Figure 8.23 Different mixer circuits: (a) single-ended mixer; (b) balanced mixer; (c) doublebalanced mixer; and (d) Gilbert cell mixer.

in Section 6.1). At low radio frequencies tapped transformers are used. The balanced and double-balanced mixers have many advantages over the singleended mixer: There is a good isolation between the signal and LO ports, the AM noise of the LO is rejected, certain spurious signals are rejected, and the compression and intercept points are higher. In a double-balanced mixer there is a good isolation also between the IF and other ports. A disadvantage of balanced and double-balanced mixers is their higher LO power requirement. The Gilbert cell mixer shown in Figure 8.23(d) is a double-balanced transistor mixer, which is widely used in integrated circuits.

8.5.2 Frequency Multipliers

In a frequency multiplier, harmonic signals at frequencies nf are generated, and using a filter one of them is selected as the output signal. This way we