MIXERS AND FREQUENCY DISCRIMINATORS

Mixers are used to convert a signal from one frequency to another. This is done by combining the original RF signal with a local oscillator (LO) signal in a non-linear device such as a Schottky-barrier diode. | MIXERS AND FREQUENCY DISCRIMINATORS Mixers are used to convert a signal from one frequency to another. This is done by combining the original RF signal with a local oscillator LO signal in a non-linear device such as a Schottky-barrier diode. The output spectrum includes The original inputs LO and RF All higher order harmonics of LO and RF The two primary sidebands LO RF m n 1 All higher order products of mLO nRF where m n are integers A DC output level The desired output frequency commonly called the intermediate frequency IF can be either the lower LO-RF or upper LO RF sideband. When a mixer is used as a down converter the lower sideband is the sideband of interest. A microwave balanced mixer makes use of the 3 dB hybrid to divide and recombine the RF and LO inputs to two mixing diodes. The 3 dB hybrid can be either the 90 or 180 type. Each has certain advantages which will be covered later. The critical requirement is that the LO and RF signals be distributed uniformly balanced to each mixer diode. Figure 1 is a typical balanced mixer block diagram. The mixer diodes are reversed relative to each other the desired frequency IF components of each diode are then in-phase while the DC outputs are positive and negative respectively. The two diode outputs are summed in a tee where the DC terms cancel and only the desired IF component exists at the IF port. _ IF Output Figure 1. Mixer Block Diagram Other types of mixers exist including the double-balanced mixer and the Ortho-Quad quadrature fed dual mixer. The relative advantages and disadvantages of each of the four types are summarized in Table 1. Table 1. Mixer Comparison Mixer Type VSWR 1 Conversion Loss 2 LO RF Isolation 3 Harmonic Suppression 4 Dynamic Range IF Bandwidth 90 Hybrid good lowest poor poor-fair high wide 180 Hybrid poor low good good high wide DoubleBalanced poor low Very good -excellent very good high extremely wide Ortho Quad .