Fairly often we receive questions (generally from grad students) about how to use the mixer as a phase detector. The basic idea is that the output of a mixer will inherit the phase from both the LO and the RF (or IF) signals. That is to say that the output signal will inherit the phase difference between the LO and input. This is why the quad hybrid on an image reject or single sideband mixer can be put on either the LO or RF port (a topic for another post). In a downconversion where the LO and RF signals are the same the output will be at DC, where the ‘phase’ translates into an amplitude. When the two signals are in phase with each other the voltage will be at a positive maximum, when they are 180° out of phase the voltage will be at a negative maximum, and when they are either 90° or 270° phased to each other the output will be purely imaginary, and the voltage will be (ideally) zero.
The wrench in the works is that the mixer is not perfect, and imperfections in the amplitude and phase balance of the baluns or mismatch in the diodes will lead to an inherent DC offset voltage even when the signals are totally in phase. This is referred to as the ‘DC offset voltage’ of the mixer. It is typically measured by terminating the RF port with a matched load and measuring the DC voltage at the IF port with a standard power LO input.
Typically it is recommended that the measurement be performed with a low pass filter at the IF port to eliminate the harmonics. The problem with this is that the mixer is designed for operation in a 50Ω environment, and a reflective low pass filter will present a very non-50Ω termination on the RF port, deteriorating the isolation that is provided by the mixer. Therefore Marki Microwave recommends that the DC offset measurement be performed with a good 50Ω termination at high frequencies. This can be provided with either an absorptive filter (such as the wavefade filters) or with a diplexer or bias tee, with the high frequency port terminated in a 50Ω load. We recommend that all phase measurements be performed in this manner, with the mixer matched at all ports.
As an example, below is the measured DC offset for our new ML1-0113L mixer using either a low pass filter or a terminated bias tee.
As you can see the 50 ohm termination makes a significant difference in measured voltage. As a point of reference, and to demonstrate how awesome the new microlithic mixers are, below are comparisons of the new mixers with other high performance mixers in the Marki line.