TECHCENTER

Single balanced mixers can remove either the LO or RF content from the IF output without the use of filters by using two diodes and a 180° hybrid coupler as a balun.

Early wideband receivers used a 90° hybrid combiner, which separated the RF and LO, but the isolation was dependent on how well the diodes were impedance matched. The 180° hybrid coupler solved this problem. This technique isolates the RF and LO ports and reduces unwanted intermodulation product. As shown in Figure 3, the RF and LO signals are applied to the sum and delta input ports of the hybrid, and the two corresponding hybrid outputs each feed a diode in turn (with one facing toward the hybrid and the other away). The outer ends of the diodes are tied together and taken as the IF output:

Figure 3: Simple block diagram of a single balanced diode mixer.

The signal applied to the 180° port of the hybrid (which can be configured to be either the LO or RF port) will be balanced and thus not appear at the IF output.

An example schematic of a single-balanced mixer is shown below in Figure 4 with the 180° hybrid at the RF and LO input ports and a low pass filter (LPF) at the IF output port (L1, C2, and C3 network):

Figure 4: Detailed schematic for a single balanced diode mixer.

The LO is applied to the 180° port and is thus balanced and suppressed from the IF port. Even so, it still drives the on/off action of the Schottky diodes to produce the mixing action. The RF signal is suppressed from the IF output by a capacitor to ground (C1) as well as a dedicated low-pass filter (LPF).

At higher LO powers, the diodes can self-bias, causing unacceptable conversion loss and isolation levels.  To avoid this, RF chokes (RFC) are shunted to ground between the coupler and the diodes.

Of the balanced mixer topologies, single-balanced mixers require the least amount of LO power. The LO or RF rejection at the IF output is typically between 20 and 30 dB.

Advantages and Disadvantages of the Single Balanced Mixer Topology

Advantages

Requires least LO of the balanced types

Suppresses AM noise from LO
(unbalanced does not)

Disadvantages

Only isolation of RF or LO without filtering

Filtering results in narrow operation band operation

Requires more LO power than unbalanced

Less linear than double-balanced

More conversion loss than double-balanced