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Quadrature Demodulation at 90 KHz

Discussion in 'Radio and Communications' started by naumank, Feb 19, 2016.

  1. naumank

    naumank New Member

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    Hello all

    I have a linear frequency modulated (LFM) signal having 90 KHz center frequency and 20 KHz bandwidth around it. My problem is to demodulate it at base band to get I and Q both (phase information required). I have searched a lot at internet but mostly found quadrature demodulator ICs have starting input frequency range in MHz. AD8333 was what i found having lowest input frequency but even it does not work for 90 KHz (as i discussed with analog devices technical staff).

    Can any one kindly guide me how to solve my problem?.

    Thanks in advance

    Best Regards
    Nauman
     
  2. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

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  3. Mikebits

    Mikebits Well-Known Member

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  4. dave

    Dave New Member

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  5. naumank

    naumank New Member

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    Thanks for your concern. Actually problem was the minimum frequency of sine wave at 4LOP pin which is 2MHz in case of AD8333. In AD8339, minimum frequency range for sine wave at 4LOP is not define or i could not found it in its data sheet.

    In case of using square wave at 4LOP pin, will not the harmonics/noise be significant in I/Q output?
     
  6. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

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    Last edited: Feb 21, 2016
  7. Mikebits

    Mikebits Well-Known Member

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    From page 3 of the data sheet:
    Local Oscillator (LO) Frequency Range: Min - .01MHz Max- 200MHz

    For LO leakage a filter is normally used. A active filter would be easy to implement.
    The mixer (analog multiplier) is a Gilbert Cell topology. You could build your own using matched transistors for the differential pairs. I think there are transistor array IC's.
     
  8. naumank

    naumank New Member

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    Thanks. Is not this LO frequency range for square wave or it is also applicable for sine wave also?
     
  9. Mikebits

    Mikebits Well-Known Member

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    This directly from the data sheet:
    LO Inputs. No internal bias; optimally biased by an LVDS driver. For best performance, these inputs should be driven differentially. If driven by a single-ended sine wave at 4LOP or 4LON, the signal level should be >0 dBm (50 Ω) with external bias resistors.

    Naumank, I don't mind helping you, but please read the data sheet first before posting your questions, as most of the answers to your questions can be found there. If you can't be bothered to read the data sheet then why should I. If your understanding of English is a problem, then I completely understand that and in that case will continue to assist you. :)

    You seem to be overly concerned with using a square wave for a LO. In the case of a Gilbert Cell (most quad mixers), the LO acts as a switch control so a square wave works best for this application. Besides, even if you input a sine wave, it will become a square wave after the internal LO divider. I suggest this again, you should research the Gilbert Cell Mixer.
    Have a look at the following link to get an idea of how these mixers work. The video has a good explanation.

     
  10. naumank

    naumank New Member

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  11. Mikebits

    Mikebits Well-Known Member

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    I almost missed your question as it was hidden in the quote (not a good idea to do that). Now I am going to answer your question with a question.

    1. As for as your second point is concerned, it means that if we apply square wave at 4LOP pin or sine wave at this pin, the I/Q outputs will be same. Am i right?

    AD8339.JPG
    Looking at the block diagram, what do you think?
    Also be advised, the LO must be driven differentially as per the data sheet.

    LO INPUT
    The LO input is a high speed, fully differential analog input that responds to differences in the input levels (and not logic levels). The LO inputs can be driven with a low common-mode voltage amplifier, such as the National Semiconductor® DS90C401 LVDS driver. The graph in Figure 22 shows the range of common-mode voltages. Logic families such as TTL or CMOS are unsuitable for direct coupling to the LO input.

    See the eval board schematic.
    AD8339LOdrive.JPG
     
  12. naumank

    naumank New Member

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    Thanks for all your help. I think i need to study its data sheet more in-depth.
     
  13. Mikebits

    Mikebits Well-Known Member

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    I can't emphasis this enough, try and learn about the Gilbert cell mixer, in fact try and build one, you will learn a great deal by doing so.
     
  14. Mikebits

    Mikebits Well-Known Member

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    I guess I need to work on my teaching style, I ran the guy off.
     
  15. naumank

    naumank New Member

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    Hello

    Your teaching style is perfect, actually i was unfortunately busy in other matters. I have few more queries for you can kindly provide some answers.

    1. How can i simulate these ICs?. I have Orcad 9.2 but it has no support for this IC. Is there any free simulation software for this type of ICs?

    2. This question may be stupid but i am wondering that this demodulation can be done using principle of "under sampling" i.e. chose the sampling frequency (lower than the nyquist criteria) such that alias of desired frequency (e.g. 420 K Hz) comes in low frequency region?

    Thanks
     
  16. Dick Cappels

    Dick Cappels Member

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