Radio Frequency Integrated Circuit Design - Webs

210 Radio Frequency Integrated Circuit Design
strongly LO voltage dependent, total noise happens to be roughly independent
of instantaneous LO voltage, and the relative value is approximately 11 (arbitrary
units). Noise from the source is dependent on instantaneous LO voltage varying
from 0 to about 1.5 (arbitrary units). This plot illustrates why maximum signal
gain and minimum noise figure is realized for a sufficiently large LO signal
such that minimal time is spent around 0V. Thus, minimum double-sideband
noise figure is
NF = 10 log Notot 11
= 10 log = 8.65 dB
No(source) 1.5
For a real, sinusoidal signal, some time is inevitably spent at 0V with a
resulting time domain waveform as shown in Figure 7.10.
In the diagram, the effective input noise is reduced down to about 1.1,
and as a result the noise figure is increased to about 10 dB.
Example 7.2 Mixer Noise and Gain with Degeneration
In this example, some equations and simulation results will be shown for noise
and gain versus degeneration.
Without consideration of input and output matching, the noise sources
associated with R S and R E of Figure 7.11 both have gain approximately equal
to the signal gain given by
Figure 7.10 Noise calculations.