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ADC32RF45: FFT response, unidentified spurs

Mark Guastaferro
Genius 11645 points

Part Number: ADC32RF45

Reviewing the data sheet, there are some questions regarding the spurs shown in the FFT responses. In particular, when looking at Figure 4 which shows the FFT for a 900 MHz input tone. 

Data sheet specs indicated SFDR at 900 MHz input is 77 dBc, excluding HD2,HD3,HD4, HD5, and interleaving spurs IL and HD2 IL.  In Figure 4 we can identify these spurs, but would like to know more about the non-HD2/HD3 spurs identified as -84 dBc and the IL spurs at -79 dBc in the figure caption:

1. Are these spurs dependent on input signal amplitude?  Will they go down in amplitude with the input, and at what rate?

2. We can identify the IL and harmonic spurs in the FFT plot.  What are the sources of the other spurs? are they clock related?  Would they scale down under certain conditions?  

  • 0
    TI__Mastermind 29220 points

    Hi,

    This is the explanation from the characterization team for that typical characteristics plot of Figure 4:

    fIN = 901.015 MHz

    fS = 3000 MHz

    A­IN = -2 dBFS

     

    Spur_Name

    Spur_Frequency (MHz)

    Spur Level (dBFS)

    Sources of the spur

    Spurs behavior across amplitude

    HD2

    1197.97

    -73.9

    Second-order harmonic distortion

    For every dB change in fundamental, level typically changes by 2 dB

    HD3

    296.95

    -75.1

    Third-order harmonic distortion

    For every dB change in fundamental, level typically changes by 3 dB

    HD5

    1494.92

    -86.0

    Fifth-order harmonic distortion

    For every dB change in fundamental, level typically changes by 5 dB

    fS/2 - HD2

    302.03

    -84.4

    Interleaving spur for HD2

    For every dB change in fundamental, level typically changes by 2 dB

    fS/4 - HD2

    447.97

    -84.3

    fS/4 + HD2

    1052.03

    -83.3

    fS/2 - HD3

    1203.05

    -83.6

    Interleaving spur for HD3

    For every dB change in fundamental, level typically changes by 3 dB

    fS/4 - HD3

    453.05

    -81.4

    fS/4 + HD3

    1046.95

    -85.7

    fS/2 - fIN

    598.98

    -80.9

    Interleaving spur for fundamental signal

    For every dB change in fundamental, level changes by 1 dB

    fS/4 + fIN

    1348.98

    -88.9

    fS/4 - fIN

    151.02

    -88.3

    Regards,

    Richard P.

  • 0 in reply to Richard Prentice
    TI__Genius 11645 points

    Hi Richard,

    Thank you for the details.  A couple of additional questions regarding the Interleave spurs in particular:

    Are the IL spurs single tones or do they follow the input (frequency wise)? If the input is a modulated signal, will the IL spur copy the input signature, and if it is related to HD2, will it follow the signature of the HD2 of the modulated signal, etc? 

  • 0 in reply to Mark Guastaferro
    TI__Mastermind 29220 points

    Hi,

    In that figure4 of the datasheet the input is a single tone, and so the spurs are single tones.   But if the input were a modulated signal of some width, then these same spurs that are a function of fin or a harmonic of fin would also respond accordingly - having a width commensurate with the width of the modulated input. 

    Regards,

    Richard P.