The operation of a QIFM can be understood by following the signal voltage vectors.
Another way of looking at the operation of a QIFM is to realize that the two IF outputs are proportional to a sin[Theta] and a cos[Theta], respectively.
The QIFM is identical in design and application to a correlator (phase discriminator) provided one of the QIFM inputs (a or b) is strong enough to act as an LO.
Since the QIFM essentially is a phase discriminator, DQIFMs also can be used as phase discriminators when the LO and RF are located at the same frequency provided there is enough power at the LO port to bias the diodes.
One technique used to accomplish this involves a Costas loop, which has a quadrature IF mixer (QIFM) as a critical component.
This paper presents the results of a design effort to realize a QIFM in a small physical size suitable for incorporation in L-band receivers in portable data applications.
A schematic diagram of the passive approach for the single-ended mixer used in the QIFM is shown in Figure 2.