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AD985x DDS MODULE part #1

Discussion in 'Homebrew and Kit Projects' started by AA7EJ, Nov 6, 2018.

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  1. AA7EJ

    AA7EJ Ham Member QRZ Page

    Risking to receive some flame, after asking Mrs Google I found this "controversial" discussions.

    Apparently NOBODY really knows the real schematic of this AD985x module, especially the function of the what appears to be output level adjustment pot.
    I appears to control ONLY SINE wave output.

    BUT - the controversy is that the module LPF was designed for WRONG AD985x chip.
    That is fine with me - since I am interested in max frequency of 30 MHz.

    However - my guess , I have not checked datsheets (in detail) , is that BOTH chips have SAME output impedance hence the LPF should be designed for SAME Z0.

    Now where does the opinion that one chip needs 200 Ohms LPF and the other 50 Ohms come from?

    As soon as I get the hardware and software put together I'll be in better position
    "arguing" this IMHO non - issue.



    73 Shirley
     
  2. K3KIC

    K3KIC Ham Member QRZ Page

    Check the data sheet description of the evaluation board (AD9851). They describe using a 200 ohm impedance filter to get full output voltage.
     

    Attached Files:

  3. AA7EJ

    AA7EJ Ham Member QRZ Page

    Thanks.
    I do not want to get QRZ mad at me so I'll refrain form posting more pictures.

    More I dig into this the better it gets.
    The chip (AD9851) functional diagram ( in datasheet) clearly shows that square wave is generated AFTER the sine wave is fed back to the chip .
    So if ever find the real schematic for the module it should show that connection AFTER the LPF!
    And yes, the LPF - for AD9851 is build for 200 Ohms Z0.
    Now - is the AD9850 module sold with (wrong) AD9851 LPF , as some claim, that needs to be verified by somebody with better eyes than mine.

    Now
    the real question - is the Z0 of 200 Ohms LPF THAT much different than one designed for Z0 50 Ohms?
    Again - max desired frequency is 30 MHz and lots of stuff can be fixed / adjusted in software anyway.
     
  4. AA3EE

    AA3EE Ham Member QRZ Page

    Well, a 4:1 SWR for starters...
    IIRC, the component values will all be different by a factor of 4 as well.
     
  5. AA7EJ

    AA7EJ Ham Member QRZ Page

    I think I was not clear in my comment.

    The AD9850 chip "analog output" (DAC) impedance is 50 to 120 kOhms.
    The AD 9851 is "120" kOhms.
    Yes - the DAC output impedance is in kiloOhms.

    The LPF of the MODULE is connected to DAC and both chips have input / output terminating resistors of 200 Ohms.

    The module output impedance is therefore 200 Ohms , hence the coupler attenuators should be designed for 200 Ohms input and 50 Ohms output = input of AD8302 phase detector.

    If the above makes sense I see only one small issue - the calculator I am using cannot calculate 2dB attenuator with input of 200 Ohms and output of 50 Ohms.
    No problem calculating 20dB attenuator.

    I am sure there must be a simpler way to make 2 dB attenuator with 200 Ohms in and 50 Ohms out.
     
  6. K3KIC

    K3KIC Ham Member QRZ Page

    Have you considered putting a single transistor amplifier after the output of the AD9850/1 module?
     
  7. VK4FFAB

    VK4FFAB Subscriber QRZ Page

    Its probably easier to remove / bypass the existing LPF and build one externally. This will also make the gain more linear as the gain drops off a fair bit from 20mhz up because of the poor lpf implementation on the module. That just seems simpler than adding attenuators, amps, buffers and agc. to get the impedance matching and linear gain.
     
    K3KIC likes this.
  8. VK4FFAB

    VK4FFAB Subscriber QRZ Page

    What you want is a 4:1 impedance transformer.
     
  9. AA7EJ

    AA7EJ Ham Member QRZ Page

    Not yet.

    I need to draw myself a full picture of all the levels at various points of the circuit.
    There seem to be only one basic design and such levels are pretty much unpublished and for all practical purposes fixed.

    For example AD8302 chip has "input" range -+ 30dB , which can be controlled / adjusted but NOT in AD8302 MODULE.

    AD8302 app note describes TWO attenuators (20dB and 2 dB) however , in mentioned design both attenuators are using SAME components - hence same , unpublished (or unreadable in schematic ) attenuators.

    The coupler "isolation /attenuation " depends on number of secondary turns...

    And yes, I do have a room / provision on the PCB to add an amp to the output of AD 9850.

    73 Shirley
     
  10. K3KIC

    K3KIC Ham Member QRZ Page

    If you were going to the trouble to remove the current filter then you might as well replace the components to make a filter to your liking.
    I've been looking at building an antenna analyzer using a AD9850/1 module and AD8307. So I'm traveling down a path similar to Shirley but slightly behind.
    I do have all the parts, including a number of the AD9850 and AD9851 modules. My plan was to add an amplifier after the module so I would end up with higher reflected levels back into the bridge.
    I have the schematic captured but haven't laid out the board. All SMD. Probably cut a test board in a month or so.
     

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