Consider first a mixer with a broadband buffer amplifier, shown in figure Figure 1:
The integrated LO buffer amplifier will typically have typically a bandwidth covering from perhaps one-tenth the LO frequency up to an octave above. The gain probably will usually peaks below the desired LO frequency, with the natural 1/f gain roll off of the active device slowly reducing the gain above the LO frequency. Since the active devices are not noiseless, their amplified noise will appear at the output of the buffer. The noise of the buffer amplifier, shaped by its frequency response, appears on its output.
The noise from the buffer amplifier travels to the mixer IF output through two paths, attenuated by the balance and LO rejection of the mixer at the IF and RF ports. The IF frequency noise at the buffer output adds directly to the output noise, while the signal leaked to the RF port will be converted to the IF output. Note that noise from the signal, image, and multiples of the LO frequency +/- IF frequency will also be converted to the IF output by the mixer’s fundamental and harmonic mixing properties. In general, the harmonic mixing has a marginal effect.
In Figure 3 the LO buffer noise spectrum is shown along with the converted signal and image band noise that appears in the IF output. The cumulative effect of the buffer noise that intrudes at the IF frequency directly is shown by the dashed curve above the converted LO noise curve. Although it would be desired to introduce a filter between the LO buffer amplifier and the mixer, it is often impractical to do. The size of the inductor would be costly in chip area, and active or R/C filters would be wasteful of LO power.
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