Re: DDS spectral "purity" ?

Jaques Anthony

Hi all,
    In a previous posting I referred to measurements that I have been making. Jim's very interesting video demonstration prompts me to put some of the results here.
     I am using a Cushman CE15 "spectrum monitor" for measurements. This is not as flexible as the true spectrum analysers that I no longer have access to, but does allow useful, if not definitive measurements to be made:

   Measured at f = 7.040 MHz
   Levels dBm
   on Boonton 92B millivoltmeter (50 Omhs)

Measurement at | f      2f      3f     4f      5f

    P101             -8     -50      *       -       -

    P102           -12     -53      *       -       -

    Output         +3     -25     -40    -56     *

   * = detectable in the noise
   - = not detectable
    This looks ok on my screen. I do hope that the system does not re-format it!
    The measurements at P101 and 102 were done with each link removed.

    Doing a frequency run from 0.5 to 30MHz does show the entertaining moving cornfield, but at a much lower level than in Jim's display, with much of it just above the noise. But with the Cushman (or my!) limitations not too much reliance should be placed on that.

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