TECHCENTER

The double balanced mixer topology has four diodes in a ring or star configuration along with two baluns (one each for the RF and LO), and provides rejection of both the LO and RF content at the IF output. This means all ports are inherently isolated from each other without the need for filtering.  This is due to the combined properties of the ring diode circuit and the wideband baluns.  

Compared to single-balanced mixers, double-balanced diode mixers have more linearity and less spurious emissions. They also tend to have better conversion efficiency and can achieve broader bandwidths since no filtering is required at the IF port. However, this mixer architecture requires a higher LO drive level, and the ports are highly sensitive to reactive terminations.

The ideal application for a double-balanced mixer is a lower-cost application where moderate LO power is available and the RF and IF frequencies do not overlap.

Figure 6: Simplified block diagram of a double-balanced diode mixer.

An example double-balanced mixer schematic for the block diagram in Figure 6 is shown in Figure 7:

Figure 7: Detailed schematic of a double-balanced diode mixer.

The IF signal is tapped off of both the LO and RF baluns. Using separate baluns for the RF and LO ports provides isolation between the RF and LO ports, reducing the level of intermodulation products compared to that of an unbalanced mixer.

Advantages and Disadvantages of the Double Balanced Diode Mixer

Advantages

Inherent isolation of both RF and LO

More linear than single-balanced

Less spurs than single-balanced

Broadband device (no filtering needed)

Less conversion loss than triple-balanced topology

Disadvantages

Requires more LO power than single-balanced

Ports sensitive to reactive terminations