Mathcad - ee217projtodonew2.mcd

Noise Correlation
Except in special cases, the optimal noise source impedance will be different from the optimal
power source impedance. Thus, when using the optimal noise source impedance, the power
gain will not be maximized. On the other hand lower power gain will increase the significance of
the noise from later stages on the noise figure of the system. Thus a tradeoff exists between the
optimal noise source impedance and optimal power source impedance for minimum overall
system noise figure. The optimal source impedance for minimum system noise figure can be
found by including the noise of the following stages, such as the mixer, in the optimal noise
source impedance calculation. In many optimal source impedance calculations, other sources of
noise are also neglected. The noise sources to be included in the source impedance calculation
are noise from the bias source, bonding pad series resistance, series resistance of inductor
degeneration from bond wires and especially from low-Q on-chip spiral inductors. The series
resistance of these spirals tend to increase optimal noise source impedance, when they are used
for emitter degneration of common emitter and common source amplifiers.
To find the noise gains, a load impedance is assumed, which will in turn affect the gains and the
optimal noise source impedance. The optimal load impedance for maximum power transfer is a
good guess for the load impedance. It becomes a better guess as the noise of following stages is
increased. Once the optimal source impedance is found using a guess for the load impedance, the
load impedance for maximum power transfer should be recalculated from the complex conjugate of
the output impedance of the device with the optimal system noise source impedance.
Thus the steps for finding optimal system noise source impedance are as follows:
1. Find S parameters of the circuit directly or indirectly by finding the Y
parameters of the device directly.
2. Find the optimal load and source impedance with the calculated S
parameters.
3. Find the gains of all noise sources and an input voltage source to the output
with a zero source impedance. Include all noise source gains, including the
noise of following stages and bias noise.
4. Find the gains of all noise sources and an input current source to the output
with a infinite source impedance.
5. Use the gains to find the input referred equivalent voltage and noise sources
and the correlation impedance.
6. Use the equivalent voltage and current noise sources to find the equivalent
series and parallel noise resistances.
7. Use the correlation impedance and noise resistances to find the optimal
source impedance.
8. Recalculate desired output impedance given the optimal system noise
source impedance.
.
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