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ADC12DJ3200EVM: ADC jitter characteristics of the ADC12DJ3200EVM

May
Genius 9880 points
Part Number: ADC12DJ3200EVM

Hi Team,

Customer have following concern about the ADC12DJ3200EVM. Please see details below.

1. Trying to compare the characteristics of jitter when undersampling and when oversampling. First, a sine wave is input from the signal generator to the ADC12DJ3200EVM, and IQ data is generated from the output data of the obtained time waveform. I would like to evaluate with Error Vector Magnitude (EVM) from the IQ data. How to do it?

2. For the sampling procedure and additional questions, please refer to attached file.

Sampling_procedure (1).pptx

Thank you in advance.

Regards
Maynard

  • 0
    TI__Mastermind 38970 points

    Hi Maynard,

    • Is it the effect of aliasing that the sampled time waveform is distorted? - RR:  the reason the sampled waveform is distorted is that you are undersampling based on your setup, to help with that, you can increase the sample point size. The second reason is that you are improperly sampling. You need to use a prime number on the Fc in order to capture the sinewave correctly. Otherwise you are always sampling the same points across the sinewave each time. See the link below. You can choose to coherently sample or noncoherently sample. This link will guide you on how to do this properly.
    • www.planetanalog.com/.../
    • What kind of calculation should be performed to perform IQ modulation after obtaining the input waveform data when a sine wave is input and plot it on the constellation diagram? RR: this is a difficult question for us to answer, we typically don't look at modulated waveforms nor do we evaluate EVM testing in the lab.

    Regards,

    Rob

  • 0 in reply to Rob Reeder
    TI__Genius 9880 points

    Hi Rob,

    Thank you for response. Customer have follow up question regarding this. Below is the details.

    1.  Does increasing the sample point size mean increasing the sampling frequency?

    I want to measure the change in the effect of jitter during undersampling and oversampling. So I don't want to change the sample point size because I want to measure the jitter even when undersampling.

    I learned about coherently sampling and noncoherently sampling of the linked materials.

    In my experiment, only the input signal from the Signal Generator is input to the ADC, not the REF signal. In this case, I thought I should choose noncoherently sampling. I used the prime numbers 99.1235MHz and 101.1235MHz instead of 100MHz in the material. Are these numbers the prime numbers? 99.1235MHz is 99123500Hz, and I wonder if it is a Prime number.

    Also, is coherent sampling impossible without using the REF signal? The input frequency calculated by HSDC GUI was used at fs = 3.2GHz and fc = 4.0GHz. (Calculated input frequency = 3.999951172GHz)
    This will still cause the same distortion as in the attached document.

    2. Second, I was thinking of measuring jitter by the phase shift from the IQ pattern. Is there a good test method to measure ADC jitter with a sine wave input, as other methods are acceptable?

    Regards,

    Maynard

  • 0 in reply to May
    TI__Mastermind 38970 points

    Hi Maynard,

    Q1: Does increasing the sample point size mean increasing the sampling frequency? RR: no, these two variables are independent

    Q2: Second, I was thinking of measuring jitter by the phase shift from the IQ pattern. Is there a good test method to measure ADC jitter with a sine wave input, as other methods are acceptable? RR: probably not the best idea, you can simply take a lower frequency FFT measurement, 10MHz, at the level you choose, for example -1dBFS, record the SNR value. Then take a higher frequency FFT measurement, one that meets your undersampling conditions (4GHz?). Again make it the same level, -1dBFS, and record the SNR value. 

    With these two values you can back-calculate the aperture jitter of the adc. Remember though you are also including the clock and/or clock signal chain you are using for this measurement. Need to substract that out if you just want the ADC's aperture jitter only.

    See page 6, figure 5 for the equation in this app note link below.

    https://www.analog.com/media/en/training-seminars/tutorials/MT-007.pdf

    Lastly, the FFT above still doesn't look correct, you may want to be a little more off of 4GHz. All of the harmonics are folding on top each other, which can provide a false measurement.

    Regards,

    Rob

  • 0 in reply to Rob Reeder
    TI__Genius 9880 points

    Hi Rob,

    Apologies for the late response. Customer have follow up questions. Below is the details.

    Q1
    I'm sorry for my lack of knowledge, but I didn't get much information when I looked up the sample point size.
    I would appreciate it if you could tell me a little about the sample point size.

    Q2
    I will try to calculate the jitter back from the two FFT measurements with reference to your reply.
    However, I think this presupposes that the FFT measurements need to be accurate.
    When the frequency of the input signal is higher than the sampling frequency, such as the undersampling condition shown in the previous question, the FFT is not measured accurately.
    I don't know how far away from 4GHz it can be measured accurately, so I would like advice.
    Also, I would like to know how to select the frequency when setting the frequency of the input signal to the prime number, which I heard in the previous question.

    Thank you in advance.

    Regards,
    Maynard

  • 0 in reply to May
    TI__Mastermind 38970 points

    Hi Maynard,

    I will take this discussion offline and respond via email.

    Regards,

    Rob