11
S and Noise Parameter Measurements
The position of the reference planes used for the mea-
surement of both S and Noise Parameter measurements is
shown in Figure 23. The reference plane can be described
as being at the center of both the gate and drain pads.
S and noise parameters are measured with a 50 ohm
microstrip test xture made with a 0.010” thickness
aluminum substrate. Both source pads are connected
directly to ground via a 0.010” thickness metal rib which
provides a very low inductance path to ground for both
source pads. The inductance associated with the addition
of printed circuit board plated through holes and source
bypass capacitors must be added to the computer circuit
simulation to properly model the e ect of grounding the
source leads in a typical ampli er design.
a matching network that will present
o
to the device with
minimal associated circuit losses. The noise gure of the
completed ampli er is equal to the noise gure of the
device plus the losses of the matching network preceding
the device. The noise gure of the device is equal to Fmin
only when the device is presented with
o
. If the re ection
coe cient of the matching network is other than
o
, then
the noise gure of the device will be greater than Fmin
based on the following equation.
NF = F
min
+ 4 R
n
|
s
–
o
|
2
Zo (|1 +
o
|
2
)(1 - |
s
|
2
)
Where Rn/Zo is the normalized noise resistance,
o
is the
optimum re ection coe cient required to produce Fmin
and
s
is the re ection coe cient of the source impedance
actually presented to the device.
The losses of the matching networks are non-zero and
they will also add to the noise gure of the device creating
a higher ampli er noise gure. The losses of the matching
networks are related to the Q of the components and as-
sociated printed circuit board loss.
o
is typically fairly
low at higher frequencies and increases as frequency is
lowered. Larger gate width devices will typically have a
lower
o
as compared to narrower gate width devices.
Typically for FETs, the higher
o
usually infers that an
impedance much higher than 50 is required for the
device to produce Fmin. At VHF frequencies and even
lower L Band frequencies, the required impedance can
be in the vicinity of several thousand ohms. Matching to
such a high impedance requires very hi-Q components
in order to minimize circuit losses. As an example at 900
MHz, when air wound coils (Q>100)are used for matching
networks, the loss can still be up to 0.25 dB which will add
directly to the noise gure of the device. Using multilayer
molded inductors with Qs in the 30 to 50 range results in
additional loss over the air wound coil. Losses as high as
0.5 dB or greater add to the typical 0.15 dB Fmin of the
device creating an ampli er noise gure of nearly 0.65 dB.
Figure 23. Position of the Reference Planes.
Gate
Pin 2
Source
Pin 3
Drain
Pin 4
Source
Pin 1
Reference
Plane
Microstrip
Transmission Lines
Px
Noise Parameter Applications Information
The Fmin values are based on a set of 16 noise gure mea-
surements made at 16 di erent impedances using an ATN
NP5 test system. From these measurements, a true Fmin
is calculated. Fmin represents the true minimum noise
gure of the device when the device is presented with an
impedance matching network that transforms the source
impedance, typically 50, to an impedance represented
by the re ection coe cient
o
. The designer must design
