I recently started taking measurement on ADL5390 evaluation board using VNA.
I treated the evaluation board as a 3port network for s-parameters , IQ RF inputs and RF output with IQ baseband DC biased for gain control setting.
When I was trying to apply different IQ baseband voltage to see the gain on RF output, the gain obtained for IBB = 0.5V and QBB = 0.5V are 3dB higher than the gain I obtained for IBB = -0.5V and QBB = 0.5V when at high frequency (>1.5GHz).
When I have IBB = QBB = 0.5V and IBB= QBB = - 0.5V, I get 10dB gain.
When I have IBB = - QBB = 0.5V and IBB = -QBB = - 0.5V, I get 5dB of gain.
According the datasheet the gain should be constant at 7dB for 2.4GHz input signal.
This problem does not occur at lower frequency (< 1GHz of input RF).
I wondering if there is something i'm doing wrong with the measurements.
Thanks for any input and help!!!