Gain control using fpga

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i'm using the DAC to generate a white noise (14 bits LFSR - generated in fpga, maybe i should decrease the number of bits. But what happens, is the noise still be white?).

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Are you using the LFSR to create uniform noise or Gaussian noise? The difference relates to the power in the power spectrum.

For a detailed analysis of LFSRs and examples of power spectra, see;

http://www.ovro.caltech.edu/~dwh/correlator/pdf/lfsr_tutorial.pdf (http://www.ovro.caltech.edu/%7edwh/correlator/pdf/lfsr_tutorial.pdf)

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I realy dont know. How do i calculate the SNR of DAC?

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The quantization noise floor will be at approximately 6*B. However, all DACs have a sinc-like output response (see examples in the above doc), so you have to inverse-filter your output signal, hence you lose SNR relative to the ideal.

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My gain should go from -19 dB to 0 dB in steps of 1 dB. I need a multiplier or a divider (what is better, start with a -19 dbm and multiply to increse the gain or start with 0 dbm and divide to achive -19 dbm?) . Does the quantization noise apply?

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The quantization noise floor does not move. If you are planning on having just a noise output, then you can probably just treat the quantization noise as part of your signal. However, if you are trying to generate band-pass filtered noise, then the quantization noise sets your noise floor.

Read these;

http://www.ovro.caltech.edu/~dwh/correlator/pdf/esc-100paper_hawkins.pdf (http://www.ovro.caltech.edu/%7edwh/correlator/pdf/esc-100paper_hawkins.pdf)

http://www.ovro.caltech.edu/~dwh/correlator/pdf/esc-100paper_hawkins.pdf (http://www.ovro.caltech.edu/%7edwh/correlator/pdf/esc-100paper_hawkins.pdf)

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Each one of the filters output must be 0 dbm (maximum) at the DAC output.

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That comment does not really makes any sense. There is only one output and hence the output 0dBm. If N signals are each contributing a fixed power level, then each signal has to have 1/N power, or an RMS that is 1/sqrt(N) smaller.

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Do you know how to calculate spurious emission?

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Of what? Do you have an antenna or board design you are trying to measure? You put it in an EMI chamber and measure it.

Cheers,

Dave