DDS Buffer amp

Discussion in '"Boat Anchor" & Classic Equipment' started by WB3JIH, Oct 16, 2017.

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  1. WB3JIH

    WB3JIH Ham Member QRZ Page

    Hey all -

    Some technical or design advice requested. I've build the N3ZI DDS for one of my tube rigs. According to the transmitter manual, the external VFO input is looking for roughly 5 volts. I built the buffer amp below, which is supposed to put out 5 volts (I've changed R2 and R3 to 220 ohms and 2.2K respectively.)
    dds-buffer-sch.jpg
    (From the N3ZI website...)

    The problem I'm having is that this isn't really cutting it for VFO drive. It's enough to get SOME drive to the transmitter, but I"m still only seeing about 3 volts pp on the scope at 5 MHz.

    I spent some time searching on this, but basically all of the buffer amps are the same design.

    Was wondering if anyone out there had built anything that would bring the output of this up to where it will adequately drive a boat anchor, or offer any suggestions on the above design.

    Jeff
    WB3JIH
     
    Last edited: Oct 16, 2017
  2. K7JEM

    K7JEM Ham Member QRZ Page

    Is the input impedance of the circuit being driven low or high? I would think that a BA would be somewhat high impedance.

    I think you could use something like a CD4069UB to amplify and buffer the output of the DDS, and get close to 12v P to P of RF at 5MHz.

    Attached schematic.
     

    Attached Files:

  3. W2WDX

    W2WDX Subscriber QRZ Page

    First question is, which radio? Second where does the VFO input on that transmitter inject the signal. This buffer amp works mostly for crystal sockets on Colpitts or Pierce type oscillators. If the VFO input on your transmitter is post oscillator there may be impedance issue or its loading the output of the buffer improperly.

    Also check to make sure the N3ZI DDS output is where is should be. If you assembled it yourself go back and make sure you performed all the voltage/current tests as the instructions dictate. Of course if the output of the DDS is low, so will the buffer amp.

    The buffer amp (and DDS) should also have good regulated power. Make sure your supply voltage is solid. Most people who have tried wall-wart power claim no joy. On mine (I have the second generation N3ZI DDS) things were a little hinky until I used a good dedicated regulated power supply. I built a VFO which includes its own PS which I built up inside the chassis providing power for both the DDS and buffer boards. It's a basic regulated supply based on a 7812 using a little Hammond transformer and bridge rectifiers.

    I also recently switched over to a buffer amp being offered on ePay which is well made and is all surface mount. It was much smaller than the breadboard amp I built up, and also provides a bit more output. Search on eBay for "DDS VFO amplifier" and it should come right up. It's $18USD including shipping.

    Mine works fine on all the boatanchors I have tried it on into the crystal socket.
     
    Last edited: Oct 16, 2017
  4. WB3JIH

    WB3JIH Ham Member QRZ Page

    Thanks for the reply. In this particular application, the transmitter is a Hammarlund HX-50. The internal VFO is feeds into a switch that permits selection of three crystal positions, the internal VFO, and a 50 ohm input external.

    The VFO output needs to be between 6 and 6.5 MHz for this rig. However, I'd also like to use it with the Swans, which are a bit more complicated...

    I'll check the power rail though.
     
  5. KL7AJ

    KL7AJ Ham Member QRZ Page

    What's the voltage at the top of R4? Could be a larger coupling capacitor on the output is called for.
     
  6. WA7PRC

    WA7PRC Ham Member QRZ Page

    The information supplied by N3ZI (http://www.pongrance.com/dds-test.html) indicates the DAC in the AD9834 IC wants to see a 200Ω load. It appears your buffer provides a heavier load, around 100Ω.

    According the AD9834 datasheet (link), VOUTMax = 0.6V and VOUTMin = 30 mV (VOUT = 570mVP-P or 0.202 VRMS).
    IOW, you need at least AV = 5 / 0.202 = 24.75 (27.9 dB) gain.

    Start with providing the 200Ω load that the DAC in the AD9834 wants to see. :)
     
  7. WB3JIH

    WB3JIH Ham Member QRZ Page

    The buffer amp above is not "mine", it is, in fact, the design on N3ZI's website.

    I also mentioned that I've changed R2 to 220 ohms.

    With that being said, what appears to have helped was 11 bifilar turns on a toroid core:
    transformer.jpg
    (From Greg Latta's website: http://faculty.frostburg.edu/phys/latta/ee/vfo/vfo.html

    So now I'm good.
     
    WA7PRC likes this.
  8. KD2AZI

    KD2AZI XML Subscriber QRZ Page

  9. AE4GM

    AE4GM Ham Member QRZ Page

    What is part number or name of torroid?. Brock
    AE4GM-- please text answer to 540.498.5706. Thanks.
     
  10. AE4GM

    AE4GM Ham Member QRZ Page

    Also . 11 turns. Is that 11 turns input to torroid and another 11 turns output? Wouldn't extra turns out increase voltage. And would it still be 50 ohms?
     

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