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Are you asking for information on how to insure that the LOs for I and Q are 90 degrees apart?
One trick is to use an analog diode ring mixer. Put one of your oscillators to the LO and the other to the RF port of the mixer. You should get 0 V out if they are 90 degrees apart. Make sure to measure the mixer output offset voltage by first making the RF input from a 50 ohm resistor. Subtract this value from the reading you get with the other LO signal to the RF.
Minicircuits makes special diode ring mixers for such phase measurements. They have low DC offsets. **broken link removed**
If you want to include the phase errors of the I Q mixers, put the same signal into both of them and use the minicircuits phase detector on the IF output signals.
Look at the AD8302 from Analog Devices. It can measure amplitude loss up to 60dB and phase differential up to 180 degrees for frequencies up to 2.7Ghz.
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