How to measure phase difference of 2 signals using IQ demodulator

[mtxx]

for one section of my project, i need to know the phase difference between two signal.
they are both 40MHz, but out of phase by a certain angle.

i have a AD8333 board from Analog Devices,
it is a dual I/Q demodulator consist of double-balanced gilbert cell mixers.

The board has 2 RF inputs, a LO input, and 4 output (I1, I2, Q1, Q2).

I have an oscilloscope to display the results.

according to wikipedia http://en.wikipedia.org/wiki/Phase_detector#Analog_phase_detector
there is a way to measure the phase difference using IQ demoludator.in this post, https://www.physicsforums.com/showthread.php?t=336619
f95toli mentioned that:
the magnitude is just given by the norm (sqrt(I^2+Q^2)) and the phase by the phase angle (arctan I/Q)

So is this method bellow correct?
to get the phase difference of both my RF signal,
i put one the signal into RF input1, and the second signal into the LO,
then i display both the I1, and Q1 in the oscilloscope, and get an get the peak to peak magniture for both the I1 and Q1, then calculate arctan I1/Q1, to get the phase difference?

 

[Baluncore]

So is this method bellow correct?

Yes, almost. But think about it differently.

Select one signal as your reference and apply it as the local oscillator. Connect the other signal to the RF input, which will then be down-converted to DC at the IQ output. The I and Q outputs can drive the x and y axes respectively of the oscilloscope to give you a polar plot. Or you can digitise the I and Q, then compute the amplitude = Sqrt(I^2+Q^2) and the phase = Atan2(I/Q).

You will have a problem with the AD8333 because it divides the LO input by 4 to generate the internal LO quadrature signal. In order to use that chip you might run a PLL VCO at 4 times the frequency of your input signals as your LO. Then apply your two input signals to the two RF channels, use the I output from one channel, through a LP filter, to control the PLL VCO frequency. The other channel outputs, I and Q will then give you the phase difference.