+ Post New Thread
Results 1 to 6 of 6
  1. #1
    Advanced Member level 5
    Points: 16,954, Level: 31
    Achievements:
    7 years registered
    neazoi's Avatar
    Join Date
    Jan 2008
    Location
    Greece
    Posts
    3,423
    Helped
    13 / 13
    Points
    16,954
    Level
    31

    AD9851 what is the output level?

    Hi, in this schematic (attached) what is the output signal level of the AD9851? Is it -5dBm? I need to verify if I am reading the datasheet correctly, that is why I am asking.
    Also, to measure the power level on a scope (at 50R), should I use the high impedance probe, since there is a 50R shunt in the output of the circuit?

    Thank you
    Last edited by neazoi; 28th November 2019 at 12:20.
    Professional engineering is the top, but amateur engineering is more fun.
    It is when you cross the barrier between these two, that things become really fascinating!

    •   AltAdvertisement

        
       

  2. #2
    Advanced Member level 5
    Points: 34,263, Level: 45

    Join Date
    Oct 2001
    Posts
    4,894
    Helped
    1453 / 1453
    Points
    34,263
    Level
    45

    Re: AD9851 what is the output level?

    Yes, the output level of the AD9851 is just below 0dBm with few dB. You need an amplifier which will provide also the necessary output impedance.

    https://telecnatron.com/articles/Var...DDS/index.html



    •   AltAdvertisement

        
       

  3. #3
    Advanced Member level 5
    Points: 16,954, Level: 31
    Achievements:
    7 years registered
    neazoi's Avatar
    Join Date
    Jan 2008
    Location
    Greece
    Posts
    3,423
    Helped
    13 / 13
    Points
    16,954
    Level
    31

    Re: AD9851 what is the output level?

    Quote Originally Posted by vfone View Post
    Yes, the output level of the AD9851 is just below 0dBm with few dB. You need an amplifier which will provide also the necessary output impedance.

    https://telecnatron.com/articles/Var...DDS/index.html
    I do not agree with the AD9850 output level versus frequency chart.
    Maybe it is measured on 200R and I used 50R, or maybe because I use the AD9851 instead.

    The point is, by testing the AD9851 circuit shown on post 1 on an analogue spectrum analyzer, I found it's output level to be flat within less than 1dB from 2-29MHz.
    I varied the frequency in MHz steps and noticed the signal peak on the analyzer. I also varied the frequency of the analyzer and checked the DDS signal at the center position of the screen at different frequencies. The signal was always flat within 1db, maybe less but I could not notice it on this old analyzer.

    I just do not have a means of measuring the output level, as this analyzer does not have markers, that is why I asked here.

    I would like to avoid external amplifiers to the DDS, because the levels are sufficient for measurements on the analyzer, and because I worry of ruining the flatness of the signal level and introducing unwanted harmonics.
    I found the ad9851 to be very clean on the 2-30MHz and I did not want to spoil this.
    Last edited by neazoi; 28th November 2019 at 15:26.
    Professional engineering is the top, but amateur engineering is more fun.
    It is when you cross the barrier between these two, that things become really fascinating!



    •   AltAdvertisement

        
       

  4. #4
    Super Moderator
    Points: 263,643, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    46,000
    Helped
    13989 / 13989
    Points
    263,643
    Level
    100

    Re: AD9851 what is the output level?

    Depends on intended operation. The generator has 50 ohm output impedance, it can be either operated with high impedance or matched 50 ohm load. In the latter case, the output level is halved.

    According to my calculation, output level with 3.9k is 500 mVpp into to internal 50 ohm termination, respectively 176,8 mV RMS. dBm measurement sense with matched load, 88.4 mV into 50 ohm is -8 dBm. The output current can be increased up to 20 mA by adjusting Rset, giving 6 dB of power margin.



  5. #5
    Advanced Member level 5
    Points: 16,954, Level: 31
    Achievements:
    7 years registered
    neazoi's Avatar
    Join Date
    Jan 2008
    Location
    Greece
    Posts
    3,423
    Helped
    13 / 13
    Points
    16,954
    Level
    31

    Re: AD9851 what is the output level?

    Quote Originally Posted by FvM View Post
    Depends on intended operation. The generator has 50 ohm output impedance, it can be either operated with high impedance or matched 50 ohm load. In the latter case, the output level is halved.

    According to my calculation, output level with 3.9k is 500 mVpp into to internal 50 ohm termination, respectively 176,8 mV RMS. dBm measurement sense with matched load, 88.4 mV into 50 ohm is -8 dBm. The output current can be increased up to 20 mA by adjusting Rset, giving 6 dB of power margin.
    So the circuit on #1 is giving around -2dBm with the 50R shunt resistor in place?
    I will have to test it but I believe that the altering of the resistor for increased level won't come without shortcomings, such as increased distortion or other things
    Professional engineering is the top, but amateur engineering is more fun.
    It is when you cross the barrier between these two, that things become really fascinating!



    •   AltAdvertisement

        
       

  6. #6
    Super Moderator
    Points: 263,643, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    46,000
    Helped
    13989 / 13989
    Points
    263,643
    Level
    100

    Re: AD9851 what is the output level?

    I believe that the altering of the resistor for increased level won't come without shortcomings, such as increased distortion or other things
    Only moderately, see SFDR over output current diagram in datasheet.

    I notice that you don't have an output low pass filter, thus carrier residuals will be present in output signal.



--[[ ]]--