Processing/Sampling IF(IQ) signals

Discussion in 'General Electronics Chat' started by himadri117, Nov 11, 2014.

  1. himadri117

    Thread Starter New Member

    Nov 11, 2014
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    0
    Signal Characteristics: The IF (I/Q) signal consists of a DC component and an AC component.

    DC Component: 2.52V; AC Component: 1.72 – 2.04 V (V P-P); BW: 20Hz - 500kHz; Power: 20dB

    Requirement: Now I want to digitize this signal for further processing. I prefer a sampling at 1 MSPS with ADC and then porting digital data to PC using a FPGA over SPI interface.

    Problem: The problem is, the signal with DC and AC component ranges from 3.5V to -1.5V with 2.5V from DC and almost 1V from positive cycle or negative cycle of the AC signal. Now I can use a coupled capacitor to block the Dcand use the AC for processing. My choice of ADC reference voltage is almost 2V-2.5V (for good resolution usability of ADC) so the signal can be amplified. But this signal have a negative part which I think is not good for a linear behavious for the ADC, as it has no negative reference voltage level. ADC reference is from 1V-5V. Using a rectifier is not a good choice from my view.

    Question: How can I digitize this signal? Also comment on my views if not correct as I have only theoretical knowledge and never worked on a practical project.
     
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    I Q signals are in quadrature, 90 degree phase shifted, not one rf and one dc. with a 90 degree phase shift, you can mathematcly detect many modes of modulation.
     
  3. himadri117

    Thread Starter New Member

    Nov 11, 2014
    10
    0
    Thanx for the reply. I know the I and Q signals are 90 deg phase shifted. I didn't mean one as DC and other as RF. Each I or Q channel has a DC and RF part.... For eg. if i take only theI channel I have the RF of 2V peak to peak and DC part of 2.5V. Now i want to digitize this signal.
     
  4. MrChips

    Moderator

    Oct 2, 2009
    12,442
    3,361
    Add a DC voltage bias to bring the signal to the mid-range of the ADC input.
     
  5. himadri117

    Thread Starter New Member

    Nov 11, 2014
    10
    0
    I made a typo... the RF range is from 3.5V to 1.5V with a DC of 2.5V....... I typed it as -1.5V
     
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