Figure 7. Gilbert Cell Mixer
The Gilbert Cell mixer shown in Figure 7 is a compact, efficient approach to combining a differential amplifier with a phase reversing switch mixer. Often the differential amplifier provides unbalanced to balanced transformation. The switching transistors also can operate with an unbalanced input. However, the best even order distortion performance and LO to IF isolation is obtained with balanced inputs. The same circuit topology works well with FET devices also. With no filter between the differential amplifier and the switching cell, the gain of the mixer has the 1/F slope of the amplifier. Shunt resistors across the RF input will improve the flatness, provide an input match, and add noise. The lack of an image frequency filter between the RF amplifier and the mixer degrades the noise figure by more than 3 dB, if the LO is above the RF frequency. If two such mixers are used in an image reject configuration, the image noise from the differential RF amplifiers will not be rejected. This is a severe disadvantage compared with a passive mixer followed by an IF amplifier of modest gain.
In figure 8, below, the block diagram of a Gilbert cell mixer is shown.
Figure 8. Block Diagram of Gilbert Cell Mixer
The image noise of the RF amplifier ahead of the power split to the 2 mixers is rejected by typically 20 to 30dB. The quadrature coupler shown in the LO path can be in the RF path instead. In the location shown, the amplitude imbalance of the coupler has little effect on the mixer performance, because the mixer sensitivity to LO variation is small.
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