Undesirable inharmonious distortions AD6679

Hello! 

For a couple of months I'm testing the AD6679 ADC and there's a problem that can not be solved, maybe you'll help with this:

In the spectrum of the digitized signal there are undesirable inharmonic distortions that depend on the frequency and level of the main signal.For example 170.3 MHz signal: 

and it's analog from the datasheet:

And one more example 340.3 MHz signal:

These distortions are present throughout the spectrum of the signal and are not harmonic. Nothing is written about this in the datasheet. Therefore, I want to know the cause of these distortions and how to get rid of them.

Best regards Petrishchev Sergey.

  • Hi Petrishchev Sergey,

    Your situation sounds similar to an example given in the following Analog Dialog article http://www.analog.com/en/analog-dialogue/articles/whats-up-with-digital-downconverters-part-2.html

    Is it possible the unidentifiable spur might be a fundamental image, DC, or 2nd/3rd harmonic that is translating around in the spectrum and not being accounted for in your analysis?

    Please provide us with the exact sample rate, input frequency, and SPI configuration (specifically the DDC) and we can attempt to simulate your conditions in our lab.

    Best Regards,

    TonyM

     

  • Hi Tony!

    I read the article and unfortunately did not find it to help with this problem. According to my calculation, these distortions are not the fundamental image, DC, or 2nd / 3d. I'll try to describe the ADC settings in more detail:
    1) Sampling frequency 500 MHz
    2) The default mode is set (after reset via the INTERFACE_CONFIG A register) without any DDC.
    3) I change the position of the digital filter (NSR mode) reg.add.0x422 = 0x28
    5) The frequency of the input signal is 170.3 MHz

    The result of measuring is:

    Sorry for mistake on screenshot 2-nd harmonic translating from Fsample/2(250MHZ)

    And the second screenshot with FM with the same parametrs:

    as you can see Undisirable distrotion have the same band as original signal so that's why i think that this distortion is non harmonic

    Best regards

    Petrishchev Sergey

  • BTW one more question about this ADC:

    I tried to measure the amplitude-frequency characteristic of the ADC and was very disappointed. I completely eliminated the effect of the input path (in my case it's a LPF and amplifier) by connecting the ADC input as shown in the figure:

    I configured the ADC in default mode and set the input resistance to 50 ohms. The harmonic signal from the generator goes to the input of the ADC with a step of 5 MHz and a power of 0 dBm. Here is the result:

    And I would like to know what amplitude-frequency characteristic really has AD6679 because in the DATASHEET this data is absent.

  • Hi,

     Sorry for this laundry list of questions but these are the types of details we need to zero-in on your reported issues.

     

    1. How much do the spurious tones move when you change the input frequency in 1MHz steps?
    2. Please give exact frequencies.
    3. How much do the spurious tones move when you change the clock frequency in 1MHz steps?
    4. Please give exact frequencies.
    5. What source/device is being used to clock the AD6679?
    6. What reference frequencies are used? Is there a crystal for the reference to the clock IC? Is the clock driven externally with a signal generator?
    7. What source/device is being used to drive the analog inputs of the AD6679?
    8. What is the filter response on the analog inputs? It does not look very good since the second harmonic aliases to 159.4MHz at a fairly high amplitude.

     

     

    Comments:

     

    1. The ADC is not generating this tone. It has to be something on the board or one of the inputs that is mixing. (Above questions moving clock/input frequencies should help identify where it is coupling from.)
    2. The tone could come from something that could be at 370.3MHz. This would alias to 129.7MHz.  Not sure what could be at 370.3MHz though. Or it could be from something running at 185.15MHz where its 2nd harmonic would be at 370.3MHz. Or 370.3MHz could be a 3rd harmonic of something at 123.433MHz...
    3. You cannot drive the analog inputs in single ended fashion and expect to get a plot of the input bandwidth. It needs to be driven differentially to get the input bandwidth.

    Best Regards,

    TonyM

  • Hi,

    1.All of the spurious tones move when I change input frequency from 170,3 Mhz to 169,3 MHz especially  spur 130,7 Mhz to 129,7 Mhz as you can see in the pictures below:

    3.Unfortunately I can not change the sampling rate by 1 MHz because I use the AD9516-1 chip to clock the ADC, but I tried to change the sampling frequencies from 500MHz to 416MHz, distortions were still present. I tried to find the input frequency so that I could see the distortion. As a result, the input frequency of 143.3 MHz, the sampling rate of 416 MHz, are shown in the figure below

    5 I use the ad9516-1 (output freq 500MHz LVPECL) with the settings shown in the figure below. The reference frequency for the AD9516-1 can be selected either a highly stable quartz oscillator mounted on the carrier board or an external reference frequency source. The selection of the source of the reference frequency for the ADC distortion is not affected.

    7 I'm using a generator R&S SMC100A

    8 LPF with cut-off frequency 450MHz

    P.S I tried to feed a differential signal from the generator directly to the input of the ADC and distortions were also present there.