MAX038 - Why a PLL and not a DAC voltage regulation ?

Discussion in 'The Projects Forum' started by simo_x, Jul 1, 2014.

  1. simo_x

    Thread Starter Member

    Dec 23, 2010
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    Hello,

    I would like to use a MAX038 IC for a function generator, but I have a doubt.

    The datasheet says that varying the voltage from FADJ pin varies \pm 2.4\,\mathrm{V} varies the output frequency from 0.3 to 1.7 %.

    At page 10 the datasheet says:

    FADJ Input: The output frequency can be modulated by FADJ, which is intended principally for fine frequency control, usually inside phase-locked loops.

    Then, a circuit using a PLL is shown at page 14.

    If the frequency fine adjustment can be set by varying the voltage to FADJ, why it is not suggested to use (for example) a DAC regulation on FADJ ?

    For my function generator, I would prefer to use a voltage regulation using a DAC.
    Do you think it is a bad idea ?

    Thank you for your suggestions.

    :)
     
  2. crutschow

    Expert

    Mar 14, 2008
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    I don't see a problem with using a DAC to generate the FADJ voltage. What are you doing with the MAX038?
     
  3. MrChips

    Moderator

    Oct 2, 2009
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    Using a DAC to control the frequency of the MAX038 would be called open loop control assuming the numerical input to the DAC is generated independently.

    A PLL is an analog closed loop control system that produces an output frequency that is synchronized in phase with another reference signal.

    A closed loop control system using a DAC may be too slow to realize the desired control.
     
    simo_x likes this.
  4. simo_x

    Thread Starter Member

    Dec 23, 2010
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    As I wrote before, a function generator.

    :)

    Perhaps you want to say open loop ?
    Howhever this could explain why the PLL is preferred instead of a voltage regultaion with a DAC.

    Now, please correct me If I am wrong. I did not completely understand how the regulation work on FADJ with the PLL..
    Maybe, the voltage on FADJ is regulated with the avarage voltage of the square wave output of the PLL ? Am I right ?

    Thank you.

    :)
     
  5. Lestraveled

    Well-Known Member

    May 19, 2014
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    What kind of function generator are you building, an open loop or a closed loop PLL?
     
  6. simo_x

    Thread Starter Member

    Dec 23, 2010
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    That's the problem.

    From the explanation of the IC' s datasheet, I understand that I can achieve the modulation also with a DAC. After the previous post by MrChips, I understand that interfacing the MAX038 with a PLL is better than using a DAC.

    But I did not understand how the frequency is modulated using the PLL output..

    :confused:

    And I don't want to design a circuit I do not completely understand.
    That's why I am asking about the behaviour of the PLL interfaced with the MAX038.

    I just know the SYNC output must be fed back to the PLL input, and the output of the PLL depends on the phase difference between the main PLL frequency oscillator and the SYNC input frequency.

    Please correct me if I am wrong.
     
    Last edited: Jul 1, 2014
  7. MrChips

    Moderator

    Oct 2, 2009
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  8. simo_x

    Thread Starter Member

    Dec 23, 2010
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    I am not an expert about PLL. I never used a PLL so let' s say that it' s something "new".

    I don' t know if it is really necessary a PLL for FADJ, but I am interested why it is used in the circuit and how the FADJ regulation works with the PLL. That' it.

    My idea is:

    Using the formula

    f_{_\text{OUT}} = {I_{_\text{IIN}} \over C_{_\text{F}}

    If I maintain a IIN current from 2\,\mu \mathrm{A} to 750\,\mu \mathrm{A}, I can achieve a frequency output from 1\,\mathrm{Hz} up to \approx 16 \mathrm{Hz} (or more) with just 3 capacitors, but I will limit the output frequency to 1 or maximum 5 MHz..

    Then I basically would adjust the FADJ voltage with a DAC because as I understand, I could use a DAC for this purpose.

    I will use an MCU for DAC and IIN control.

    Do you think it is better to use a PLL for FADJ or do you think the DAC is fine ?

    Thank you for the AN. :)
     
    Last edited: Jul 1, 2014
  9. MrChips

    Moderator

    Oct 2, 2009
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    What is the purpose and advantage of using a DAC?
    Why not simply use a pot?
     
  10. simo_x

    Thread Starter Member

    Dec 23, 2010
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    Because I can digitally program the voltage with the DAC followed by an op amp.

    I think the digital control would belong to more precision than using a potentiometer/variable resistor.
     
  11. Lestraveled

    Well-Known Member

    May 19, 2014
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    A PLL control loop gives you the following abilities that open loop does not:
    - Able to control the frequency to sub Hertz resolution,: 1Hz, .1Hz or .01Hz.
    - Able to phase lock the frequency to another frequency source.

    A PLL is far more complicated than a open loop generator. If you do not need what a PLL gives you, then do not do it. Stay open loop.

    Mark
     
  12. alfacliff

    Well-Known Member

    Dec 13, 2013
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    why a pll and not dac voltage regulation? stability, a pll can nold the frequency where its supposed to be, a dac cannot, without a phase / frequency detector to set the dac output, and if you had that, it would be a pll.
     
  13. MrChips

    Moderator

    Oct 2, 2009
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    You want to build a simple function generator using an XR8038 or similar chip, right?

    A DAC can give you a digital input to the chip but you still will not be able to control the frequency reliably. There is a functional relationship between the digital input to the DAC and the frequency output of the 8038. This relationship is not necessarily linear and will drift over time and temperature.

    I would choose a variable resistor to control the output frequency.
     
  14. DickCappels

    Moderator

    Aug 21, 2008
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    According to the datasheet, frequency as a function of the fine adjust voltage is pretty linear, except at the ends of the range. It is conceivable that you can calibrate the frequency with respect to the value into the DAC.

    One advantage of using a DAC over a pot is that you won't have to worry about noisy pots (mine function generator suffers from that).

    If you use a DAC, you will be stepping the frequency instead of sweeping it. That might give you some unexpected side effects.
     
  15. simo_x

    Thread Starter Member

    Dec 23, 2010
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    Hello guys,

    Thank you for your explanations about the purpose of the PLL.

    I will do some test (with the DAC and the PLL) and I will decide depending of the results.

    :)
     
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