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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.

  • Hi PMH,
    I have another question about the AD9864 RSSI. Is there a 24dBm constant gain inside AD9864? For example, I disable VGA and DVAG, also disable the 16dB attenuator, and input a signal with -50dBm. Then I log the SSI IQ data looks like below: (16bit I and 16bit Q, iterative)
    0000 04c6 0000 03ef 0000 040a 0000 04a6 0000 033f 0000 0543 0000 0254 0000 05b0 0000 0162 0000 05fb 0000 0067 0000 0624 0000 ff6b 0000 0624 0000 fe70 0000 05f7
    following the RSSI calculation formula: 10log(I^2+Q^2) = -26dbm. looks like there is a 24dB diff.
    I also tried on other input strength and get the similar result: the calculated RSSI is 24dbm higher than the input signal.
    I am wondering if this is caused by AD9864, but I can't find any description about this in document. Could you help to explain this? Thanks.
  • FormerMember
    0 FormerMember
on Apr 13, 2018 3:35 PM

Hello,

Difficult for me to decipher sequence.  

The AD9864 provides constant gain in the region where the AGC is not active.  When AGC is active (i.e. 12 dB ATTEN portion only), the AGC will reduce the internal analog gain by 12 dB during the AGC operating range. Once you run out of ATTEN range.........you should see a dB per dB increase in the FUND power (on FFT) as you increase the IF input.

If you are trying to estimate the IF absolute input power than you will need to consider the AGC setting  along with the FUND power in the FFT.  

Page 34 of datasheet 2nd Column

A linear estimate of the received signal strength is performed at
the output of the first decimation stage (DEC1) and output of
the DVGA (if enabled), as discussed in the AGC section. This
data is available as a 6-bit RSSI field within an SSI frame with 60
corresponding to a full-scale signal for a given AGC attenuation
setting. The RSSI field is updated at fCLK/60 and can be used
with the 8-bit attenuation field (or AGCG attenuation setting)
to determine the absolute signal strength. Note that the RSSI data
must be post filtered to remove the ac ripple component that is
dependent on the frequency offset relative to the IF frequency.

Regards.

  • Hi PMH,

    'The AD9864 provides constant gain in the region where the AGC is not active.' AGC is disabled in my setting, AGCG is also set to "0", so do you know the exact value of the constant gain?

    BTW, with above setting, when input IF signal is about -40dbm, the ATTN and SSI value on SSI frame is about 0x0009; with -50dbm input, the value is 0x0002; with -60dbm input, the value is 0x0000. Do you know how to convert these value to the true IF input power?

    Thanks.

  • Hello,

    Difficult to specify the gain of the AD9864 (or any ADC) in terms of dB.  It is for this reason we specify a full-scale value in term of dBm which corresponds to a 0 dBFS input level.  In the case of the AD9864 (being an IF subsystem with mixe and sigma-delta ADC)............we refer to this as an IF Input Clip Point which is specified as being -2 dB below the 0 dBFS level.   Anyways, the 0 dBFS level for  Max and Min VGA attenuation is around  -17 and -29 dBm. 

    With a ATTN and SSI value of  about 0x0009...........................this would indicate that ATTEN value is 0 hence the 0 dBFS level is -9.   The max value that the SSI reading can achieve is 60 (out of 63 for 6-but word) and this value can only be achieved if you place an IF exactly at the IF center such that DC is read from I and Q while adjusting phase to reach value of 60.   Consider this the "peak power" which is 3 dB higher than the rms power of a CW waveform.   If we normalize to a CW waveform slightly offset from the IF center than max rms value is 42.     A value of 9 out of 42 corresponds to -1 

    For an SSI reading of 9 out of 63...........this would indicate that the full-scale is 13.4 dBFS below -29 dBm..............or approximately -42 dBm.

    Regards.