TABLE OF CONTENTS For a given LO frequency, there are two different RFs, ( ? LO + ? IF) and ( ? LO - ? IF) that can result in the same IF. Depending on the application, one of these frequencies is the desired RF frequency, while the other is the image frequency. In receiver applications, this implies that there are two frequencies that will down-convert to the same IF. In the case of up-conversion, two RF frequency bands, referred to as the upper and lower sidebands, are generated. In many applications, these frequencies are close together and it is not feasible to isolate them using filters. Instead, hybrids are used to isolate them by introducing different phase shifts to the two frequencies in the mixer circuit.
Figure 7.47 a shows the block diagram of the phasing-type image-rejection mixer. It consists of two identical mixers, usually double-balanced, which are fed with phase-quadrature RF input signals and an in-phase LO. The two downconverted sideband signals can then isolated at the outputs of the 90 hybrid, with the unwanted sideband terminated in a resistive load.
The block diagram of the phasing-type single-sideband modulator is shown in Figure 7.47 b. The RF input signals and the modulating signals are fed in phase-quadrature, while the mixer outputs are combined with an in-phase power combiner. As shown, the single-sideband modulator...
