clip point" (being -2 dBFS) is specified as nominal -31 dBm hence -29 dBm corresponds to 0 dBFS full-scale.   Hence, for a -37 dBm signal, I would expect a high SNR-dBc around 86.1 dBc (=94.1 dBFS - 8 dBFS) assuming phase noise is not an issue.   Reducing the signal to -90 dBm corresponds to a reduction of 53 dB hence one would expect the SNR-dBc to be around 33.1 dBc.

    Unfortunately I do not have script to extract I and Q data so difficult to generate FFT plot with measured SNR with Visual Analog software tool we use.  Feel free to resend data in I + j Q format and I could measure SNR and observe FFT spectrum.

    Answer to other questions are as follows:

    1) Not sure why ATTEN=0x29 when input signal is -70 dBm with or without AGC since this falls well below the AGC threshold set by the AGCR field in 0x06.  Note that an  If you disable AGC by setting 0x06 to 0x00.......you should also set AGCG setting in 0x04 and 0x03 to "0" so no attenuation is applied to input signals (when testing with signals below IF clip point of -31 dBm).   I would expect that the ATTEN field in the SSI data stream would reflect the settings in the AGCG field (0x06) when AGC is disabled.  Note.............the RSSI field  looks correct since it has insufficient resolution (being 6-bits) to detect signals that low.  I would expect the RSSI field to start changing "0" value around -65 dBm signal level (with ATTEN=0x00).

    2) My advice is to not to use the DVGA function since it really has minimal benefit. Only under large signal conditions when AGC causes ATTEN to increase from 0 to 12 dB will one see a slight improvement in SNR...........BUT one could argue that one will have more than sufficient SNR performance to demodulate a "large desired signal" or in case if it is narrowband blocker at 25 KHz offset.................the phase noise of blocker will dominate over thermal noise hence no real benefit.  Lastly, thermal of the RF-to-IF front end will likely be several dB higher than the AD9864's NF.   Perhaps best to operate with 0x06=0x01 instead of 0x81.

    Regards.

Reply
  • Hello,

    It now appears that you are capturing data correctly.  

    Referring to figure 67 of the AD9864 Rev A datasheet,  the BW=10 KHz curve suggests that you should achieve an SNR-dBFS of around 94.1 dBFS when the VGA attenuation is set to 0 dB.  Note that this assumes no contribution from any prior driver stage in front of the AD9864.   The "IF clip point" (being -2 dBFS) is specified as nominal -31 dBm hence -29 dBm corresponds to 0 dBFS full-scale.   Hence, for a -37 dBm signal, I would expect a high SNR-dBc around 86.1 dBc (=94.1 dBFS - 8 dBFS) assuming phase noise is not an issue.   Reducing the signal to -90 dBm corresponds to a reduction of 53 dB hence one would expect the SNR-dBc to be around 33.1 dBc.

    Unfortunately I do not have script to extract I and Q data so difficult to generate FFT plot with measured SNR with Visual Analog software tool we use.  Feel free to resend data in I + j Q format and I could measure SNR and observe FFT spectrum.

    Answer to other questions are as follows:

    1) Not sure why ATTEN=0x29 when input signal is -70 dBm with or without AGC since this falls well below the AGC threshold set by the AGCR field in 0x06.  Note that an  If you disable AGC by setting 0x06 to 0x00.......you should also set AGCG setting in 0x04 and 0x03 to "0" so no attenuation is applied to input signals (when testing with signals below IF clip point of -31 dBm).   I would expect that the ATTEN field in the SSI data stream would reflect the settings in the AGCG field (0x06) when AGC is disabled.  Note.............the RSSI field  looks correct since it has insufficient resolution (being 6-bits) to detect signals that low.  I would expect the RSSI field to start changing "0" value around -65 dBm signal level (with ATTEN=0x00).

    2) My advice is to not to use the DVGA function since it really has minimal benefit. Only under large signal conditions when AGC causes ATTEN to increase from 0 to 12 dB will one see a slight improvement in SNR...........BUT one could argue that one will have more than sufficient SNR performance to demodulate a "large desired signal" or in case if it is narrowband blocker at 25 KHz offset.................the phase noise of blocker will dominate over thermal noise hence no real benefit.  Lastly, thermal of the RF-to-IF front end will likely be several dB higher than the AD9864's NF.   Perhaps best to operate with 0x06=0x01 instead of 0x81.

    Regards.

Children
No Data