Radio Frequency Integrated Circuit Design - Webs

Mixers
Figure 7.9 Mixer noise shown at various LO levels.
In order to determine noise figure from Figure 7.9, the relative value of
the total noise compared to the noise from the source must be determined.
Very conveniently, the calculated noise figure is approximately the same when
calculated using a slowly swept dc voltage at the LO input or with an actual
LO signal. With a slowly swept dc voltage, the mixer becomes equivalent to a
cascode amplifier and the LO input serves as a gain-controlling signal. When
used as a mixer, any noise (or signal) is mixed to two output frequencies, thus
reducing the output level. This results in the mixer having less gain than the
equivalent differential pair. However, noise from both the RF and the image
frequency is mixed to the IF, resulting in a doubling of noise power at the
output. Thus, the noise prediction based on a swept LO analysis is very close
to that predicted using an actual LO signal.
There are a few other points to consider. One is that with a mixer treated
as an amplifier, output noise is calculated at f RF, so if some output filtering is
included, for example, with capacitors across R C , the noise will be reduced.
However, the ratio of total noise to noise due to the source can still be correct.
Another point is that in this analysis, noise that has been mixed from higher
LO harmonics has not been included. However, as will be shown in Example
7.3, these end up not being very important, so the error is not severe. Some
of these issues and points will be illustrated in the next three examples.
Example 7.1 Mixer Noise Figure Determination
For the mixer simulation results shown in Figure 7.9, estimate the mixer noise
figure.
Solution
The noise figure is given by the ratio of total noise to noise from the source.
In this example, while individual components of noise from most sources are
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