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signal generator question

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Khin Thet Mar

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anyone can advice me?
i'm now doing a project of signal generator by using DDS technique.
i've used PIC 16F877 with 24MHz clock.
analog output waveform using R-2R ladder network.
but the output waveform that i got is not smooth at 40KHz and above.
one thing i'd like to know is how to get the smoother 100KHz analog output waveform.
thanks in advance.

khin thet mar
myanmar
 

Hi,
I hope you are using a LPF network after the DAC. To get it to 100kHz and also get 40kHz is slightly difficult. What is the resolution you expect.
B R M
 

What is your DAC sample rate? I'm guessing it's slower than 24MHz. As brmadhukar suggests, you need a low pass filter after the DAC to reject sampling images (they look like stair steps on an oscilloscope). Design a filter that passes the frequencies you want, and strongly attenuates the sample rate and higher.
 

Thanks for your advise.
I've used 8-bits DAC & 24-bits phase accumulator wordlength.
How much resolution will be got the output analog waveform to be smooth?
 

Your third sentance is unclear, so this may not answer your question.

An 8-bit DAC generates quantization noise approximately 48 dB below the desired sinewave. If you build a very good low-pass filter, the sinewave will look beautiful and smooth on an ordinary oscilloscope, however the quantization noise will be easily visible on a spectrum analyzer.
 

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