In the LTC5589 Datasheet (5589f) on Page 30, there is a small section on using the LTC5589 as a vector modulator (in other words, to control the amplitude and phase of an existing RF signal applied to the LO port). It says "The phase and gain can be set by DC values at the baseband inputs in combination with the settings of registers 0x00 to 0x08." I was thinking this meant that I could provide fixed DC values at the baseband inputs, and use only the LTC5589 internal registers to control the "vector", but after trying out the eval board, I think I am concluding that one would need to provide adjustable DC values externally, not simply fixed DC values.
My setup consists of a VNA hooked up with Port 1 to the LO port and Port 2 to the RF port, and looking at S21 magnitude and phase. I am applying 1.2V DC (from an LDO) to BBMI and BBMQ, and 1.6V (from an LDO) to BBPQ and BBPI. I believe I could go a little higher in differential voltage but 1.2V is the lowest I can go with LDOs I have on hand and I wanted to stay at the 1.4V common mode voltage.
After getting a reasonable result on the VNA with the default GUI settings, I put those VNA traces in memory and started changing the various registers in the GUI to see if I could get any meaningful phase shifts relative to the default state. I was unable to find any changes to the registers that resulted in anything other than small phase shifts (except for of course changing the LO frequency but I don't think that is a valid approach for what I'm trying to accomplish).
I'm hoping the author of this applications section, or someone who has implemented a vector modulator (for phase/amplitude control of a non-CW RF signal into the LO port, not for I/Q modulation of a CW at the LO port) with this device (or similar) can chime in and confirm my conclusion that I will need to have control over the DC values at the baseband inputs in order to use this in such a manner.