QI am interested in a build a Vector Network Analyzer but AD8302 can only
detect +-90 degrees.
To make a good Vector Network Analyzer its not necessary to detect +-180
AIN fact the AD8302 allows you to measure 0 to 180deg phase but there is no
simple way to determine direction of phase.
One possibility is to temporarily insert a known phase shift into one of the
signal paths and then observe the way in which the VPHS output changes. When
slope of the VPHS output is determined, then one can determine in which
quadrant the angle lies.
For narrow band applications (which obviously does not describe this
application) two AD8302's can be used. The INPA's for each of the AD8302's are
connected to the same signal, via transmission lines of equal electrical
length. One of the INPB inputs is connected to the other signal of interest.
The second INPB is connected to the same signal, but via a transmission line
that is 90 degrees linger. Then, the VPHS outputs can be compared to each
other to determine the phase angle anywhere between 0 and 360 degrees.