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Hardware Mixer Calibration

Question asked by emcsnoopy on Dec 16, 2010
Latest reply on Dec 21, 2010 by enash

I am trying to develop a package which contains an IQ mixer (probably around the 4-8GHz band), and automatically calibrates the mixer (corrects the IQ plane for offset, skew, and rotation). 


The calibration needs to be quite good ideally >60dB contrast so off really needs to be off.


I am currently working on the first part, the offset which contributes to leakage of the LO when the device is nominally off. 


We currently do this using a suite of test equipment.  We use a spectrum analyzer to measure the leakage, and adjust the (1GS/s) dac that is driving the IQ voltages to minimize the measured power.  This works well enough to achieve the desired contrast but uses >$50k worth of test equipment.  Performing a one time calibration works to some extent but is not good enough due to drifts. 


My plan was to try and somehow lock a VCO or other cheap tunable frequency source (I'd be interested to hear what might work best for a wide band tunable source) to an 10 MHz offset.  Then use the VCO as a local oscillator to mix down, and bandpass the leakage, and then feed it into a log power detector.  From there using a gradient descent type algorithm for nulling the leakage. 


I've tested a relatively cheap and easy solution for the detection if provided with an appropriate LO, but currently I am using a $30k generator as the LO so I haven't gained much.  The next step is to make the oscillator which can lock to a specific offset.  Or alternatively to develop a better path for measuring the leakage power. 


I'm very interested in any ideas as to how to improve the approach or implement a cheap slave oscillator at an offset with respect to one which is provided.