The FET without drain voltage can also be used to design and implement passive sub-harmonic mixers without additional DC power consumption. In order to reduce the area, some designs adopt the way of bending transmission lines, but also lead to the limited bandwidth of 30 GHz and 20 GHz respectively. The most typical circuit such as 78–114 GHz sub-harmonic mixer utilizes quarter-wavelength open-stub and short-stub at each end of the diodes pair to achieve good isolation between LO and RF ports, but it also brings a large chip area of 3 mm 2. The sub-harmonic mixer with diode has simple circuit structure and zero power consumption, so it is widely used in millimeter wave band. The mixer based on active device can achieve better frequency conversion gain, but it will be accompanied by larger noise and additional DC power consumption. The sub-harmonic mixer is usually realized by using the nonlinear characteristics of active transistor such as field effect transistor (FET) and bipolar transistor or varactor diode device.
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