DDS module, checking/measuring quality of sine wave across frequency range

Discussion in 'Homework Help' started by ninjaman, Oct 28, 2014.

  1. ninjaman

    Thread Starter Member

    May 18, 2013

    I have an AD9850 DDS module from ebay. the cheap one. I want to use it to generate a sine wave with frequency from 1-30MHZ. I have to write a technical spec or list of measurable for a project at college. my tutor is making this a little difficult for me as im a student and not qualified engineer. I don't know what kind of things to include in the list of measurable. I have looked at other low cost antenna analyzers to find their technical spec which is something like the following
    frequency generation and control:
    • 1-60MHz
    • source impedance 50 ohms
    • stability +/- 100ppm
    • spectral purity harmonic down >- TBD dB beyond 60MHz
    usable measurement range
    • SWR 1.0 to 9.99
    • impedance approx. 5 to 2000 ohms
    RF output
    • adjustable 2.0 volts pp (typ)
    power supply
    • external 13.8 to 19 volts DC, 500mA
    • internal; 8xAAA 1000mAh NiMH cells
    • charging time 12 hours (charge rate 0.1c)
    • pushbuttons (5) mode, band, config, scan, up, down
    • switch, power on
    • RF out BNC
    • usb mini b receptacle
    • external power 2.1mm power jack (centre pin positive)
    this is the technical spec. my tutor wants me to write things like I will measure this.... and whatever it is. so if I have a source impedance of 50ohms how will I measure it. I thought it would be ok to use a ohm meter.
    but I have a few problems.
    the DDS module is the main one. comparing the frequency output with the display frequency. that shouldn't be too difficult. I would measure the frequency output and compare that by. output divided by display. I think this will give percentage error after x100.
    then the quality of the sine wave, on here it says stability and spectral purity. I don't know how to measure these things, I found this and now don't know what im doing


    I have read other things about mirror spurs and dB attenuation which is more confusing. this is an HNC course which is the equivalent of a first year degree (four years total).
    I am whining some because I have very little knowledge of electronics. I completed a level 3 course and intermediate amateur radio license but this stuff is way above my head.

    can anyone please give a suggestion about what I could look for to describe what I am going to measure and how I could do it. I did think that it would be a case of connecting an oscilloscope and looking at the sine wave over the frequency range and seeing if it drops off or changes shape at all because of distortion. I expect it to stay the same shape over the full range to 30 MHz because most other projects on the internet say that its that stable. so I don't know what to do!

    any help would be great!

    all the best
  2. WBahn


    Mar 31, 2012
    Perhaps the problem is that you are taking on too ambitious a project for the level you are at and the level of this course. Perhaps you should find a project for which you CAN reasonably come up with measurable specifications.
  3. MrChips


    Oct 2, 2009
    To measure the output impedance, measure the output voltage with no load attached.
    Connect a load resistor and vary the resistance until you get half of the no-load voltage. The output impedance will be equal to the resistance value of the load resistor.

    You cannot measure the distortion or spectral purity by looking at the waveform on an oscilloscope.
    The simplest way to examine the spectral purity is to use an oscilloscope that has a FFT function built-in.
  4. WBahn


    Mar 31, 2012
    Doesn't that make some assumptions about the circuitry, namely that there is no feedback regulation looking at the output signal?