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AFE7900EVM: Auto calculation of coherent frequencies

Joanne Gu
Prodigy 160 points
Part Number: AFE7900EVM


I saw the calculation method from this articlehttps://www.ti.com/lit/an/slaa510/slaa510.pdf?ts=1637632948673&ref_url=https%253A%252F%252Fwww.ti.com%252Fdata-converters%252Fadc-circuit%252Fhigh-speed%252Flearning-center.html

By testing AFE7900EVM, captured samples(N)=65536, Fs=2949.12MHz, Fin(initial)=1.73GHz

M=N×Fin/Fs=38444

Fin(final)= Fs×M/N= 2949.12M×38444/65536=1.72998GHz

But the result of auto calculation for FFT is 1.73000525GHz(see the picture below).

Why was two results different?

  • 0
    TI__Genius 10875 points

    Hi Joanne,

    I tested this in HSDC Pro and see that it is calculating the coherent frequency as 1.72999875GHz. Can you try closing and reopening HSDC Pro to see if you get the same? Also, when you are using the formula you mentioned you should use the data rate, 245.75MHz, as the Fs. The reason that HSDC Pro only sees sampling rate of 245.76MHz and has no knowledge of the 2949.12MHz clock on the AFE side.

    1729.99875/245.76*65536 = 461333.0

    HSDC Pro finds an M such that it's a prime integer.

    Regards,

    Vijay