Impedance measurement using I/Q demodulation

Discussion in 'Wireless & RF Design' started by Phat, Feb 20, 2013.

  1. Phat

    Thread Starter New Member

    Dec 31, 2012

    I hope I put this in the right forum category. Since I am asking about demodulation and common techniques in RF applications I figured this would be the place to ask.

    I want to measure an unknown impedance using an I/Q demodulator.


    I was thinking of using a DDS (Direct Digital Synthesizer) as the sine wave signal generator. Splitting this signal to a reference signal and an excitation signal. The reference signal will go directly to the LO input while the excitation signal will go through a "black box" with an unknown impedance (RC, no L), and then to the RF input. The signal out from the "black box" will probably be attenuated and phase shifted. I am interested in getting the phase shift phi = arctan(Q/I) and amplitude A = sqrt(I^2 + Q^2).

    The AD8333 has this block diagram:


    The I and Q outputs are currents, so I would use a current-to-voltage converter to get this converted to a voltage. Assuming the voltage is alternating I would low pass filter the alternating voltage at each output (I and Q) and sample this value for a time (100ms) with an ADC and calculate the average on my micro controller. I am targeting measurements in the close to 0 to 300 000Hz intervall.

    Does it sounds like the AD8333 will fit my needs?