Topic: FMC230: How To Operate In The Second Nyquist Zone?  (Read 10354 times)

chehovin May 20, 2014, 03:43 PM

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 For our application we would like to operate the FMC230 DAC in the second Nyquist zone.   We are seeing 39 dB of attenuation in the Second Nyquist Zone on the FMC230 when compared to the First Nyquist Zone.  That is too much for our application.     

How do we reduce the attenuation in the second Nyquist zone? 

Test Setup:
- External Clock = 1333.333333 MHz
- DAC digital input is a single sine wave at 100 MHz

Results as observed on a spectrum analyzer:

freq. (MHz)   Nyquist Zone Amplitude (dBm)
100                       1                3.6
1233.333              2             -35.7
1433.333              3             -40.0
2566.666              4             -24.0

There is another forum thread that might be related titled "Unexpected Amplitude Transfer from FMC230"  but I was not able to access it because it was moved to another forum for which I don't have access.  Your help is appreciated.



peter May 21, 2014, 06:12 PM (#1)

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Hello,


These amplitudes looks like normal sinc rollof.


See also Figure 155 in the datasheet of the AD9219.


Best Regards,
Peter

chehovin May 21, 2014, 06:42 PM (#2)

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The solution is to put the DAC in Mix Mode. 

Added one line of code to the "DAC Init" portion of the function "FMC230_dac_init" located in the source file "fmc230_dac.cpp" in the software reference design for the FMC230.

The line of code is
rc = sipif_writesipreg(bar_dac+0x19, 0x01); Sleep(2); //Enable Mix Mode to operate in second Nyquist Zone

Results as observed on a spectrum analyzer:

freq. (MHz)   Nyquist Zone Amplitude (dBm)
100                       1             -29
1233.333              2                1
1433.333              3               -2
2566.666              4             -43


peter May 22, 2014, 12:05 PM (#3)

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Hi, glad you figured that out. I was assuming you needed both Nyquist zones simultaneously.


Best Regards,
Peter

peter May 23, 2014, 12:48 PM (#4)

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This topic is being closed because the issue is considered as resolved by 4DSP. Feel free to create a new topic for any further inquiries.