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Altera_Forum
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16 years ago

Output amplitute from an NCO

Am i right in thinking that the output amplitutde from the NCO is not constant over different frequencies?

25 Replies

  • Altera_Forum's avatar
    Altera_Forum
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    16 years ago

    Amplitude depends on how you want it to be and is not related to frequency. Check your nco IP mif files for maximum/minimum.

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    Altera_Forum
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    16 years ago

    When i sweep the frequency the amplitude is changing though, could it be to do with the dac?

  • Altera_Forum's avatar
    Altera_Forum
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    16 years ago

    Hard to say. you better give more details about your design from NCO to DAC to the way you see amplitude. Do you use any fft?

    You can exclude NCO by looking at max/min mif values per each sweep point.
  • Altera_Forum's avatar
    Altera_Forum
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    16 years ago

    I am sorry I can't run any projects at the time being.

    I suggest the easiest way is to use nios or signaltap(or simulation) after adding this simple piece of code on your sine data: 

    
-- get max data
process(clear,clk)
begin
if clear = '1' then
    temp <= (others => '0');
elsif rising_edge(clk) then
    if temp < data then
        temp <= data;
    end if;
end if;
end process;

    you can use nios to clear temp or read its value per each frequency.

    Remember to make temp and data signed type
  • Altera_Forum's avatar
    Altera_Forum
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    16 years ago

    It is definatly something to do with the DAC / ADC, as i just run it without using the dac and is fine.

  • Altera_Forum's avatar
    Altera_Forum
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    16 years ago

    check your DAC number system. It is either 2's complement or offset binary.

    if offset binary you need to do one more thing:

    invert the sign bit

    I don't know what is ADC doing here.
  • Altera_Forum's avatar
    Altera_Forum
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    16 years ago

    I have already inverted sign bit and is outputing a wave fine. The issue is when frequency sweeping the amplitude is changing.

    Is there any files for the frequency response of the DAC?

    I mention the ADC as I guess it will have a similar issue when feeding a signal back into the card.
  • Altera_Forum's avatar
    Altera_Forum
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    16 years ago

    DACs do not have frequency response per se. It just converts digits to a ladder of values.

    If you inject any data into DAC, it should be converted to the ladder of corresponding digits. There is no way the DAC will discriminate against the frequencies. Assuming you are not violating the DAC sampling speed. Make sure your sine data are at DAC's sampling frequency and that you are not violating the sampling rule. I don't think you are violating because you generate them in the digits(not from ADC) so make sure your DACs sampling frequncy is same as that of your sine data.
  • Altera_Forum's avatar
    Altera_Forum
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    16 years ago

    Hold on...It could be you are having sinc effect of DAC's hold device.

    DACs normally have inverse sinc filter to compensate for that.

    But it could be you are right, as you get closer to Fs the amplitude will droop and becomes zero at Fs(in theory). So there is no problem now. Basically I missed the fact that your DAC is basic without inverse sinc correction.