| Popis: |
A theory for a mixer using four MOSFETs in a broadband balanced-bridge configuration has been developed in terms of both ideal models and approximate representations for departures from the ideal case. An interfering signal is assumed to be present in order to analyze intermodulation and cross-modulation distortions. The mathematics of a MOSFET mixer are developed and compared with the conversion properties and intermodulation limitations of the simple nonlinear-resistance mixer. Measurements are made in the 10-200 MHz range and close agreement with the theory is found for conversion loss. Disagreement in RF and LO isolations is attributed to stray capacitances and impedance mismatching. It is also shown that circuit-board and transformer design greatly affects performance of a mixer. A comparison of MOSFET and hot-carrier diode double balanced mixers is given and it is indicated that the MOSFET mixer is superior in conversion loss and RF and LO isolations. |
